G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin. This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.\nAnother instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. Incidentally, the high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.\nAdditionally, it was found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. Yet, it was hypothesized that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.\nSickle cell anemia, Beta-thalassemia, and cystic fibrosis are some other examples of the role antagonistic pleiotropy may play in genetic disorders."}],"string":"[\n {\n \"doc_id\": 8660851524417328000,\n \"text\": \"Role in disease[edit]\\nThe survival of many serious genetic disorders in our long evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are defined by the existence of deleterious alleles, then natural selection acting over evolutionary time would result in a lower frequency of mutations than are currently observed. In a recent article, Carter and Nguyen identify several genetic disorders, arguing that far from being a rare phenomenon, antagonistic pleiotropy might be a fundamental mechanism for the survival of these non-optimal alleles.\\nIn one of these studies, 99 individuals with Laron syndrome (a rare form of dwarfism) were monitored alongside their non-dwarf kin for a period of ten years. Patients with Laron syndrome possess one of three genotypes for the growth hormone receptor gene (GHR). Most patients have an A->G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin. This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.\\nAnother instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. Incidentally, the high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.\\nAdditionally, it was found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. Yet, it was hypothesized that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.\\nSickle cell anemia, Beta-thalassemia, and cystic fibrosis are some other examples of the role antagonistic pleiotropy may play in genetic disorders.\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":3717481448095056000,"text":" S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008."},{"doc_id":6800680407069684000,"text":" \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28"},{"doc_id":4946908657905475000,"text":"redity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911. ^ Shelomi M, Zeuss"},{"doc_id":6917128559766396000,"text":" and human lactase genes\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria"},{"doc_id":1910979479660340500,"text":".16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911. ^ Shelomi M, Zeuss D (5 April 2017). \"Bergmann's and Allen's Rules in Native European and Mediterranean Phas"},{"doc_id":7451854849775404000,"text":"Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911.\n\nShelomi M, Zeuss D (5 April 2017). \"Bergmann's and Allen's Rules in Native European and Mediterranean Phasmatodea\". Frontiers in Ecology and Evolution. 5."},{"doc_id":9116245427462050000,"text":". Protective effects of the sickle cell gene against malaria morbidity and mortality. Lancet. 2002;359:1311–1312. [PubMed] [Google Scholar]\nAlawieh A., Musharrafieh U., Jaber A., Berry A., Ghosn N., Bizri A.R. Revisiting leishmaniasis in the time of war: the Syrian conflict and the Lebanese outbreak. Int. J. Infect. Dis. 2014;29:115–119. [PubMed] [Google Scholar]\nAlthaus C.L"},{"doc_id":4961714778855227000,"text":" G, England PR, Bradley DG, Jann OC, Bertorelle G, et al. (December 2003). \"Gene-culture coevolution between cattle milk protein genes and human lactase genes\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011"},{"doc_id":5861845573567613000,"text":" Evans D.B., Jha P., Mills A., Musgrove P., editors. Disease Control Priorities in Developing Countries. The International Bank for Reconstruction and Development/The World Bank Group; Washington, DC: 2006. [Google Scholar]\nLanghorne J., Ndungu F.M., Sponaas A.M., Marsh K. Immunity to malaria: more questions than answers. Nat. Immunol. 2008;9:725–732. [PubMed] [Google Scholar]\nLeavell H.R., Clark E.G. McG"},{"doc_id":469742901370423600,"text":"Beja-Pereira A, Luikart G, England PR, Bradley DG, Jann OC, Bertorelle G, et al. (December 2003). \"Gene-culture coevolution between cattle milk protein genes and human lactase genes\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396.\n\nHedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–"},{"doc_id":4422283637412871000,"text":"Whitty CJM, Ansah E. Malaria control stalls in high incidence areas. BMJ.\n2019 May 21; 365 :l2216. [ [ PubMed : 31113780\n](https://pubmed.ncbi.nlm.nih.gov/31113780) ]"},{"doc_id":2533275001596672000,"text":"* [ Folic acid supplementation and malaria susceptibility and severity among people taking antifolate antimalarial drugs in endemic areas. ](/pubmed/36321557) [Cochrane Database Syst Rev. 2022]"},{"doc_id":4987334270306259000,"text":" 5600404. PMID 28873402. ^ Beja-Pereira A, Luikart G, England PR, Bradley DG, Jann OC, Bertorelle G, et al. (December 2003). \"Gene-culture coevolution between cattle milk protein genes and human lactase genes\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". He"},{"doc_id":4866019419213672000,"text":" include both innate, genetic factors and acquired factors such as the specific immunity that develops following exposure or vaccination. The malaria resistance afforded carriers of the sickle cell trait exemplifies how genetics can influence susceptibility to infectious disease (Aidoo et al., 2002). Susceptibility is also affected by extremes of age, stress, pregnancy, nutritional status, and underlying diseases. These latter factors can impact immunity to infection, as illustrated by immunologically naïve infant populations, aging populations experiencing immune senescence, and immunocompromised HIV/AIDS patients.\nMechanical and chemical surface barriers such as the skin"},{"doc_id":8721903729592441000,"text":" cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at high altitudes. Some populations have evolved highly unique adaptations to very specific environmental conditions, such as those advantageous to ocean-dwelling lifestyles and freediving in the"},{"doc_id":7579552577826821000,"text":"Riehle, M. A., _et al_. Using bacteria to express and display anti-\n_Plasmodium_ molecules in the mosquito midgut. _International Journal of\nParasitology_ **37**, 595–603 (2007)"},{"doc_id":4789399353139791000,"text":".\n\n^ Manczinger M, Boross G, Kemény L, Müller V, Lenz TL, Papp B, et al. (January 2019). \"Pathogen diversity drives the evolution of generalist MHC-II alleles in human populations\". PLOS Biology. 17 (1): e3000131. doi:10.1371/journal.pbio.3000131. PMC 6372212. PMID 30703088.\n\n^ Zeisset I, Beebee TJ (2014). \"Dr"},{"doc_id":7099069620372744000,"text":"There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other"},{"doc_id":3006593300305258500,"text":" hair colors; from top left, clockwise: black, brown, blonde, white, red. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky"},{"doc_id":1394281673211026200,"text":" evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at"},{"doc_id":970114010689562600,"text":" The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at high altitudes. Some populations have evolved highly unique adaptations"},{"doc_id":712862380789366700,"text":"* Malaria. (2022).\n[ https://www.who.int/news-room/fact-sheets/detail/malaria\n](https://www.who.int/news-room/fact-sheets/detail/malaria)"},{"doc_id":180497508201615230,"text":" pathogens drive genetic diversity: MHC, mechanisms and misunderstandings\". Proceedings. Biological Sciences. 277 (1684): 979–88. doi:10.1098/rspb.2009.2084. PMC 2842774. PMID 20071384."},{"doc_id":4944825111087627000,"text":", clockwise: black, brown, blonde, white, red. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall"},{"doc_id":491145170522689500,"text":" sleep patterns.\nThere is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with"},{"doc_id":7291511111192631000,"text":"-2 infections, and genome sequences from 659 patients with severe COVID-19 revealed genetic variants that appear to be associated with life-threatening disease. One gene identified in these studies is type I interferon (IFN). Autoantibodies against type I IFNs were found in up to 13.7% of patients with life-threatening COVID-19, indicating that a complex interaction between genetics and the immune system is important for natural resistance to Covid.\nSimilarly, mutations in the ERAP2 gene, encoding endoplasmic reticulum aminopeptid"},{"doc_id":1787966951908398000,"text":"Folic acid supplementation and malaria susceptibility and severity among\npeople taking antifolate antimalarial drugs in endemic areas."},{"doc_id":4924544718779407000,"text":"These issues come into focus when you consider the devastating malaria\nepidemic that kills one to three million people worldwide every year. The\npathogens that cause malaria are protozoan parasites from the genus\n_Plasmodium_ that are transmitted through the bite of mosquitoes of the genus\n_Anopheles_. Both of these animal genera contain hundreds of species,\nalthough only a few are involved in transmitting malaria in humans. Recent\ngenetic studies of the symbiotic bacteria in the midgut of the _Anopheles\nstephensi_ mosquitoes have yielded"},{"doc_id":5012708765966004000,"text":" microbe. The evolution of the MHC polymorphism ensures that a population will not succumb to a new pathogen or a mutated one, because at least some individuals will be able to develop an adequate immune response to win over the pathogen. The variations in the MHC molecules (responsible for the polymorphism) are the result of the inheritance of different MHC molecules, and they are not induced by recombination, as it is the case for the antigen receptors.\nBecause of the high levels of allelic diversity found within its genes, MHC has also attracted the attention of many evolutionary biologists"},{"doc_id":4161775671760185300,"text":"[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)"},{"doc_id":6254006696981987000,"text":" up to 13.7% of patients with life-threatening COVID-19, indicating that a complex interaction between genetics and the immune system is important for natural resistance to Covid.\nSimilarly, mutations in the ERAP2 gene, encoding endoplasmic reticulum aminopeptidase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria Yersinia pestis. People who inherited two copies of a complete variant of the gene were twice as likely to have survived the plague as those who inherited two copies of a truncated variant"},{"doc_id":3336316715439913500,"text":" size is partly determined by genes and is also significantly influenced by environmental factors such as diet, exercise, and sleep patterns. A variety of human hair colors; from top left, clockwise: black, brown, blonde, white, red. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that"},{"doc_id":8842831987032849000,"text":":10.4049/jimmunol.168.1.260. PMID 11751970. S2CID 36838421.\n\n^ Yamaguchi T, Dijkstra JM (April 2019). \"Major Histocompatibility Complex (MHC) Genes and Disease Resistance in Fish\". Cells. 8 (4): 378. doi:10.3390/cells8040378. PMC 6523485. PMID 31027287.\n\n^ Abi Rached L, McDermott MF, Pontarotti P ("},{"doc_id":7517740263136706000,"text":" of major human infectious diseases. Nature. 2007;447:279–283. [PMC free article] [PubMed] [Google Scholar]\nYasuoka J., Levins R. Impact of deforestation and agricultural development on anopheline ecology and malaria epidemiology. Am. J. Trop. Med. Hyg. 2007;76:450–460. [PubMed] [Google Scholar]\nYu I.T., Li Y., Wong T.W., Tam W., Chan A.T., Lee J.H., Leung D.Y., Ho T. Evidence"},{"doc_id":6283416201445990000,"text":" has become clear that there are genetic factor which determine susceptibility to infection. For instance, up to 40% of SARS-CoV-2 infections may be asymptomatic, suggesting that many people are naturally protected from disease. Large genetic studies have defined risk factors for severe SARS-CoV-2 infections, and genome sequences from 659 patients with severe COVID-19 revealed genetic variants that appear to be associated with life-threatening disease. One gene identified in these studies is type I interferon (IFN). Autoantibodies against type I IFNs were found in"},{"doc_id":4985945625302089000,"text":":10.3390/cancers12051155. PMC 7281430. PMID 32375397.\n\n^ \"HLA Alleles Numbers\". hla.alleles.org.\n\n^ van Oosterhout C (February 2009). \"A new theory of MHC evolution: beyond selection on the immune genes\". Proceedings. Biological Sciences. 276 (1657): 657–65. doi:10.1098/rspb.2008.1299. PMC 2660941. PMID 18986972"},{"doc_id":3486976231575510500,"text":" against these may get much sicker from COVID‑19. Genetic screening is able to detect interferon effector genes. Some genetic variants are risk factors in specific populations. For instance, an allele of the DOCK2 gene (dedicator of cytokinesis 2 gene) is a common risk factor in Asian populations but much less common in Europe. The mutation leads to lower expression of DOCK2 especially in younger patients with severe Covid. In fact, many other genes and genetic variants have been found that determine the outcome of SARS-CoV-2 infections.\nWhile very"},{"doc_id":1055589053584834800,"text":"Kebede A., Mccann J.C., Kiszewski A.E., Ye-Ebiyo Y. New evidence of the effects of agro-ecologic change on malaria transmission. Am. J. Trop. Med. Hyg. 2005;73:676–680. [PubMed] [Google Scholar]\nKeiser J., De Castro M.C., Maltese M.F., Bos R., Tanner M., Singer B.H., Utzinger J. Effect of irrigation and large dams on the burden of malaria on a global and regional scale"},{"doc_id":1348784263469054700,"text":" (1): 57–106. doi : 10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \"Evidence for variable selective pressures at MC1R\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 107334"},{"doc_id":5835360718990907000,"text":" Nature of Selection on the Major Histocompatibility Complex\". Critical Reviews in Immunology. 37 (2–6): 75–120. doi:10.1615/CritRevImmunol.v37.i2-6.10. PMID 29773018.\n\n^ Sommer S (October 2005). \"The importance of immune gene variability (MHC) in evolutionary ecology and conservation\". Frontiers in Zoology. 2 (16): 16. doi:10.1186/1742-9994-2-16. PMC 12825"},{"doc_id":6676066334518801000,"text":" ](https://slcc.pressbooks.pub/collegebiology1/chapter/variation-adaptation/)"},{"doc_id":6546051547658558000,"text":" Biol. Med. 2014;57:198–207. [PubMed] [Google Scholar]\nHalpin J. Avian flu from an occupational health perspective. Arch. Environ. Occup. 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As pathogenic pressure on the previously common alleles decreases, their frequency in the population stabilizes"},{"doc_id":7145942413818299000,"text":"Guerra CA, Kang SY, Citron DT, Hergott DEB, Perry M, Smith J, Phiri WP, Osá\nNfumu JO, Mba Eyono JN, Battle KE, Gibson HS, García GA, Smith DL. Human\nmobility patterns and malaria importation on Bioko Island. Nat Commun. 2019\nMay 27; 10 (1):2332. [ [ PMC free article : PMC6536527\n](/pmc/articles/PMC6536527/) ]"},{"doc_id":2111678933010816500,"text":"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H"},{"doc_id":6707359235402285000,"text":"Malaria\nJapanese encephalitis\nLyme\nMalaria\nHendra\tSteinmann et al. (2006)"},{"doc_id":5810698596454523000,"text":" : 10.1038/nature09298. PMC 3173859. PMID 20811451. ^ Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \"Insights into human genetic variation and population history from 929 diverse genomes\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa"},{"doc_id":6235040381659362000,"text":". doi : 10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \"Evidence for variable selective pressures at MC1R\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 10733465. ^ Robin A ("},{"doc_id":7056037267678778000,"text":" promising results: _Enterobacter\nagglomerans_ bacteria were genetically engineered to display two anti-\n_Plasmodium_ effector molecules that kill the parasite before it is\ntransmitted to humans (Riehle _et al._, 2007). Now consider the genetic and\nphysiological differences between the wild-type and genetically modified\n_Anopheles stephensi_ mosquitoes: they are still the same species by all major\nstandards of species definition, yet what a difference it would make for\nhumankind if the _Plasmodium-_ resistant"},{"doc_id":7639397133823105000,"text":"Search PMC Full-Text Archive\nSearch PMC Full-Text Archive\nSearch in PMC\nAdvanced Search User Guide\nJournal List PLoS Genet v.12(7); 2016 Jul PMC4956268\nAs a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health.\nLearn more: PMC Disclaimer | PMC Copyright Notice\nLogo of plosgen\nPLoS Genet. 2016 Jul; 12"},{"doc_id":5044981846658660000,"text":"* Raphael Vallat [ ORCID: orcid.org/0000-0003-1779-7653 ](http://orcid.org/0000-0003-1779-7653) 1 ,\n* Sarah E. Berry [ ORCID: orcid.org/0000-0002-5819-5109 ](http://orcid.org/0000-0002-5819-5109) 2 ,\n* Neli Tsereteli 3 ,\n* Joan Capdevila 4 ,"},{"doc_id":6009779149809428000,"text":" of the United States of America. 104 (41): 16221–6. doi:10.1073/pnas.0704580104. PMC 1999395. PMID 17911263.\n\n^ Shum BP, Guethlein L, Flodin LR, Adkison MA, Hedrick RP, Nehring RB, et al. (March 2001). \"Modes of salmonid MHC class I and II evolution differ from the primate paradigm\". Journal of Immunology. 166 (5): 3297–308. doi"},{"doc_id":8327998045110626000,"text":"ase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria Yersinia pestis. People who inherited two copies of a complete variant of the gene were twice as likely to have survived the plague as those who inherited two copies of a truncated variant.\nSusceptibility also determined the epidemiology of infection, given that different populations have different genetic and environmental conditions that affect infections."},{"doc_id":1587330908315665200,"text":" sexual reproduction.\nThe clearance of the pathogens, either treatment-induced or spontaneous, it can be influenced by the genetic variants carried by the individual patients. For instance, for genotype 1 hepatitis C treated with Pegylated interferon-alpha-2a or Pegylated interferon-alpha-2b (brand names Pegasys or PEG-Intron) combined with ribavirin, it has been shown that genetic polymorphisms near the human IL28B gene, encoding interferon lambda 3, are associated with significant differences in the treatment-induced clearance of the"},{"doc_id":7414764089986002000,"text":"### Plasmodium"},{"doc_id":7008811410189903000,"text":" of its carriers. The\nmost ancestral strain is found in Africa, and the data from seven other\ndistinct geographical variants allowed scientists to develop a dispersal\nmodel that corresponds to the pattern of human migration (Falush, 2003). As\nglobalization continues to increase the rates of microbial transmission, it\nbecomes more and more crucial to study these organisms, for they play a major\nrole in regulating our body's functions."},{"doc_id":7195437749761732000,"text":" PLOS Genetics are provided here courtesy of PLOS\nOTHER FORMATS\nPDF (939K)\nACTIONS\nCite\nCollections\nSHARE"},{"doc_id":6522456750091133000,"text":"et demonstrated the existence of MHC genes in humans and described the first human leucocyte antigen, the protein which we call now HLA-A2. Some years later Baruj Benacerraf showed that polymorphic MHC genes not only determine an individual’s unique constitution of antigens but also regulate the interaction among the various cells of the immunological system. These three scientists have been awarded the 1980 Nobel Prize in Physiology or Medicine for their discoveries concerning “genetically determined structures on the cell surface that regulate immunological reactions”.\nThe first fully sequenced"},{"doc_id":6571952563565801000,"text":" virus. This finding, originally reported in Nature, showed that genotype 1 hepatitis C patients carrying certain genetic variant alleles near the IL28B gene are more possibly to achieve sustained virological response after the treatment than others. Later report from Nature demonstrated that the same genetic variants are also associated with the natural clearance of the genotype 1 hepatitis C virus."},{"doc_id":7853884882958839000,"text":"nature09298. PMC 3173859. PMID 20811451. ^ Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \"Insights into human genetic variation and population history from 929 diverse genomes\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa, the Americas, and O"},{"doc_id":4000437132722955000,"text":" P.; Elferink, R. Oude (2002). [ \"Mammalian ABC transporters in health and disease\" ](https://pure.uva.nl/ws/files/3499814/42885_202387y.pdf) (PDF) . _Annual Review of Biochemistry_. **71** : 537–592. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1146/annurev.biochem.71.1023"},{"doc_id":2974077984675988000,"text":"2): 101–9. doi:10.1016/s0165-0378(97)00056-9. PMID 9421795.\n\n^ Ober C, Weitkamp LR, Cox N, Dytch H, Kostyu D, Elias S (September 1997). \"HLA and mate choice in humans\". American Journal of Human Genetics. 61 (3): 497–504. doi:10.1086/515511. PMC 1715964. PMID 9326314.\n\n^ Chaix R,"},{"doc_id":8274408650100361000,"text":"journal=Nature&doi=10.1038%2Fnature13972&volume=515&pages=365-370&publication_year=2014&author=Stergachis%2CAB)"},{"doc_id":8520953987562449000,"text":" Cao C, Donnelly P (September 2008). \"Is mate choice in humans MHC-dependent?\". PLOS Genetics. 4 (9): e1000184. doi:10.1371/journal.pgen.1000184. PMC 2519788. PMID 18787687.\n\n^ Derti A, Cenik C, Kraft P, Roth FP (April 2010). \"Absence of evidence for MHC-dependent mate selection within HapMap populations\". PLOS Genetics. 6 (4): e1000925. doi:10"},{"doc_id":3474157058676721700,"text":"rhagic fever. Lancet. 2011;377:849–862. [PMC free article] [PubMed] [Google Scholar]\nGama J.A., Abby S.S., Vieira-Silva S., Dionisio F., Rocha E.P. Immune subversion and quorum-sensing shape the variation in infectious dose among bacterial pathogens. PLoS Pathog. 2012;8:e1002503. [PMC free article] [PubMed] [Google Scholar]\nGibbs E.P. The evolution of One"},{"doc_id":5136385584355986000,"text":" the initial hybridised population may have experienced up to a 94% reduction in fitness compared to contemporary humans. By this measure, Neanderthals may have substantially increased in fitness. A 2017 study focusing on archaic genes in Turkey found associations with coeliac disease, malaria severity and Costello syndrome. Nonetheless, some genes may have helped modern East Asians adapt to the environment; the putatively Neanderthal Val92Met variant of the MC1R gene, which may be weakly associated with red hair and UV radiation sensitivity, is primarily found in East Asian, rather than European, individuals. Some genes related"},{"doc_id":8746695212421639000,"text":"The protozoa _Plasmodium_ _genus_ causes the tropical disease [ malaria\n](https://www.medicalnewstoday.com/articles/150670). The parasite primarily\n[ transmits ](https://www.who.int/news-room/fact-sheets/detail/malaria)\nthrough [ mosquito bites ](https://www.medicalnewstoday.com/articles/311485)."},{"doc_id":6621003551263229000,"text":"[ Article ](https://doi.org/10.1111%2Fj.1440-1843.2011.02037.x) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=21848707)\n[ Google Scholar\n](http://scholar.google.com/scholar_lookup?&title=Phenotypes%20of%20patients%20with%20"},{"doc_id":2841831592050604000,"text":"Ilardo MA, Moltke I, Korneliussen TS, Cheng J, Stern AJ, Racimo F, et al. (April 2018). \"Physiological and Genetic Adaptations to Diving in Sea Nomads\". Cell. 173 (3): 569–580.e15. doi : 10.1016/j.cell.2018.03.054. PMID 29677510.\n\nRogers AR, Iltis D, Wooding S (2004). \"Genetic variation at the MC1R locus and the time since loss of"},{"doc_id":4740884425898056000,"text":"24. PMID 16435885.\n\n^ \"IPD-MHC Database\". EMBL-EBI.\n\n^ Kulski JK, Shiina T, Anzai T, Kohara S, Inoko H (December 2002). \"Comparative genomic analysis of the MHC: the evolution of class I duplication blocks, diversity and complexity from shark to man\". Immunological Reviews. 190: 95–122. doi:10.1034/j.1600-065x.2002.19008.x. PMID 12493009."},{"doc_id":6060117423807330000,"text":"The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are called MHC molecules.\nThe name of this locus comes from its discovery through the study of transplanted tissue compatibility. Later studies revealed that tissue rejection due to incompatibility is only a facet of the full function of MHC molecules: binding an antigen derived from self-proteins, or from pathogens, and bringing the antigen presentation to the cell surface for recognition by the appropriate T-cells. MHC molecules mediate the interactions of leukocytes, also called white blood cells (WBCs), with other leukocytes or with body cells. The MHC determines donor compatibility for organ transplant, as well as one's susceptibility to autoimmune diseases.\nIn a cell, protein molecules of the host's own phenotype or of other biologic entities are continually synthesized and degraded. Each MHC molecule on the cell surface displays a small peptide (a molecular fraction of a protein) called an epitope. The presented self-antigens prevent an organism's immune system from targeting its own cells. The presentation of pathogen-derived proteins results in the elimination of the infected cell by the immune system.\nDiversity of an individual's self-antigen presentation, mediated by MHC self-antigens, is attained in at least three ways: (1) an organism's MHC repertoire is polygenic (via multiple, interacting genes); (2) MHC expression is codominant (from both sets of inherited alleles); (3) MHC gene variants are highly polymorphic (diversely varying from organism to organism within a species). Sexual selection has been observed in male mice choosing to mate with females with different MHCs. Also, at least for MHC I presentation, there has been evidence of antigenic peptide splicing, which can combine peptides from different proteins, vastly increasing antigen diversity."},{"doc_id":8183121990202397000,"text":"51. ** ^ ** Krebs, J. E.; Goldstein, E. S.; Kilpatrick, ST (2011). [ _Lewin's GENES X_ ](https://archive.org/details/lewinsgenesx0000unse/page/289). Boston: Jones and Bartlett Publishers. pp. [ 289–292 ](https://archive.org/details/lewinsgenesx0000unse/page/289). [ ISBN ](/wiki/ISBN_\\(identifier\\) \"ISBN \\(identifier"},{"doc_id":657536276632004200,"text":" ^ Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \"Insights into human genetic variation and population history from 929 diverse genomes\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa, the Americas, and Oceania each harbor tens to hundreds of thousands of private, common genetic variants."},{"doc_id":6536929371804143000,"text":" ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 4455760 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4455760). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 26009770 ](https://pubmed.ncbi.nlm.nih.gov/26009770).\n39. ** ^ ** Larsson, Max; Sawada, Ke"},{"doc_id":4323140975966511600,"text":" the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \"Evidence for variable selective pressures at MC1R\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 10733465. ^ Robin A (1991). Biological Perspectives on Human Pigmentation. Cambridge: Cambridge University Press. ^ \"The Science Behind the Human Genome Project\""},{"doc_id":1976914264601072400,"text":"mb.2006.07.045 ](https://doi.org/10.1016%2Fj.jmb.2006.07.045). [ PMC ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 3662476 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3662476). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 16934836 ](https"},{"doc_id":5332197148514614000,"text":"1. [ Stabilizing Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-1)\n2. [ Directional Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-2)\n3. [ Diversifying Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-3"},{"doc_id":1343602846445218300,"text":", which were introduced to and slowly selected out of the much larger modern human population; the initial hybridised population may have experienced up to a 94% reduction in fitness compared to contemporary humans. By this measure, Neanderthals may have substantially increased in fitness. A 2017 study focusing on archaic genes in Turkey found associations with coeliac disease, malaria severity and Costello syndrome. Nonetheless, some genes may have helped modern East Asians adapt to the environment; the putatively Neanderthal Val92Met variant of the MC1R gene, which may be weakly associated with red hair and UV radiation sensitivity"},{"doc_id":7752196112672118000,"text":" HLA allele is named with a number. For instance, for a given individual, his haplotype might be HLA-A2, HLA-B5, HLA-DR3, etc... Each heterozygous individual will have two MHC haplotypes, one each from the paternal and maternal chromosomes.\nThe MHC genes are highly polymorphic; many different alleles exist in the different individuals inside a population. The polymorphism is so high, in a mixed population (nonendogamic), no two individuals have exactly the same set of MHC molecules, with the exception"},{"doc_id":1572121283436197600,"text":"Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \"Insights into human genetic variation and population history from 929 diverse genomes\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa, the Americas, and Oceania each harbor tens to hundreds of thousands of private, common genetic variants."},{"doc_id":5871790602362253000,"text":"In sexual mate selection[edit]\n\nMain article: Major histocompatibility complex and sexual selection\n\nSee also: Interpersonal compatibility\nMHC molecules enable immune system surveillance of the population of protein molecules in a host cell, and greater MHC diversity permits greater diversity of antigen presentation. In 1976, Yamazaki et al demonstrated a sexual selection mate choice by male mice for females of a different MHC. Similar results have been obtained with fish. Some data find lower rates of early pregnancy loss in human couples of dissimilar MHC genes.\nMHC may be related to mate choice"},{"doc_id":4675850447830746000,"text":"When this answer was published in 2009, Jared was a Ph.D. candidate in the\nDepartment of Genetics, studying yeast carbon metabolism and genome evolution\nin Gavin Sherlock’s laboratory. Jared wrote this answer while participating in\nthe Stanford at The Tech program.\n\nAsk a Geneticist Home"},{"doc_id":1332471960494971400,"text":""},{"doc_id":8327005706154174000,"text":" NG, Chaplin G (July 2000). \"The evolution of human skin coloration\" (PDF). Journal of Human Evolution. 39 (1): 57–106. doi : 10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \"Evidence for variable selective pressures at MC1R\". American Journal of Human Genetics. 66 (4"},{"doc_id":3496377408504689700,"text":"Proteins[edit]\nT-cell receptor complexed with MHC-I and MHC-II\nMHC class I[edit]\nMain article: MHC class I\nMHC class I molecules are expressed in some nucleated cells and also in platelets—in essence all cells but red blood cells. It presents epitopes to killer T cells, also called cytotoxic T lymphocytes (CTLs). A CTL expresses CD8 receptors, in addition to T-cell receptors (TCR)s. When a CTL's CD8 receptor docks to a MHC class I molecule, if the CTL's TCR fits the epitope within the MHC class I molecule, the CTL triggers the cell to undergo programmed cell death by apoptosis. Thus, MHC class I helps mediate cellular immunity, a primary means to address intracellular pathogens, such as viruses and some bacteria, including bacterial L forms, bacterial genus Mycoplasma, and bacterial genus Rickettsia. In humans, MHC class I comprises HLA-A, HLA-B, and HLA-C molecules.\nThe first crystal structure of Class I MHC molecule, human HLA-A2, was published in 1989. The structure revealed that MHC-I molecules are heterodimers, they have polymorphic heavy α-subunit whose gene occurs inside the MHC locus and small invariant β2 microglobulin subunit whose gene is located usually outside of it. Polymorphic heavy chain of MHC-I molecule contains N-terminal extra-cellular region composed by three domains, α1, α2, and α3, transmembrane helix to hold MHC-I molecule on the cell surface and short cytoplasmic tail. Two domains, α1 and α2 form deep peptide-binding groove between two long α-helices and the floor of the groove formed by eight β-strands. Immunoglobulin-like domain α3 involved in the interaction with CD8 co-receptor. β2 microglobulin provides stability of the complex and participates in the recognition of peptide-MHC class I complex by CD8 co-receptor. The peptide is non-covalently bound to MHC-I, it is held by the several pockets on the floor of the peptide-binding groove. Amino acid side-chains that are most polymorphic in human alleles fill up the central and widest portion of the binding groove, while conserved side-chains are clustered at the narrower ends of the groove.\nSchematic view of MHC class I and MHC class II molecules\nClassical MHC molecules present epitopes to the TCRs of CD8+ T lymphocytes. Nonclassical molecules (MHC class IB) exhibit limited polymorphism, expression patterns, and presented antigens; this group is subdivided into a group encoded within MHC loci (e.g., HLA-E, -F, -G), as well as those not (e.g., stress ligands such as ULBPs, Rae1, and H60); the antigen/ligand for many of these molecules remain unknown, but they can interact with each of CD8+ T cells, NKT cells, and NK cells. The evolutionary oldest nonclassical MHC class I lineage in human was deduced to be the lineage that includes the CD1 and PROCR (alias EPCR) molecules and this lineage may have been established before the origin of tetrapod species. However, the only nonclassical MHC class I lineage for which evidence exists that it was established before the evolutionary separation of Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish plus tetrapods) is lineage Z of which members are found, together in each species with classical MHC class I, in lungfish and throughout ray-finned fishes; why the Z lineage was well conserved in ray-finned fish but lost in tetrapods is not understood.\nMHC class II[edit]\n\nMain article: MHC class II\nMHC class II can be conditionally expressed by all cell types, but normally occurs only on \"professional\" antigen-presenting cells (APCs): macrophages, B cells, and especially dendritic cells (DCs). An APC takes up an antigenic protein, performs antigen processing, and returns a molecular fraction of it—a fraction termed the epitope—and displays it on the APC's surface coupled within an MHC class II molecule (antigen presentation). On the cell's surface, the epitope can be recognized by immunologic structures like T-cell receptors (TCRs). The molecular region which binds to the epitope is the paratope.\nOn surfaces of helper T cells are CD4 receptors, as well as TCRs. When a naive helper T cell's CD4 molecule docks to an APC's MHC class II molecule, its TCR can meet and bind the epitope coupled within the MHC class II. This event primes the naive T cell. According to the local milieu, that is, the balance of cytokines secreted by APCs in the microenvironment, the naive helper T cell (Th0) polarizes into either a memory Th cell or an effector Th cell of phenotype either type 1 (Th1), type 2 (Th2), type 17 (Th17), or regulatory/suppressor (Treg), as so far identified, the Th cell's terminal differentiation.\nMHC class II thus mediates immunization to—or, if APCs polarize Th0 cells principally to Treg cells, immune tolerance of—an antigen. The polarization during primary exposure to an antigen is key in determining a number of chronic diseases, such as inflammatory bowel diseases and asthma, by skewing the immune response that memory Th cells coordinate when their memory recall is triggered upon secondary exposure to similar antigens. B cells express MHC class II to present antigens to Th0, but when their B cell receptors bind matching epitopes, interactions which are not mediated by MHC, these activated B cells secrete soluble immunoglobulins: antibody molecules mediating humoral immunity.\nClass II MHC molecules are also heterodimers, genes for both α and β subunits are polymorphic and located within MHC class II subregion. Peptide-binding groove of MHC-II molecules is forms by N-terminal domains of both subunits of the heterodimer, α1 and β1, unlike MHC-I molecules, where two domains of the same chain are involved. In addition, both subunits of MHC-II contain transmembrane helix and immunoglobulin domains α2 or β2 that can be recognized by CD4 co-receptors. In this way MHC molecules chaperone which type of lymphocytes may bind to the given antigen with high affinity, since different lymphocytes express different T-Cell Receptor (TCR) co-receptors.\nMHC class II molecules in humans have five to six isotypes. Classical molecules present peptides to CD4+ lymphocytes. Nonclassical molecules, accessories, with intracellular functions, are not exposed on cell membranes, but in internal membranes, assisting with the loading of antigenic peptides onto classic MHC class II molecules. The important nonclassical MHC class II molecule DM is only found from the evolutionary level of lungfish, although also in more primitive fishes both classical and nonclassical MHC class II are found.\nSr.No\n\nFeature\n\nClass I MHC\n\nClass II MHC\n\n\n1\n\nConstituting polypeptide chains\n\nα chain (45KDa in humans)\nβ2 chain (12 KDa in humans)\n\n\nα chain (30–34 KDa in humans)\nβ chain (26–29 KDa in humans)\n\n\n\n2\n\nAntigen binding domain\n\nα1and α2 domains\n\nα1 and β1 domains\n\n\n3\n\nBinds protein antigens of\n\n8–10 amino acids residues\n\n13–18 amino acids residues\n\n\n4\n\nPeptide bending cleft\n\nFloor formed by β sheets and sides by α\nhelices, blocked at both the ends\n\n\nFloor formed by β sheets and sides by α\nhelices, opened at both the ends\n\n\n\n5\n\nAntigenic peptide motifs\ninvolved in binding\n\n\nAnchor residues located at amino and\ncarbon terminal ends\n\n\nAnchor residues located almost uniformly\nalong the peptide\n\n\n\n6\n\nPresents antigenic peptide to\n\nCD8+ T cells\n\nCD4+ T cells\nMHC class III[edit]\n\nMain article: MHC class III\nClass III molecules have physiologic roles unlike classes I and II, but are encoded between them in the short arm of human chromosome 6. Class III molecules include several secreted proteins with immune functions: components of the complement system (such as C2, C4, and B factor), cytokines (such as TNF-α, LTA, and LTB), and heat shock proteins.\nFunction[edit]\nMHC is the tissue-antigen that allows the immune system (more specifically T cells) to bind to, recognize, and tolerate itself (autorecognition). MHC is also the chaperone for intracellular peptides that are complexed with MHCs and presented to T cell receptors (TCRs) as potential foreign antigens. MHC interacts with TCR and its co-receptors to optimize binding conditions for the TCR-antigen interaction, in terms of antigen binding affinity and specificity, and signal transduction effectiveness.\nEssentially, the MHC-peptide complex is a complex of auto-antigen/allo-antigen. Upon binding, T cells should in principle tolerate the auto-antigen, but activate when exposed to the allo-antigen. Disease states occur when this principle is disrupted.\nAntigen presentation: MHC molecules bind to both T cell receptor and CD4/CD8 co-receptors on T lymphocytes, and the antigen epitope held in the peptide-binding groove of the MHC molecule interacts with the variable Ig-Like domain of the TCR to trigger T-cell activation\nAutoimmune reaction: Having some MHC molecules increases the risk of autoimmune diseases more than having others. HLA-B27 is an example. It is unclear how exactly having the HLA-B27 tissue type increases the risk of ankylosing spondylitis and other associated inflammatory diseases, but mechanisms involving aberrant antigen presentation or T cell activation have been hypothesized.\nTissue allorecognition: MHC molecules in complex with peptide epitopes are essentially ligands for TCRs. T cells become activated by binding to the peptide-binding grooves of any MHC molecule that they were not trained to recognize during positive selection in the thymus."},{"doc_id":4446130463120729000,"text":" sicker from COVID‑19. Genetic screening is able to detect interferon effector genes. Some genetic variants are risk factors in specific populations. For instance, an allele of the DOCK2 gene (dedicator of cytokinesis 2 gene) is a common risk factor in Asian populations but much less common in Europe. The mutation leads to lower expression of DOCK2 especially in younger patients with severe Covid. In fact, many other genes and genetic variants have been found that determine the outcome of SARS-CoV-2 infections.\nChildren\n\nSee also: Impact"},{"doc_id":2448606410365659600,"text":"doi.org/10.1073%2Fpnas.0502893102). [ PMC ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 1093311 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1093311). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 15888555 ](https://pubmed.ncbi.nlm.nih.gov"},{"doc_id":7037905041389609000,"text":"1. [ Epistasis ](https://slcc.pressbooks.pub/collegebiology1/chapter/polygenic-inheritance-and-epistasis/#chapter-1076-section-1)\n2. [ Polygenic Inheritance ](https://slcc.pressbooks.pub/collegebiology1/chapter/polygenic-inheritance-and-epistasis/#chapter-1076-section-2)\n22. XXI . Energy and the ATP Cycle"},{"doc_id":5174268529100157000,"text":"%20association%20loci%20for%20human%20diseases%20and%20traits&journal=Proc.%20Natl%20Acad.%20Sci.%20USA&doi=10.1073%2Fpnas.0903103106&volume=106&pages=9362-9367&publication_year=2009&author=Hindorff%2CLA)"},{"doc_id":1139994221072065300,"text":"Notes and references[edit]\n^ Hull P (August 1970). \"Notes on Dr Snell's observations concerning the H-2 locus polymorphism\". Heredity. 25 (3): 461–5. doi:10.1038/hdy.1970.47. PMID 5275401.\n\n^ Janeway Jr CA, Travers P, Walport M, et al. (2001). \"The Major Histocompatibility Complex and Its Functions\". Immunobiology: The Immune System in Health and Disease (5th ed.)."},{"doc_id":6739160733741488000,"text":", and remain circulating in a large population. Genetic drift is also a major driving force in some species. It is possible that the combined effects of some or all of these factors cause the genetic diversity.\nMHC diversity has also been suggested as a possible indicator for conservation, because large, stable populations tend to display greater MHC diversity, than smaller, isolated populations. Small, fragmented populations that have experienced a population bottleneck typically have lower MHC diversity. For example, relatively low MHC diversity has been observed in the cheetah (Acinonyx jubatus), Eurasian beaver"},{"doc_id":6078984594418064000,"text":"In transplant rejection[edit]\nIn a transplant procedure, as of an organ or stem cells, MHC molecules themselves act as antigens and can provoke immune response in the recipient, thus causing transplant rejection. MHC molecules were identified and named after their role in transplant rejection between mice of different strains, though it took over 20 years to clarify MHC's role in presenting peptide antigens to cytotoxic T lymphocytes (CTLs).\nEach human cell expresses six MHC class I alleles (one HLA-A, -B, and -C allele from each parent) and six to eight MHC class II alleles (one HLA-DP and -DQ, and one or two HLA-DR from each parent, and combinations of these). The MHC variation in the human population is high, at least 350 alleles for HLA-A genes, 620 alleles for HLA-B, 400 alleles for DR, and 90 alleles for DQ. Any two individuals who are not identical twins, triplets, or higher order multiple births, will express differing MHC molecules. All MHC molecules can mediate transplant rejection, but HLA-C and HLA-DP, showing low polymorphism, seem least important.\nWhen maturing in the thymus, T lymphocytes are selected for their TCR incapacity to recognize self antigens, yet T lymphocytes can react against the donor MHC's peptide-binding groove, the variable region of MHC holding the presented antigen's epitope for recognition by TCR, the matching paratope. T lymphocytes of the recipient take the incompatible peptide-binding groove as nonself antigen.\nTransplant rejection has various types known to be mediated by MHC (HLA):\nHyperacute rejection occurs when, before the transplantation, the recipient has preformed anti-HLA antibodies, perhaps by previous blood transfusions (donor tissue that includes lymphocytes expressing HLA molecules), by anti-HLA generated during pregnancy (directed at the father's HLA displayed by the fetus), or by previous transplantation;\nAcute cellular rejection occurs when the recipient's T lymphocytes are activated by the donor tissue, causing damage via mechanisms such as direct cytotoxicity from CD8 cells.\nAcute humoral rejection and chronic disfunction occurs when the recipient's anti-HLA antibodies form directed at HLA molecules present on endothelial cells of the transplanted tissue.\nIn all of the above situations, immunity is directed at the transplanted organ, sustaining lesions. A cross-reaction test between potential donor cells and recipient serum seeks to detect presence of preformed anti-HLA antibodies in the potential recipient that recognize donor HLA molecules, so as to prevent hyperacute rejection. In normal circumstances, compatibility between HLA-A, -B, and -DR molecules is assessed. The higher the number of incompatibilities, the lower the five-year survival rate. Global databases of donor information enhance the search for compatible donors.\nThe involvement in allogeneic transplant rejection appears to be an ancient feature of MHC molecules, because also in fish associations between transplant rejections and (mis-)matching of MHC class I and MHC class II were observed."},{"doc_id":4143018406264960000,"text":"ifier\\)\") : [ 10.1016/j.abb.2006.05.001 ](https://doi.org/10.1016%2Fj.abb.2006.05.001). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 16806044 ](https://pubmed.ncbi.nlm.nih.gov/16806044).\n33. ** ^ ** Kamenetsky, M.; Middelhaufe, S.; Bank,"},{"doc_id":7354962069222019000,"text":"Evolutionary diversity[edit]\nMost mammals have MHC variants similar to those of humans, who bear great allelic diversity, especially among the nine classical genes—seemingly due largely to gene duplication—though human MHC regions have many pseudogenes. The most diverse loci, namely HLA-A, HLA-B, and HLA-C, have roughly 6000, 7200, and 5800 known alleles, respectively. Many HLA alleles are ancient, sometimes of closer homology to a chimpanzee MHC alleles than to some other human alle"},{"doc_id":2820692476841372700,"text":" 20167026 ](https://pubmed.ncbi.nlm.nih.gov/20167026). [ S2CID ](/wiki/S2CID_\\(identifier\\) \"S2CID \\(identifier\\)\") [ 11617147 ](https://api.semanticscholar.org/CorpusID:11617147).\n30. ** ^ ** Bernstein, H.; Hopf, F. A.; Michod, R. E. (1987). \"The molecular basis of the evolution of sex\". _"},{"doc_id":2767495083226295000,"text":":\nEach person carries 2 alleles of each of the 3 class-I genes, (HLA-A, HLA-B and HLA-C), and so can express six different types of MHC-I (see figure).\nIn the class-II locus, each person inherits a pair of HLA-DP genes (DPA1 and DPB1, which encode α and β chains), a couple of genes HLA-DQ (DQA1 and DQB1, for α and β chains), one gene HLA-DRα ("},{"doc_id":7748368743253828000,"text":". PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \"Evidence for variable selective pressures at MC1R\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 10733465. ^ Robin A (1991). Biological Perspectives on Human Pigmentation. Cambridge: Cambridge University Press."},{"doc_id":112755929557492580,"text":"HLA biology[edit]\nCodominant expression of HLA genes\n\nMain article: Human leukocyte antigen\nHuman MHC class I and II are also called human leukocyte antigen (HLA). To clarify the usage, some of the biomedical literature uses HLA to refer specifically to the HLA protein molecules and reserves MHC for the region of the genome that encodes for this molecule, but this is not a consistent convention.\nThe most studied HLA genes are the nine classical MHC genes: HLA-A, HLA-B, HLA-"}],"string":"[\n {\n \"doc_id\": 3717481448095056000,\n \"text\": \" S2CID 20415396. ^ Hedrick PW (October 2011). \\\"Population genetics of malaria resistance in humans\\\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\\\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.\"\n },\n {\n \"doc_id\": 6800680407069684000,\n \"text\": \" \\\"Population genetics of malaria resistance in humans\\\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\\\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28\"\n },\n {\n \"doc_id\": 4946908657905475000,\n \"text\": \"redity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\\\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911. ^ Shelomi M, Zeuss\"\n },\n {\n \"doc_id\": 6917128559766396000,\n \"text\": \" and human lactase genes\\\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \\\"Population genetics of malaria resistance in humans\\\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\"\n },\n {\n \"doc_id\": 1910979479660340500,\n \"text\": \".16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\\\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911. ^ Shelomi M, Zeuss D (5 April 2017). \\\"Bergmann's and Allen's Rules in Native European and Mediterranean Phas\"\n },\n {\n \"doc_id\": 7451854849775404000,\n \"text\": \"Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\\\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911.\\n\\nShelomi M, Zeuss D (5 April 2017). \\\"Bergmann's and Allen's Rules in Native European and Mediterranean Phasmatodea\\\". Frontiers in Ecology and Evolution. 5.\"\n },\n {\n \"doc_id\": 9116245427462050000,\n \"text\": \". Protective effects of the sickle cell gene against malaria morbidity and mortality. Lancet. 2002;359:1311–1312. [PubMed] [Google Scholar]\\nAlawieh A., Musharrafieh U., Jaber A., Berry A., Ghosn N., Bizri A.R. Revisiting leishmaniasis in the time of war: the Syrian conflict and the Lebanese outbreak. Int. J. Infect. Dis. 2014;29:115–119. [PubMed] [Google Scholar]\\nAlthaus C.L\"\n },\n {\n \"doc_id\": 4961714778855227000,\n \"text\": \" G, England PR, Bradley DG, Jann OC, Bertorelle G, et al. (December 2003). \\\"Gene-culture coevolution between cattle milk protein genes and human lactase genes\\\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \\\"Population genetics of malaria resistance in humans\\\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011\"\n },\n {\n \"doc_id\": 5861845573567613000,\n \"text\": \" Evans D.B., Jha P., Mills A., Musgrove P., editors. Disease Control Priorities in Developing Countries. The International Bank for Reconstruction and Development/The World Bank Group; Washington, DC: 2006. [Google Scholar]\\nLanghorne J., Ndungu F.M., Sponaas A.M., Marsh K. Immunity to malaria: more questions than answers. Nat. Immunol. 2008;9:725–732. [PubMed] [Google Scholar]\\nLeavell H.R., Clark E.G. McG\"\n },\n {\n \"doc_id\": 469742901370423600,\n \"text\": \"Beja-Pereira A, Luikart G, England PR, Bradley DG, Jann OC, Bertorelle G, et al. (December 2003). \\\"Gene-culture coevolution between cattle milk protein genes and human lactase genes\\\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396.\\n\\nHedrick PW (October 2011). \\\"Population genetics of malaria resistance in humans\\\". Heredity. 107 (4): 283–\"\n },\n {\n \"doc_id\": 4422283637412871000,\n \"text\": \"Whitty CJM, Ansah E. Malaria control stalls in high incidence areas. BMJ.\\n2019 May 21; 365 :l2216. [ [ PubMed : 31113780\\n](https://pubmed.ncbi.nlm.nih.gov/31113780) ]\"\n },\n {\n \"doc_id\": 2533275001596672000,\n \"text\": \"* [ Folic acid supplementation and malaria susceptibility and severity among people taking antifolate antimalarial drugs in endemic areas. ](/pubmed/36321557) [Cochrane Database Syst Rev. 2022]\"\n },\n {\n \"doc_id\": 4987334270306259000,\n \"text\": \" 5600404. PMID 28873402. ^ Beja-Pereira A, Luikart G, England PR, Bradley DG, Jann OC, Bertorelle G, et al. (December 2003). \\\"Gene-culture coevolution between cattle milk protein genes and human lactase genes\\\". Nature Genetics. 35 (4): 311–313. doi : 10.1038/ng1263. PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \\\"Population genetics of malaria resistance in humans\\\". He\"\n },\n {\n \"doc_id\": 4866019419213672000,\n \"text\": \" include both innate, genetic factors and acquired factors such as the specific immunity that develops following exposure or vaccination. The malaria resistance afforded carriers of the sickle cell trait exemplifies how genetics can influence susceptibility to infectious disease (Aidoo et al., 2002). Susceptibility is also affected by extremes of age, stress, pregnancy, nutritional status, and underlying diseases. These latter factors can impact immunity to infection, as illustrated by immunologically naïve infant populations, aging populations experiencing immune senescence, and immunocompromised HIV/AIDS patients.\\nMechanical and chemical surface barriers such as the skin\"\n },\n {\n \"doc_id\": 8721903729592441000,\n \"text\": \" cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at high altitudes. Some populations have evolved highly unique adaptations to very specific environmental conditions, such as those advantageous to ocean-dwelling lifestyles and freediving in the\"\n },\n {\n \"doc_id\": 7579552577826821000,\n \"text\": \"Riehle, M. A., _et al_. Using bacteria to express and display anti-\\n_Plasmodium_ molecules in the mosquito midgut. _International Journal of\\nParasitology_ **37**, 595–603 (2007)\"\n },\n {\n \"doc_id\": 4789399353139791000,\n \"text\": \".\\n\\n^ Manczinger M, Boross G, Kemény L, Müller V, Lenz TL, Papp B, et al. (January 2019). \\\"Pathogen diversity drives the evolution of generalist MHC-II alleles in human populations\\\". PLOS Biology. 17 (1): e3000131. doi:10.1371/journal.pbio.3000131. PMC 6372212. PMID 30703088.\\n\\n^ Zeisset I, Beebee TJ (2014). \\\"Dr\"\n },\n {\n \"doc_id\": 7099069620372744000,\n \"text\": \"There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other\"\n },\n {\n \"doc_id\": 3006593300305258500,\n \"text\": \" hair colors; from top left, clockwise: black, brown, blonde, white, red. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky\"\n },\n {\n \"doc_id\": 1394281673211026200,\n \"text\": \" evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at\"\n },\n {\n \"doc_id\": 970114010689562600,\n \"text\": \" The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at high altitudes. Some populations have evolved highly unique adaptations\"\n },\n {\n \"doc_id\": 712862380789366700,\n \"text\": \"* Malaria. (2022).\\n[ https://www.who.int/news-room/fact-sheets/detail/malaria\\n](https://www.who.int/news-room/fact-sheets/detail/malaria)\"\n },\n {\n \"doc_id\": 180497508201615230,\n \"text\": \" pathogens drive genetic diversity: MHC, mechanisms and misunderstandings\\\". Proceedings. Biological Sciences. 277 (1684): 979–88. doi:10.1098/rspb.2009.2084. PMC 2842774. PMID 20071384.\"\n },\n {\n \"doc_id\": 4944825111087627000,\n \"text\": \", clockwise: black, brown, blonde, white, red. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall\"\n },\n {\n \"doc_id\": 491145170522689500,\n \"text\": \" sleep patterns.\\nThere is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with\"\n },\n {\n \"doc_id\": 7291511111192631000,\n \"text\": \"-2 infections, and genome sequences from 659 patients with severe COVID-19 revealed genetic variants that appear to be associated with life-threatening disease. One gene identified in these studies is type I interferon (IFN). Autoantibodies against type I IFNs were found in up to 13.7% of patients with life-threatening COVID-19, indicating that a complex interaction between genetics and the immune system is important for natural resistance to Covid.\\nSimilarly, mutations in the ERAP2 gene, encoding endoplasmic reticulum aminopeptid\"\n },\n {\n \"doc_id\": 1787966951908398000,\n \"text\": \"Folic acid supplementation and malaria susceptibility and severity among\\npeople taking antifolate antimalarial drugs in endemic areas.\"\n },\n {\n \"doc_id\": 4924544718779407000,\n \"text\": \"These issues come into focus when you consider the devastating malaria\\nepidemic that kills one to three million people worldwide every year. The\\npathogens that cause malaria are protozoan parasites from the genus\\n_Plasmodium_ that are transmitted through the bite of mosquitoes of the genus\\n_Anopheles_. Both of these animal genera contain hundreds of species,\\nalthough only a few are involved in transmitting malaria in humans. Recent\\ngenetic studies of the symbiotic bacteria in the midgut of the _Anopheles\\nstephensi_ mosquitoes have yielded\"\n },\n {\n \"doc_id\": 5012708765966004000,\n \"text\": \" microbe. The evolution of the MHC polymorphism ensures that a population will not succumb to a new pathogen or a mutated one, because at least some individuals will be able to develop an adequate immune response to win over the pathogen. The variations in the MHC molecules (responsible for the polymorphism) are the result of the inheritance of different MHC molecules, and they are not induced by recombination, as it is the case for the antigen receptors.\\nBecause of the high levels of allelic diversity found within its genes, MHC has also attracted the attention of many evolutionary biologists\"\n },\n {\n \"doc_id\": 4161775671760185300,\n \"text\": \"[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)\"\n },\n {\n \"doc_id\": 6254006696981987000,\n \"text\": \" up to 13.7% of patients with life-threatening COVID-19, indicating that a complex interaction between genetics and the immune system is important for natural resistance to Covid.\\nSimilarly, mutations in the ERAP2 gene, encoding endoplasmic reticulum aminopeptidase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria Yersinia pestis. People who inherited two copies of a complete variant of the gene were twice as likely to have survived the plague as those who inherited two copies of a truncated variant\"\n },\n {\n \"doc_id\": 3336316715439913500,\n \"text\": \" size is partly determined by genes and is also significantly influenced by environmental factors such as diet, exercise, and sleep patterns. A variety of human hair colors; from top left, clockwise: black, brown, blonde, white, red. There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that\"\n },\n {\n \"doc_id\": 8842831987032849000,\n \"text\": \":10.4049/jimmunol.168.1.260. PMID 11751970. S2CID 36838421.\\n\\n^ Yamaguchi T, Dijkstra JM (April 2019). \\\"Major Histocompatibility Complex (MHC) Genes and Disease Resistance in Fish\\\". Cells. 8 (4): 378. doi:10.3390/cells8040378. PMC 6523485. PMID 31027287.\\n\\n^ Abi Rached L, McDermott MF, Pontarotti P (\"\n },\n {\n \"doc_id\": 7517740263136706000,\n \"text\": \" of major human infectious diseases. Nature. 2007;447:279–283. [PMC free article] [PubMed] [Google Scholar]\\nYasuoka J., Levins R. Impact of deforestation and agricultural development on anopheline ecology and malaria epidemiology. Am. J. Trop. Med. Hyg. 2007;76:450–460. [PubMed] [Google Scholar]\\nYu I.T., Li Y., Wong T.W., Tam W., Chan A.T., Lee J.H., Leung D.Y., Ho T. Evidence\"\n },\n {\n \"doc_id\": 6283416201445990000,\n \"text\": \" has become clear that there are genetic factor which determine susceptibility to infection. For instance, up to 40% of SARS-CoV-2 infections may be asymptomatic, suggesting that many people are naturally protected from disease. Large genetic studies have defined risk factors for severe SARS-CoV-2 infections, and genome sequences from 659 patients with severe COVID-19 revealed genetic variants that appear to be associated with life-threatening disease. One gene identified in these studies is type I interferon (IFN). Autoantibodies against type I IFNs were found in\"\n },\n {\n \"doc_id\": 4985945625302089000,\n \"text\": \":10.3390/cancers12051155. PMC 7281430. PMID 32375397.\\n\\n^ \\\"HLA Alleles Numbers\\\". hla.alleles.org.\\n\\n^ van Oosterhout C (February 2009). \\\"A new theory of MHC evolution: beyond selection on the immune genes\\\". Proceedings. Biological Sciences. 276 (1657): 657–65. doi:10.1098/rspb.2008.1299. PMC 2660941. PMID 18986972\"\n },\n {\n \"doc_id\": 3486976231575510500,\n \"text\": \" against these may get much sicker from COVID‑19. Genetic screening is able to detect interferon effector genes. Some genetic variants are risk factors in specific populations. For instance, an allele of the DOCK2 gene (dedicator of cytokinesis 2 gene) is a common risk factor in Asian populations but much less common in Europe. The mutation leads to lower expression of DOCK2 especially in younger patients with severe Covid. In fact, many other genes and genetic variants have been found that determine the outcome of SARS-CoV-2 infections.\\nWhile very\"\n },\n {\n \"doc_id\": 1055589053584834800,\n \"text\": \"Kebede A., Mccann J.C., Kiszewski A.E., Ye-Ebiyo Y. New evidence of the effects of agro-ecologic change on malaria transmission. Am. J. Trop. Med. Hyg. 2005;73:676–680. [PubMed] [Google Scholar]\\nKeiser J., De Castro M.C., Maltese M.F., Bos R., Tanner M., Singer B.H., Utzinger J. Effect of irrigation and large dams on the burden of malaria on a global and regional scale\"\n },\n {\n \"doc_id\": 1348784263469054700,\n \"text\": \" (1): 57–106. doi : 10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \\\"Evidence for variable selective pressures at MC1R\\\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 107334\"\n },\n {\n \"doc_id\": 5835360718990907000,\n \"text\": \" Nature of Selection on the Major Histocompatibility Complex\\\". Critical Reviews in Immunology. 37 (2–6): 75–120. doi:10.1615/CritRevImmunol.v37.i2-6.10. PMID 29773018.\\n\\n^ Sommer S (October 2005). \\\"The importance of immune gene variability (MHC) in evolutionary ecology and conservation\\\". Frontiers in Zoology. 2 (16): 16. doi:10.1186/1742-9994-2-16. PMC 12825\"\n },\n {\n \"doc_id\": 6676066334518801000,\n \"text\": \" ](https://slcc.pressbooks.pub/collegebiology1/chapter/variation-adaptation/)\"\n },\n {\n \"doc_id\": 6546051547658558000,\n \"text\": \" Biol. Med. 2014;57:198–207. [PubMed] [Google Scholar]\\nHalpin J. Avian flu from an occupational health perspective. Arch. Environ. Occup. Health. 2005;60:62–69. [PubMed] [Google Scholar]\\nHay S.I., Smith D.L., Snow R.W. Measuring malaria endemicity from intense to interrupted transmission. Lancet Infect. Dis. 2008;8:369–378. [PMC free article] [PubMed] [Google Scholar]\\nHennessy T.W\"\n },\n {\n \"doc_id\": 8072226226985974000,\n \"text\": \"les of the same gene.\\nMHC allelic diversity has challenged evolutionary biologists for explanation. Most posit balancing selection (see polymorphism (biology)), which is any natural selection process whereby no single allele is absolutely most fit, such as frequency-dependent selection and heterozygote advantage. Pathogenic coevolution, as a type of balancing selection, posits that common alleles are under greatest pathogenic pressure, driving positive selection of uncommon alleles—moving targets, so to say, for pathogens. As pathogenic pressure on the previously common alleles decreases, their frequency in the population stabilizes\"\n },\n {\n \"doc_id\": 7145942413818299000,\n \"text\": \"Guerra CA, Kang SY, Citron DT, Hergott DEB, Perry M, Smith J, Phiri WP, Osá\\nNfumu JO, Mba Eyono JN, Battle KE, Gibson HS, García GA, Smith DL. Human\\nmobility patterns and malaria importation on Bioko Island. Nat Commun. 2019\\nMay 27; 10 (1):2332. [ [ PMC free article : PMC6536527\\n](/pmc/articles/PMC6536527/) ]\"\n },\n {\n \"doc_id\": 2111678933010816500,\n \"text\": \"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H\"\n },\n {\n \"doc_id\": 6707359235402285000,\n \"text\": \"Malaria\\nJapanese encephalitis\\nLyme\\nMalaria\\nHendra\\tSteinmann et al. (2006)\"\n },\n {\n \"doc_id\": 5810698596454523000,\n \"text\": \" : 10.1038/nature09298. PMC 3173859. PMID 20811451. ^ Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \\\"Insights into human genetic variation and population history from 929 diverse genomes\\\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa\"\n },\n {\n \"doc_id\": 6235040381659362000,\n \"text\": \". doi : 10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \\\"Evidence for variable selective pressures at MC1R\\\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 10733465. ^ Robin A (\"\n },\n {\n \"doc_id\": 7056037267678778000,\n \"text\": \" promising results: _Enterobacter\\nagglomerans_ bacteria were genetically engineered to display two anti-\\n_Plasmodium_ effector molecules that kill the parasite before it is\\ntransmitted to humans (Riehle _et al._, 2007). Now consider the genetic and\\nphysiological differences between the wild-type and genetically modified\\n_Anopheles stephensi_ mosquitoes: they are still the same species by all major\\nstandards of species definition, yet what a difference it would make for\\nhumankind if the _Plasmodium-_ resistant\"\n },\n {\n \"doc_id\": 7639397133823105000,\n \"text\": \"Search PMC Full-Text Archive\\nSearch PMC Full-Text Archive\\nSearch in PMC\\nAdvanced Search User Guide\\nJournal List PLoS Genet v.12(7); 2016 Jul PMC4956268\\nAs a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health.\\nLearn more: PMC Disclaimer | PMC Copyright Notice\\nLogo of plosgen\\nPLoS Genet. 2016 Jul; 12\"\n },\n {\n \"doc_id\": 5044981846658660000,\n \"text\": \"* Raphael Vallat [ ORCID: orcid.org/0000-0003-1779-7653 ](http://orcid.org/0000-0003-1779-7653) 1 ,\\n* Sarah E. Berry [ ORCID: orcid.org/0000-0002-5819-5109 ](http://orcid.org/0000-0002-5819-5109) 2 ,\\n* Neli Tsereteli 3 ,\\n* Joan Capdevila 4 ,\"\n },\n {\n \"doc_id\": 6009779149809428000,\n \"text\": \" of the United States of America. 104 (41): 16221–6. doi:10.1073/pnas.0704580104. PMC 1999395. PMID 17911263.\\n\\n^ Shum BP, Guethlein L, Flodin LR, Adkison MA, Hedrick RP, Nehring RB, et al. (March 2001). \\\"Modes of salmonid MHC class I and II evolution differ from the primate paradigm\\\". Journal of Immunology. 166 (5): 3297–308. doi\"\n },\n {\n \"doc_id\": 8327998045110626000,\n \"text\": \"ase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria Yersinia pestis. People who inherited two copies of a complete variant of the gene were twice as likely to have survived the plague as those who inherited two copies of a truncated variant.\\nSusceptibility also determined the epidemiology of infection, given that different populations have different genetic and environmental conditions that affect infections.\"\n },\n {\n \"doc_id\": 1587330908315665200,\n \"text\": \" sexual reproduction.\\nThe clearance of the pathogens, either treatment-induced or spontaneous, it can be influenced by the genetic variants carried by the individual patients. For instance, for genotype 1 hepatitis C treated with Pegylated interferon-alpha-2a or Pegylated interferon-alpha-2b (brand names Pegasys or PEG-Intron) combined with ribavirin, it has been shown that genetic polymorphisms near the human IL28B gene, encoding interferon lambda 3, are associated with significant differences in the treatment-induced clearance of the\"\n },\n {\n \"doc_id\": 7414764089986002000,\n \"text\": \"### Plasmodium\"\n },\n {\n \"doc_id\": 7008811410189903000,\n \"text\": \" of its carriers. The\\nmost ancestral strain is found in Africa, and the data from seven other\\ndistinct geographical variants allowed scientists to develop a dispersal\\nmodel that corresponds to the pattern of human migration (Falush, 2003). As\\nglobalization continues to increase the rates of microbial transmission, it\\nbecomes more and more crucial to study these organisms, for they play a major\\nrole in regulating our body's functions.\"\n },\n {\n \"doc_id\": 7195437749761732000,\n \"text\": \" PLOS Genetics are provided here courtesy of PLOS\\nOTHER FORMATS\\nPDF (939K)\\nACTIONS\\nCite\\nCollections\\nSHARE\"\n },\n {\n \"doc_id\": 6522456750091133000,\n \"text\": \"et demonstrated the existence of MHC genes in humans and described the first human leucocyte antigen, the protein which we call now HLA-A2. Some years later Baruj Benacerraf showed that polymorphic MHC genes not only determine an individual’s unique constitution of antigens but also regulate the interaction among the various cells of the immunological system. These three scientists have been awarded the 1980 Nobel Prize in Physiology or Medicine for their discoveries concerning “genetically determined structures on the cell surface that regulate immunological reactions”.\\nThe first fully sequenced\"\n },\n {\n \"doc_id\": 6571952563565801000,\n \"text\": \" virus. This finding, originally reported in Nature, showed that genotype 1 hepatitis C patients carrying certain genetic variant alleles near the IL28B gene are more possibly to achieve sustained virological response after the treatment than others. Later report from Nature demonstrated that the same genetic variants are also associated with the natural clearance of the genotype 1 hepatitis C virus.\"\n },\n {\n \"doc_id\": 7853884882958839000,\n \"text\": \"nature09298. PMC 3173859. PMID 20811451. ^ Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \\\"Insights into human genetic variation and population history from 929 diverse genomes\\\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa, the Americas, and O\"\n },\n {\n \"doc_id\": 4000437132722955000,\n \"text\": \" P.; Elferink, R. Oude (2002). [ \\\"Mammalian ABC transporters in health and disease\\\" ](https://pure.uva.nl/ws/files/3499814/42885_202387y.pdf) (PDF) . _Annual Review of Biochemistry_. **71** : 537–592. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1146/annurev.biochem.71.1023\"\n },\n {\n \"doc_id\": 2974077984675988000,\n \"text\": \"2): 101–9. doi:10.1016/s0165-0378(97)00056-9. PMID 9421795.\\n\\n^ Ober C, Weitkamp LR, Cox N, Dytch H, Kostyu D, Elias S (September 1997). \\\"HLA and mate choice in humans\\\". American Journal of Human Genetics. 61 (3): 497–504. doi:10.1086/515511. PMC 1715964. PMID 9326314.\\n\\n^ Chaix R,\"\n },\n {\n \"doc_id\": 8274408650100361000,\n \"text\": \"journal=Nature&doi=10.1038%2Fnature13972&volume=515&pages=365-370&publication_year=2014&author=Stergachis%2CAB)\"\n },\n {\n \"doc_id\": 8520953987562449000,\n \"text\": \" Cao C, Donnelly P (September 2008). \\\"Is mate choice in humans MHC-dependent?\\\". PLOS Genetics. 4 (9): e1000184. doi:10.1371/journal.pgen.1000184. PMC 2519788. PMID 18787687.\\n\\n^ Derti A, Cenik C, Kraft P, Roth FP (April 2010). \\\"Absence of evidence for MHC-dependent mate selection within HapMap populations\\\". PLOS Genetics. 6 (4): e1000925. doi:10\"\n },\n {\n \"doc_id\": 3474157058676721700,\n \"text\": \"rhagic fever. Lancet. 2011;377:849–862. [PMC free article] [PubMed] [Google Scholar]\\nGama J.A., Abby S.S., Vieira-Silva S., Dionisio F., Rocha E.P. Immune subversion and quorum-sensing shape the variation in infectious dose among bacterial pathogens. PLoS Pathog. 2012;8:e1002503. [PMC free article] [PubMed] [Google Scholar]\\nGibbs E.P. The evolution of One\"\n },\n {\n \"doc_id\": 5136385584355986000,\n \"text\": \" the initial hybridised population may have experienced up to a 94% reduction in fitness compared to contemporary humans. By this measure, Neanderthals may have substantially increased in fitness. A 2017 study focusing on archaic genes in Turkey found associations with coeliac disease, malaria severity and Costello syndrome. Nonetheless, some genes may have helped modern East Asians adapt to the environment; the putatively Neanderthal Val92Met variant of the MC1R gene, which may be weakly associated with red hair and UV radiation sensitivity, is primarily found in East Asian, rather than European, individuals. Some genes related\"\n },\n {\n \"doc_id\": 8746695212421639000,\n \"text\": \"The protozoa _Plasmodium_ _genus_ causes the tropical disease [ malaria\\n](https://www.medicalnewstoday.com/articles/150670). The parasite primarily\\n[ transmits ](https://www.who.int/news-room/fact-sheets/detail/malaria)\\nthrough [ mosquito bites ](https://www.medicalnewstoday.com/articles/311485).\"\n },\n {\n \"doc_id\": 6621003551263229000,\n \"text\": \"[ Article ](https://doi.org/10.1111%2Fj.1440-1843.2011.02037.x) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=21848707)\\n[ Google Scholar\\n](http://scholar.google.com/scholar_lookup?&title=Phenotypes%20of%20patients%20with%20\"\n },\n {\n \"doc_id\": 2841831592050604000,\n \"text\": \"Ilardo MA, Moltke I, Korneliussen TS, Cheng J, Stern AJ, Racimo F, et al. (April 2018). \\\"Physiological and Genetic Adaptations to Diving in Sea Nomads\\\". Cell. 173 (3): 569–580.e15. doi : 10.1016/j.cell.2018.03.054. PMID 29677510.\\n\\nRogers AR, Iltis D, Wooding S (2004). \\\"Genetic variation at the MC1R locus and the time since loss of\"\n },\n {\n \"doc_id\": 4740884425898056000,\n \"text\": \"24. PMID 16435885.\\n\\n^ \\\"IPD-MHC Database\\\". EMBL-EBI.\\n\\n^ Kulski JK, Shiina T, Anzai T, Kohara S, Inoko H (December 2002). \\\"Comparative genomic analysis of the MHC: the evolution of class I duplication blocks, diversity and complexity from shark to man\\\". Immunological Reviews. 190: 95–122. doi:10.1034/j.1600-065x.2002.19008.x. PMID 12493009.\"\n },\n {\n \"doc_id\": 6060117423807330000,\n \"text\": \"The major histocompatibility complex (MHC) is a large locus on vertebrate DNA containing a set of closely linked polymorphic genes that code for cell surface proteins essential for the adaptive immune system. These cell surface proteins are called MHC molecules.\\nThe name of this locus comes from its discovery through the study of transplanted tissue compatibility. Later studies revealed that tissue rejection due to incompatibility is only a facet of the full function of MHC molecules: binding an antigen derived from self-proteins, or from pathogens, and bringing the antigen presentation to the cell surface for recognition by the appropriate T-cells. MHC molecules mediate the interactions of leukocytes, also called white blood cells (WBCs), with other leukocytes or with body cells. The MHC determines donor compatibility for organ transplant, as well as one's susceptibility to autoimmune diseases.\\nIn a cell, protein molecules of the host's own phenotype or of other biologic entities are continually synthesized and degraded. Each MHC molecule on the cell surface displays a small peptide (a molecular fraction of a protein) called an epitope. The presented self-antigens prevent an organism's immune system from targeting its own cells. The presentation of pathogen-derived proteins results in the elimination of the infected cell by the immune system.\\nDiversity of an individual's self-antigen presentation, mediated by MHC self-antigens, is attained in at least three ways: (1) an organism's MHC repertoire is polygenic (via multiple, interacting genes); (2) MHC expression is codominant (from both sets of inherited alleles); (3) MHC gene variants are highly polymorphic (diversely varying from organism to organism within a species). Sexual selection has been observed in male mice choosing to mate with females with different MHCs. Also, at least for MHC I presentation, there has been evidence of antigenic peptide splicing, which can combine peptides from different proteins, vastly increasing antigen diversity.\"\n },\n {\n \"doc_id\": 8183121990202397000,\n \"text\": \"51. ** ^ ** Krebs, J. E.; Goldstein, E. S.; Kilpatrick, ST (2011). [ _Lewin's GENES X_ ](https://archive.org/details/lewinsgenesx0000unse/page/289). Boston: Jones and Bartlett Publishers. pp. [ 289–292 ](https://archive.org/details/lewinsgenesx0000unse/page/289). [ ISBN ](/wiki/ISBN_\\\\(identifier\\\\) \\\"ISBN \\\\(identifier\"\n },\n {\n \"doc_id\": 657536276632004200,\n \"text\": \" ^ Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \\\"Insights into human genetic variation and population history from 929 diverse genomes\\\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa, the Americas, and Oceania each harbor tens to hundreds of thousands of private, common genetic variants.\"\n },\n {\n \"doc_id\": 6536929371804143000,\n \"text\": \" ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 4455760 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4455760). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 26009770 ](https://pubmed.ncbi.nlm.nih.gov/26009770).\\n39. ** ^ ** Larsson, Max; Sawada, Ke\"\n },\n {\n \"doc_id\": 4323140975966511600,\n \"text\": \" the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \\\"Evidence for variable selective pressures at MC1R\\\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 10733465. ^ Robin A (1991). Biological Perspectives on Human Pigmentation. Cambridge: Cambridge University Press. ^ \\\"The Science Behind the Human Genome Project\\\"\"\n },\n {\n \"doc_id\": 1976914264601072400,\n \"text\": \"mb.2006.07.045 ](https://doi.org/10.1016%2Fj.jmb.2006.07.045). [ PMC ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 3662476 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3662476). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 16934836 ](https\"\n },\n {\n \"doc_id\": 5332197148514614000,\n \"text\": \"1. [ Stabilizing Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-1)\\n2. [ Directional Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-2)\\n3. [ Diversifying Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-3\"\n },\n {\n \"doc_id\": 1343602846445218300,\n \"text\": \", which were introduced to and slowly selected out of the much larger modern human population; the initial hybridised population may have experienced up to a 94% reduction in fitness compared to contemporary humans. By this measure, Neanderthals may have substantially increased in fitness. A 2017 study focusing on archaic genes in Turkey found associations with coeliac disease, malaria severity and Costello syndrome. Nonetheless, some genes may have helped modern East Asians adapt to the environment; the putatively Neanderthal Val92Met variant of the MC1R gene, which may be weakly associated with red hair and UV radiation sensitivity\"\n },\n {\n \"doc_id\": 7752196112672118000,\n \"text\": \" HLA allele is named with a number. For instance, for a given individual, his haplotype might be HLA-A2, HLA-B5, HLA-DR3, etc... Each heterozygous individual will have two MHC haplotypes, one each from the paternal and maternal chromosomes.\\nThe MHC genes are highly polymorphic; many different alleles exist in the different individuals inside a population. The polymorphism is so high, in a mixed population (nonendogamic), no two individuals have exactly the same set of MHC molecules, with the exception\"\n },\n {\n \"doc_id\": 1572121283436197600,\n \"text\": \"Bergström A, McCarthy SA, Hui R, Almarri MA, Ayub Q, Danecek P, et al. (March 2020). \\\"Insights into human genetic variation and population history from 929 diverse genomes\\\". Science. 367 (6484): eaay5012. doi : 10.1126/science.aay5012. PMC 7115999. PMID 32193295. Populations in central and southern Africa, the Americas, and Oceania each harbor tens to hundreds of thousands of private, common genetic variants.\"\n },\n {\n \"doc_id\": 5871790602362253000,\n \"text\": \"In sexual mate selection[edit]\\n\\nMain article: Major histocompatibility complex and sexual selection\\n\\nSee also: Interpersonal compatibility\\nMHC molecules enable immune system surveillance of the population of protein molecules in a host cell, and greater MHC diversity permits greater diversity of antigen presentation. In 1976, Yamazaki et al demonstrated a sexual selection mate choice by male mice for females of a different MHC. Similar results have been obtained with fish. Some data find lower rates of early pregnancy loss in human couples of dissimilar MHC genes.\\nMHC may be related to mate choice\"\n },\n {\n \"doc_id\": 4675850447830746000,\n \"text\": \"When this answer was published in 2009, Jared was a Ph.D. candidate in the\\nDepartment of Genetics, studying yeast carbon metabolism and genome evolution\\nin Gavin Sherlock’s laboratory. Jared wrote this answer while participating in\\nthe Stanford at The Tech program.\\n\\nAsk a Geneticist Home\"\n },\n {\n \"doc_id\": 1332471960494971400,\n \"text\": \"\"\n },\n {\n \"doc_id\": 8327005706154174000,\n \"text\": \" NG, Chaplin G (July 2000). \\\"The evolution of human skin coloration\\\" (PDF). Journal of Human Evolution. 39 (1): 57–106. doi : 10.1006/jhev.2000.0403. PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \\\"Evidence for variable selective pressures at MC1R\\\". American Journal of Human Genetics. 66 (4\"\n },\n {\n \"doc_id\": 3496377408504689700,\n \"text\": \"Proteins[edit]\\nT-cell receptor complexed with MHC-I and MHC-II\\nMHC class I[edit]\\nMain article: MHC class I\\nMHC class I molecules are expressed in some nucleated cells and also in platelets—in essence all cells but red blood cells. It presents epitopes to killer T cells, also called cytotoxic T lymphocytes (CTLs). A CTL expresses CD8 receptors, in addition to T-cell receptors (TCR)s. When a CTL's CD8 receptor docks to a MHC class I molecule, if the CTL's TCR fits the epitope within the MHC class I molecule, the CTL triggers the cell to undergo programmed cell death by apoptosis. Thus, MHC class I helps mediate cellular immunity, a primary means to address intracellular pathogens, such as viruses and some bacteria, including bacterial L forms, bacterial genus Mycoplasma, and bacterial genus Rickettsia. In humans, MHC class I comprises HLA-A, HLA-B, and HLA-C molecules.\\nThe first crystal structure of Class I MHC molecule, human HLA-A2, was published in 1989. The structure revealed that MHC-I molecules are heterodimers, they have polymorphic heavy α-subunit whose gene occurs inside the MHC locus and small invariant β2 microglobulin subunit whose gene is located usually outside of it. Polymorphic heavy chain of MHC-I molecule contains N-terminal extra-cellular region composed by three domains, α1, α2, and α3, transmembrane helix to hold MHC-I molecule on the cell surface and short cytoplasmic tail. Two domains, α1 and α2 form deep peptide-binding groove between two long α-helices and the floor of the groove formed by eight β-strands. Immunoglobulin-like domain α3 involved in the interaction with CD8 co-receptor. β2 microglobulin provides stability of the complex and participates in the recognition of peptide-MHC class I complex by CD8 co-receptor. The peptide is non-covalently bound to MHC-I, it is held by the several pockets on the floor of the peptide-binding groove. Amino acid side-chains that are most polymorphic in human alleles fill up the central and widest portion of the binding groove, while conserved side-chains are clustered at the narrower ends of the groove.\\nSchematic view of MHC class I and MHC class II molecules\\nClassical MHC molecules present epitopes to the TCRs of CD8+ T lymphocytes. Nonclassical molecules (MHC class IB) exhibit limited polymorphism, expression patterns, and presented antigens; this group is subdivided into a group encoded within MHC loci (e.g., HLA-E, -F, -G), as well as those not (e.g., stress ligands such as ULBPs, Rae1, and H60); the antigen/ligand for many of these molecules remain unknown, but they can interact with each of CD8+ T cells, NKT cells, and NK cells. The evolutionary oldest nonclassical MHC class I lineage in human was deduced to be the lineage that includes the CD1 and PROCR (alias EPCR) molecules and this lineage may have been established before the origin of tetrapod species. However, the only nonclassical MHC class I lineage for which evidence exists that it was established before the evolutionary separation of Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish plus tetrapods) is lineage Z of which members are found, together in each species with classical MHC class I, in lungfish and throughout ray-finned fishes; why the Z lineage was well conserved in ray-finned fish but lost in tetrapods is not understood.\\nMHC class II[edit]\\n\\nMain article: MHC class II\\nMHC class II can be conditionally expressed by all cell types, but normally occurs only on \\\"professional\\\" antigen-presenting cells (APCs): macrophages, B cells, and especially dendritic cells (DCs). An APC takes up an antigenic protein, performs antigen processing, and returns a molecular fraction of it—a fraction termed the epitope—and displays it on the APC's surface coupled within an MHC class II molecule (antigen presentation). On the cell's surface, the epitope can be recognized by immunologic structures like T-cell receptors (TCRs). The molecular region which binds to the epitope is the paratope.\\nOn surfaces of helper T cells are CD4 receptors, as well as TCRs. When a naive helper T cell's CD4 molecule docks to an APC's MHC class II molecule, its TCR can meet and bind the epitope coupled within the MHC class II. This event primes the naive T cell. According to the local milieu, that is, the balance of cytokines secreted by APCs in the microenvironment, the naive helper T cell (Th0) polarizes into either a memory Th cell or an effector Th cell of phenotype either type 1 (Th1), type 2 (Th2), type 17 (Th17), or regulatory/suppressor (Treg), as so far identified, the Th cell's terminal differentiation.\\nMHC class II thus mediates immunization to—or, if APCs polarize Th0 cells principally to Treg cells, immune tolerance of—an antigen. The polarization during primary exposure to an antigen is key in determining a number of chronic diseases, such as inflammatory bowel diseases and asthma, by skewing the immune response that memory Th cells coordinate when their memory recall is triggered upon secondary exposure to similar antigens. B cells express MHC class II to present antigens to Th0, but when their B cell receptors bind matching epitopes, interactions which are not mediated by MHC, these activated B cells secrete soluble immunoglobulins: antibody molecules mediating humoral immunity.\\nClass II MHC molecules are also heterodimers, genes for both α and β subunits are polymorphic and located within MHC class II subregion. Peptide-binding groove of MHC-II molecules is forms by N-terminal domains of both subunits of the heterodimer, α1 and β1, unlike MHC-I molecules, where two domains of the same chain are involved. In addition, both subunits of MHC-II contain transmembrane helix and immunoglobulin domains α2 or β2 that can be recognized by CD4 co-receptors. In this way MHC molecules chaperone which type of lymphocytes may bind to the given antigen with high affinity, since different lymphocytes express different T-Cell Receptor (TCR) co-receptors.\\nMHC class II molecules in humans have five to six isotypes. Classical molecules present peptides to CD4+ lymphocytes. Nonclassical molecules, accessories, with intracellular functions, are not exposed on cell membranes, but in internal membranes, assisting with the loading of antigenic peptides onto classic MHC class II molecules. The important nonclassical MHC class II molecule DM is only found from the evolutionary level of lungfish, although also in more primitive fishes both classical and nonclassical MHC class II are found.\\nSr.No\\n\\nFeature\\n\\nClass I MHC\\n\\nClass II MHC\\n\\n\\n1\\n\\nConstituting polypeptide chains\\n\\nα chain (45KDa in humans)\\nβ2 chain (12 KDa in humans)\\n\\n\\nα chain (30–34 KDa in humans)\\nβ chain (26–29 KDa in humans)\\n\\n\\n\\n2\\n\\nAntigen binding domain\\n\\nα1and α2 domains\\n\\nα1 and β1 domains\\n\\n\\n3\\n\\nBinds protein antigens of\\n\\n8–10 amino acids residues\\n\\n13–18 amino acids residues\\n\\n\\n4\\n\\nPeptide bending cleft\\n\\nFloor formed by β sheets and sides by α\\nhelices, blocked at both the ends\\n\\n\\nFloor formed by β sheets and sides by α\\nhelices, opened at both the ends\\n\\n\\n\\n5\\n\\nAntigenic peptide motifs\\ninvolved in binding\\n\\n\\nAnchor residues located at amino and\\ncarbon terminal ends\\n\\n\\nAnchor residues located almost uniformly\\nalong the peptide\\n\\n\\n\\n6\\n\\nPresents antigenic peptide to\\n\\nCD8+ T cells\\n\\nCD4+ T cells\\nMHC class III[edit]\\n\\nMain article: MHC class III\\nClass III molecules have physiologic roles unlike classes I and II, but are encoded between them in the short arm of human chromosome 6. Class III molecules include several secreted proteins with immune functions: components of the complement system (such as C2, C4, and B factor), cytokines (such as TNF-α, LTA, and LTB), and heat shock proteins.\\nFunction[edit]\\nMHC is the tissue-antigen that allows the immune system (more specifically T cells) to bind to, recognize, and tolerate itself (autorecognition). MHC is also the chaperone for intracellular peptides that are complexed with MHCs and presented to T cell receptors (TCRs) as potential foreign antigens. MHC interacts with TCR and its co-receptors to optimize binding conditions for the TCR-antigen interaction, in terms of antigen binding affinity and specificity, and signal transduction effectiveness.\\nEssentially, the MHC-peptide complex is a complex of auto-antigen/allo-antigen. Upon binding, T cells should in principle tolerate the auto-antigen, but activate when exposed to the allo-antigen. Disease states occur when this principle is disrupted.\\nAntigen presentation: MHC molecules bind to both T cell receptor and CD4/CD8 co-receptors on T lymphocytes, and the antigen epitope held in the peptide-binding groove of the MHC molecule interacts with the variable Ig-Like domain of the TCR to trigger T-cell activation\\nAutoimmune reaction: Having some MHC molecules increases the risk of autoimmune diseases more than having others. HLA-B27 is an example. It is unclear how exactly having the HLA-B27 tissue type increases the risk of ankylosing spondylitis and other associated inflammatory diseases, but mechanisms involving aberrant antigen presentation or T cell activation have been hypothesized.\\nTissue allorecognition: MHC molecules in complex with peptide epitopes are essentially ligands for TCRs. T cells become activated by binding to the peptide-binding grooves of any MHC molecule that they were not trained to recognize during positive selection in the thymus.\"\n },\n {\n \"doc_id\": 4446130463120729000,\n \"text\": \" sicker from COVID‑19. Genetic screening is able to detect interferon effector genes. Some genetic variants are risk factors in specific populations. For instance, an allele of the DOCK2 gene (dedicator of cytokinesis 2 gene) is a common risk factor in Asian populations but much less common in Europe. The mutation leads to lower expression of DOCK2 especially in younger patients with severe Covid. In fact, many other genes and genetic variants have been found that determine the outcome of SARS-CoV-2 infections.\\nChildren\\n\\nSee also: Impact\"\n },\n {\n \"doc_id\": 2448606410365659600,\n \"text\": \"doi.org/10.1073%2Fpnas.0502893102). [ PMC ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 1093311 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1093311). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 15888555 ](https://pubmed.ncbi.nlm.nih.gov\"\n },\n {\n \"doc_id\": 7037905041389609000,\n \"text\": \"1. [ Epistasis ](https://slcc.pressbooks.pub/collegebiology1/chapter/polygenic-inheritance-and-epistasis/#chapter-1076-section-1)\\n2. [ Polygenic Inheritance ](https://slcc.pressbooks.pub/collegebiology1/chapter/polygenic-inheritance-and-epistasis/#chapter-1076-section-2)\\n22. XXI . Energy and the ATP Cycle\"\n },\n {\n \"doc_id\": 5174268529100157000,\n \"text\": \"%20association%20loci%20for%20human%20diseases%20and%20traits&journal=Proc.%20Natl%20Acad.%20Sci.%20USA&doi=10.1073%2Fpnas.0903103106&volume=106&pages=9362-9367&publication_year=2009&author=Hindorff%2CLA)\"\n },\n {\n \"doc_id\": 1139994221072065300,\n \"text\": \"Notes and references[edit]\\n^ Hull P (August 1970). \\\"Notes on Dr Snell's observations concerning the H-2 locus polymorphism\\\". Heredity. 25 (3): 461–5. doi:10.1038/hdy.1970.47. PMID 5275401.\\n\\n^ Janeway Jr CA, Travers P, Walport M, et al. (2001). \\\"The Major Histocompatibility Complex and Its Functions\\\". Immunobiology: The Immune System in Health and Disease (5th ed.).\"\n },\n {\n \"doc_id\": 6739160733741488000,\n \"text\": \", and remain circulating in a large population. Genetic drift is also a major driving force in some species. It is possible that the combined effects of some or all of these factors cause the genetic diversity.\\nMHC diversity has also been suggested as a possible indicator for conservation, because large, stable populations tend to display greater MHC diversity, than smaller, isolated populations. Small, fragmented populations that have experienced a population bottleneck typically have lower MHC diversity. For example, relatively low MHC diversity has been observed in the cheetah (Acinonyx jubatus), Eurasian beaver\"\n },\n {\n \"doc_id\": 6078984594418064000,\n \"text\": \"In transplant rejection[edit]\\nIn a transplant procedure, as of an organ or stem cells, MHC molecules themselves act as antigens and can provoke immune response in the recipient, thus causing transplant rejection. MHC molecules were identified and named after their role in transplant rejection between mice of different strains, though it took over 20 years to clarify MHC's role in presenting peptide antigens to cytotoxic T lymphocytes (CTLs).\\nEach human cell expresses six MHC class I alleles (one HLA-A, -B, and -C allele from each parent) and six to eight MHC class II alleles (one HLA-DP and -DQ, and one or two HLA-DR from each parent, and combinations of these). The MHC variation in the human population is high, at least 350 alleles for HLA-A genes, 620 alleles for HLA-B, 400 alleles for DR, and 90 alleles for DQ. Any two individuals who are not identical twins, triplets, or higher order multiple births, will express differing MHC molecules. All MHC molecules can mediate transplant rejection, but HLA-C and HLA-DP, showing low polymorphism, seem least important.\\nWhen maturing in the thymus, T lymphocytes are selected for their TCR incapacity to recognize self antigens, yet T lymphocytes can react against the donor MHC's peptide-binding groove, the variable region of MHC holding the presented antigen's epitope for recognition by TCR, the matching paratope. T lymphocytes of the recipient take the incompatible peptide-binding groove as nonself antigen.\\nTransplant rejection has various types known to be mediated by MHC (HLA):\\nHyperacute rejection occurs when, before the transplantation, the recipient has preformed anti-HLA antibodies, perhaps by previous blood transfusions (donor tissue that includes lymphocytes expressing HLA molecules), by anti-HLA generated during pregnancy (directed at the father's HLA displayed by the fetus), or by previous transplantation;\\nAcute cellular rejection occurs when the recipient's T lymphocytes are activated by the donor tissue, causing damage via mechanisms such as direct cytotoxicity from CD8 cells.\\nAcute humoral rejection and chronic disfunction occurs when the recipient's anti-HLA antibodies form directed at HLA molecules present on endothelial cells of the transplanted tissue.\\nIn all of the above situations, immunity is directed at the transplanted organ, sustaining lesions. A cross-reaction test between potential donor cells and recipient serum seeks to detect presence of preformed anti-HLA antibodies in the potential recipient that recognize donor HLA molecules, so as to prevent hyperacute rejection. In normal circumstances, compatibility between HLA-A, -B, and -DR molecules is assessed. The higher the number of incompatibilities, the lower the five-year survival rate. Global databases of donor information enhance the search for compatible donors.\\nThe involvement in allogeneic transplant rejection appears to be an ancient feature of MHC molecules, because also in fish associations between transplant rejections and (mis-)matching of MHC class I and MHC class II were observed.\"\n },\n {\n \"doc_id\": 4143018406264960000,\n \"text\": \"ifier\\\\)\\\") : [ 10.1016/j.abb.2006.05.001 ](https://doi.org/10.1016%2Fj.abb.2006.05.001). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 16806044 ](https://pubmed.ncbi.nlm.nih.gov/16806044).\\n33. ** ^ ** Kamenetsky, M.; Middelhaufe, S.; Bank,\"\n },\n {\n \"doc_id\": 7354962069222019000,\n \"text\": \"Evolutionary diversity[edit]\\nMost mammals have MHC variants similar to those of humans, who bear great allelic diversity, especially among the nine classical genes—seemingly due largely to gene duplication—though human MHC regions have many pseudogenes. The most diverse loci, namely HLA-A, HLA-B, and HLA-C, have roughly 6000, 7200, and 5800 known alleles, respectively. Many HLA alleles are ancient, sometimes of closer homology to a chimpanzee MHC alleles than to some other human alle\"\n },\n {\n \"doc_id\": 2820692476841372700,\n \"text\": \" 20167026 ](https://pubmed.ncbi.nlm.nih.gov/20167026). [ S2CID ](/wiki/S2CID_\\\\(identifier\\\\) \\\"S2CID \\\\(identifier\\\\)\\\") [ 11617147 ](https://api.semanticscholar.org/CorpusID:11617147).\\n30. ** ^ ** Bernstein, H.; Hopf, F. A.; Michod, R. E. (1987). \\\"The molecular basis of the evolution of sex\\\". _\"\n },\n {\n \"doc_id\": 2767495083226295000,\n \"text\": \":\\nEach person carries 2 alleles of each of the 3 class-I genes, (HLA-A, HLA-B and HLA-C), and so can express six different types of MHC-I (see figure).\\nIn the class-II locus, each person inherits a pair of HLA-DP genes (DPA1 and DPB1, which encode α and β chains), a couple of genes HLA-DQ (DQA1 and DQB1, for α and β chains), one gene HLA-DRα (\"\n },\n {\n \"doc_id\": 7748368743253828000,\n \"text\": \". PMID 10896812. Archived from the original (PDF) on 14 January 2012. ^ Harding RM, Healy E, Ray AJ, Ellis NS, Flanagan N, Todd C, et al. (April 2000). \\\"Evidence for variable selective pressures at MC1R\\\". American Journal of Human Genetics. 66 (4): 1351–1361. doi : 10.1086/302863. PMC 1288200. PMID 10733465. ^ Robin A (1991). Biological Perspectives on Human Pigmentation. Cambridge: Cambridge University Press.\"\n },\n {\n \"doc_id\": 112755929557492580,\n \"text\": \"HLA biology[edit]\\nCodominant expression of HLA genes\\n\\nMain article: Human leukocyte antigen\\nHuman MHC class I and II are also called human leukocyte antigen (HLA). To clarify the usage, some of the biomedical literature uses HLA to refer specifically to the HLA protein molecules and reserves MHC for the region of the genome that encodes for this molecule, but this is not a consistent convention.\\nThe most studied HLA genes are the nine classical MHC genes: HLA-A, HLA-B, HLA-\"\n }\n]","isConversational":false}}},{"rowIdx":92,"cells":{"query":{"kind":"string","value":"I'm studying the life cycle and transmission of malaria and have a specific question regarding the role of mosquitoes in spreading the disease. How do mosquitoes act as vectors for the *Plasmodium* parasite, and what mechanisms allow *Plasmodium* to survive and multiply within the mosquito before being transmitted to humans? Any insights or references to detailed studies on the interaction between *Plasmodium* and mosquitoes would be greatly appreciated."},"positive_passages":{"kind":"list like","value":[{"doc_id":4161775671760185300,"text":"[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)"}],"string":"[\n {\n \"doc_id\": 4161775671760185300,\n \"text\": \"[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":7579552577826821000,"text":"Riehle, M. A., _et al_. Using bacteria to express and display anti-\n_Plasmodium_ molecules in the mosquito midgut. _International Journal of\nParasitology_ **37**, 595–603 (2007)"},{"doc_id":712862380789366700,"text":"* Malaria. (2022).\n[ https://www.who.int/news-room/fact-sheets/detail/malaria\n](https://www.who.int/news-room/fact-sheets/detail/malaria)"},{"doc_id":4924544718779407000,"text":"These issues come into focus when you consider the devastating malaria\nepidemic that kills one to three million people worldwide every year. The\npathogens that cause malaria are protozoan parasites from the genus\n_Plasmodium_ that are transmitted through the bite of mosquitoes of the genus\n_Anopheles_. Both of these animal genera contain hundreds of species,\nalthough only a few are involved in transmitting malaria in humans. Recent\ngenetic studies of the symbiotic bacteria in the midgut of the _Anopheles\nstephensi_ mosquitoes have yielded"},{"doc_id":7414764089986002000,"text":"### Plasmodium"},{"doc_id":482585183638768960,"text":"Box 3\nHosts, vectors, and vehicles involved in the life cycle of infectious agents transmitted indirectly\nDefinitive host: A host in which a parasite reproduces sexually. Humans are definitive hosts for roundworms. By strict definition, mosquitoes are the definitive host of malaria as they are the organism in which sexual reproduction of the agent protozoa, Plasmodium spp., occurs."},{"doc_id":5861845573567613000,"text":" Evans D.B., Jha P., Mills A., Musgrove P., editors. Disease Control Priorities in Developing Countries. The International Bank for Reconstruction and Development/The World Bank Group; Washington, DC: 2006. [Google Scholar]\nLanghorne J., Ndungu F.M., Sponaas A.M., Marsh K. Immunity to malaria: more questions than answers. Nat. Immunol. 2008;9:725–732. [PubMed] [Google Scholar]\nLeavell H.R., Clark E.G. McG"},{"doc_id":4422283637412871000,"text":"Whitty CJM, Ansah E. Malaria control stalls in high incidence areas. BMJ.\n2019 May 21; 365 :l2216. [ [ PubMed : 31113780\n](https://pubmed.ncbi.nlm.nih.gov/31113780) ]"},{"doc_id":278312632305110720,"text":" is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.\nPersistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.\nTransmission[edit]\n\nMain article: Transmission (medicine)\nA southern house mosquito (Culex quinquefasciatus) is a vector that transmits the pathogens that cause West Nile fever and avian malaria among others.\nFor infecting organisms to survive and repeat the infection cycle in other hosts, they"},{"doc_id":1320637855125465900,"text":"thropods\". _Insect Molecular Biology_. **19** (Suppl 1): 165–180. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1111/j.1365-2583.2009.00948.x ](https://doi.org/10.1111%2Fj.1365-2583.2009.00948.x). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") ["},{"doc_id":1362592987580663000,"text":"Biological transmission occurs when multiplication and/or development of a pathogenic agent within a vector (e.g., biological vector or intermediate host) is required for the agent to become infectious to humans. The time that is necessary for these events to occur is known as the extrinsic incubation period; in contrast to the intrinsic incubation period which is the time required for an exposed human host to become infectious. Indirect transmission by mosquito vectors is the primary mode of transmission of a large number of viruses (arthropod-borne viruses or arboviruses) of public health concern ("},{"doc_id":6707359235402285000,"text":"Malaria\nJapanese encephalitis\nLyme\nMalaria\nHendra\tSteinmann et al. (2006)"},{"doc_id":6899342711634635000,"text":" PMID 14634648. S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365"},{"doc_id":7145942413818299000,"text":"Guerra CA, Kang SY, Citron DT, Hergott DEB, Perry M, Smith J, Phiri WP, Osá\nNfumu JO, Mba Eyono JN, Battle KE, Gibson HS, García GA, Smith DL. Human\nmobility patterns and malaria importation on Bioko Island. Nat Commun. 2019\nMay 27; 10 (1):2332. [ [ PMC free article : PMC6536527\n](/pmc/articles/PMC6536527/) ]"},{"doc_id":6800680407069684000,"text":" \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28"},{"doc_id":3110736927014209500,"text":" disease prevention. Bull. World Health Organ. 2005;83:942–947. [PMC free article] [PubMed] [Google Scholar]\nVasilakis N., Cardosa J., Hanley K.A., Holmes E.C., Weaver S.C. Fever from the forest: prospects for the continued emergence of sylvatic dengue virus and its impact on public health. Nat. Rev. Microbiol. 2011;9:532–541. [PMC free article] [PubMed] [Google Scholar]\nVos"},{"doc_id":5535637625853454000,"text":"identifier\\) \"Doi \\(identifier\\)\") : [ 10.1146/annurev.bi.49.070180.004305 ](https://doi.org/10.1146%2Fannurev.bi.49.070180.004305). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 6250450 ](https://pubmed.ncbi.nlm.nih.gov/6250450).\n4. ** ^ **"},{"doc_id":8564780383626280000,"text":"insects/)\n* [ Mosquitoes: Friend or Foe? ](https://www.floridamuseum.ufl.edu/exhibits/lease/mosquitoes-friend-or-foe/)"},{"doc_id":3632965364524991500,"text":"Biological vector: A vector (often arthropod) in which an infectious organism must develop or multiply before the vector can transmit the organism to a susceptible host. Aedes spp. mosquitoes are a biological vector for dengue, chikungunya, and Zika."},{"doc_id":7517740263136706000,"text":" of major human infectious diseases. Nature. 2007;447:279–283. [PMC free article] [PubMed] [Google Scholar]\nYasuoka J., Levins R. Impact of deforestation and agricultural development on anopheline ecology and malaria epidemiology. Am. J. Trop. Med. Hyg. 2007;76:450–460. [PubMed] [Google Scholar]\nYu I.T., Li Y., Wong T.W., Tam W., Chan A.T., Lee J.H., Leung D.Y., Ho T. Evidence"},{"doc_id":5802792367038120000,"text":"ically distinct from that causing initial dengue infection (Screaton et al., 2015)."},{"doc_id":3717481448095056000,"text":" S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008."},{"doc_id":4946908657905475000,"text":"redity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911. ^ Shelomi M, Zeuss"},{"doc_id":6222474132400071000,"text":"[ Creature Cast ](http://www.nature.com/scitable/blog/creature-cast)"},{"doc_id":9121594639098404000,"text":"I think this paper will satisfy you, everyzhing int he one place:\n[ https://jeb.biologists.org/content/218/4/506\n](https://jeb.biologists.org/content/218/4/506)"},{"doc_id":6851750948176027000,"text":"[ Article ](https://doi.org/10.7554%2FeLife.07871) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=26473971)\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4608052) [\nGoogle Scholar\n](http://scholar.google.com/"},{"doc_id":1398732066174818000,"text":"Med] [Google Scholar]\nAnyamba A., Linthicum K.J., Small J., Britch S.C., Pak E., De La Rocque S., Formenty P., Hightower A.W., Breiman R.F., Chretien J.P., Tucker C.J., Schnabel D., Sang R., Haagsma K., Latham M., Lewandowski H.B., Magdi S.O., Mohamed M.A., Nguku P.M., Reynes J.M., Swanepoel R. 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[ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 14641005 ](https://pubmed.ncbi.nlm.nih.gov/14641005).\n21. ^ _**a** _ _**b** _ _**c** _ Lodish, H.; Berk, A.; Matsudair"},{"doc_id":2429127711217836500,"text":" mating.\nSource: Modification of table from WHO, 2012. Handbook for Integrated Vector Management. WHO Press, France.\nDiarrheal diseases primarily result from oral contact with water, food, or other vehicles contaminated with pathogenic agents originating from human or animal feces. Most (∼88%) of diarrhea-associated deaths are attributable to unsafe drinking water, inadequate sanitation, and insufficient hygiene (Black et al., 2003; Prüss-Üstün et al., 2008). Interruption of fecal–oral transmission through provision of safe water and adequate sanitation, and promotion"},{"doc_id":8184988300445025000,"text":"(https://pure.rug.nl/ws/files/6771142/2011MolMicrobiolAjon.pdf) (PDF) . _Molecular Microbiology_. **82** (4). Wiley: 807–817. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1111/j.1365-2958.2011.07861.x ](https://doi.org/10.1111%2Fj.1365-2958"},{"doc_id":6686516171026289000,"text":"* [ ](http://www.nature.com/scitable/blog/viruses101)"},{"doc_id":7056037267678778000,"text":" promising results: _Enterobacter\nagglomerans_ bacteria were genetically engineered to display two anti-\n_Plasmodium_ effector molecules that kill the parasite before it is\ntransmitted to humans (Riehle _et al._, 2007). Now consider the genetic and\nphysiological differences between the wild-type and genetically modified\n_Anopheles stephensi_ mosquitoes: they are still the same species by all major\nstandards of species definition, yet what a difference it would make for\nhumankind if the _Plasmodium-_ resistant"},{"doc_id":2052100685612085200,"text":"Mechanical transmission does not require pathogen multiplication or development within a living organism. It occurs when an infectious agent is physically transferred by a live entity (mechanical vector) or inanimate object (vehicle) to a susceptible host. Classic examples of diseases spread by mechanical vector transmission are shigellosis (transmission of Shigella spp. on the appendages of flies) and plague (transmission of Yersinia pestis by fleas). Many diarrheal diseases are transmitted by the fecal-oral route with food and water often acting as vehicles (Figure 7 )."},{"doc_id":7456314755400268000,"text":" example, many hemipteran bugs have piercing and sucking mouthparts, adapted for feeding on plant sap, while species in groups such as fleas, lice, and mosquitoes are hematophagous, feeding on the blood of animals.\nA parasitoid wasp ovipositing into an aphid\n\n\n\nPlant parasite or micropredator: a coreid bug sucking plant sap\n\n\n\nHuman head-lice are directly transmitted obligate ectoparasites."},{"doc_id":2111678933010816500,"text":"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H"},{"doc_id":7906178860567034000,"text":"* [ Reference & Cite ](https://bio.libretexts.org/?refmodal)"},{"doc_id":2820692476841372700,"text":" 20167026 ](https://pubmed.ncbi.nlm.nih.gov/20167026). [ S2CID ](/wiki/S2CID_\\(identifier\\) \"S2CID \\(identifier\\)\") [ 11617147 ](https://api.semanticscholar.org/CorpusID:11617147).\n30. ** ^ ** Bernstein, H.; Hopf, F. A.; Michod, R. E. (1987). \"The molecular basis of the evolution of sex\". _"},{"doc_id":4050011035651363000,"text":"[\n](/articles/10.1186/s12931-022-02141-3/figures/2)"},{"doc_id":1663613504998063900,"text":"372978). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 7968924 ](https://pubmed.ncbi.nlm.nih.gov/7968924).\n50. ** ^ ** Lodé, T. (2012). [ \"Have Sex or Not? Lessons from Bacteria\" ](https://doi.org/10.1159%2F000342879). _Sexual Development_. **6** (6): 325–328. [ doi"},{"doc_id":622762634218896600,"text":"[ Viruses101 ](http://www.nature.com/scitable/blog/viruses101)"},{"doc_id":1055589053584834800,"text":"Kebede A., Mccann J.C., Kiszewski A.E., Ye-Ebiyo Y. New evidence of the effects of agro-ecologic change on malaria transmission. Am. J. Trop. Med. Hyg. 2005;73:676–680. [PubMed] [Google Scholar]\nKeiser J., De Castro M.C., Maltese M.F., Bos R., Tanner M., Singer B.H., Utzinger J. Effect of irrigation and large dams on the burden of malaria on a global and regional scale"},{"doc_id":8543636506476261000,"text":"[\n](/articles/10.1186/s12931-022-02141-3/figures/1)"},{"doc_id":1976914264601072400,"text":"mb.2006.07.045 ](https://doi.org/10.1016%2Fj.jmb.2006.07.045). [ PMC ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 3662476 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3662476). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 16934836 ](https"},{"doc_id":7291930789071169000,"text":"[\n](/articles/10.1186/s40101-022-00317-w/figures/1)"},{"doc_id":6723334600733621000,"text":"Targeting Indirectly Transmitted Infectious Diseases VBDs comprise approximately 17% of the global burden of infectious diseases (Townson et al., 2005). For some diseases (e.g., dengue, Zika, Chagas), chemoprophylaxis and immunoprophylaxis are not prevention and control options, leaving vector control as the primary means of preventing disease transmission. Integrated vector management is defined by the WHO as, “a rational decision-making process to optimize the use of resources for vector control” (WHO, 2012). There are four major categories of"},{"doc_id":5437687107991960000,"text":" of insect metamorphosis\" ](http://www.insecta.ufv.br/Entomologia/ent/disciplina/ban%20160/AULAT/aula8/truman.pdf) (PDF) . _[ Nature ](/wiki/Nature_\\(journal\\) \"Nature \\(journal\\)\") _. **401** (6752): 447–452. [ Bibcode ](/wiki/Bibcode_\\(identifier\\) \"Bibcode \\(identifier\\)\") : [ 1999Natur"},{"doc_id":3122746632115011600,"text":" can also be quite different. For example, outbreaks of Ebola arise following an initial bush animal-to-human transmission event, and subsequent human-to-human transmission is often limited (Feldmann and Geisbert, 2011). In contrast, the four dengue viruses originally emerged from a sylvatic cycle between non-human primates and mosquitoes, and are now sustained by a continuous human-mosquito-human cycle of transmission with outbreaks occurring as a result of infected individuals entering into naïve populations (Vasilakis et al., 2011). Thus, while Ebola outbreak prevention efforts would"},{"doc_id":7451854849775404000,"text":"Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911.\n\nShelomi M, Zeuss D (5 April 2017). \"Bergmann's and Allen's Rules in Native European and Mediterranean Phasmatodea\". Frontiers in Ecology and Evolution. 5."},{"doc_id":2013480756726217000,"text":" via aerobic respiration when the host ATP pool is depleted [158]."},{"doc_id":2130377825308731600,"text":" females ova. The ova develop into eggs that have a\ncovering called the [ chorion ](/wiki/Chorion \"Chorion\"), which forms before\ninternal fertilization. Insects have very diverse mating and reproductive\nstrategies most often resulting in the male depositing a [ spermatophore\n](/wiki/Spermatophore \"Spermatophore\") within the female, which she stores\nuntil she is ready for egg fertilization. After fertilization, and the\nformation of a zygote, and"},{"doc_id":4269165874862062000,"text":"[\n](/articles/10.1186/s12931-022-02141-3/figures/5)"},{"doc_id":7251020604647786000,"text":"[\n](/articles/10.1186/s12931-022-02141-3/figures/3)"},{"doc_id":7023781858110910000,"text":"[\n](/articles/10.1186/s12931-022-02141-3/figures/4)"},{"doc_id":6763275328965136000,"text":" pili formation\" ](https://doi.org/10.1111%2Fj.1365-2958.2008.06459.x). _Molecular Microbiology_. **70** (4). Wiley: 938–952. 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[ PMID ](/wiki/PMID_\\(identifier\\)"},{"doc_id":110430267741693570,"text":"Mechanical vector: A vector (often arthropod) that transmits an infectious organism from one host to another but is not essential to the life cycle of the organism. The house fly is a mechanical vector in the diarrheal disease shigellosis as it carries feces contaminated with the Shigella spp. bacterium to a susceptible person."},{"doc_id":1262575473485306000,"text":"/S2CID_\\(identifier\\) \"S2CID \\(identifier\\)\") [ 42880145 ](https://api.semanticscholar.org/CorpusID:42880145). [ Archived ](https://web.archive.org/web/20211010101112/https://pure.rug.nl/ws/files/6771142/2011MolMicrobiolAjon.pdf) (PDF) from the original on 10 October 2021 . Retrieved 13 December 2019 ."},{"doc_id":1910979479660340500,"text":".16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection: the red cell and malaria\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911. ^ Shelomi M, Zeuss D (5 April 2017). \"Bergmann's and Allen's Rules in Native European and Mediterranean Phas"},{"doc_id":5044981846658660000,"text":"* Raphael Vallat [ ORCID: orcid.org/0000-0003-1779-7653 ](http://orcid.org/0000-0003-1779-7653) 1 ,\n* Sarah E. Berry [ ORCID: orcid.org/0000-0002-5819-5109 ](http://orcid.org/0000-0002-5819-5109) 2 ,\n* Neli Tsereteli 3 ,\n* Joan Capdevila 4 ,"},{"doc_id":4143018406264960000,"text":"ifier\\)\") : [ 10.1016/j.abb.2006.05.001 ](https://doi.org/10.1016%2Fj.abb.2006.05.001). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 16806044 ](https://pubmed.ncbi.nlm.nih.gov/16806044).\n33. ** ^ ** Kamenetsky, M.; Middelhaufe, S.; Bank,"},{"doc_id":8455510814863896000,"text":"e.g., West Nile, Zika). A number of NTDs are also transmitted by biological vectors including lymphatic filariasis (a.k.a. elephantiasis) by mosquitoes. Ticks are biological vectors for many bacterial etiological agents (e.g., Lyme disease and ehrlichiosis), and the parasitic agent causing babesiosis. The infectious agent of the helminthic NTDs, schistosomiasis, and dracunculiasis are transmitted indirectly via intermediate freshwater snail and copepod hosts, respectively."},{"doc_id":7124345023361765000,"text":"[ Microbe Matters ](http://www.nature.com/scitable/blog/microbe-matters)"},{"doc_id":6109593316207363000,"text":"othecae.\nFor males, the reproductive system consists of one or two testes, suspended in the body cavity by tracheae. The testes contain sperm tubes or follicles in a membranous sac. These connect to a duct that leads to the outside. The terminal portion of the duct may be sclerotized to form the intromittent organ, the aedeagus.\nRespiratory\n\nMain article: Respiratory system of insects\nThe tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body,"},{"doc_id":8895542693010381000,"text":"Category\tMethod\tChagas disease\tDengue\tTrypanosomiasis\tJapanese encephalitis\tLeishmaniasis\tLymphatic filariasis\tMalaria\tOnchocerciasis\tSchistosomiasis\tTrachoma\nEnvironmental\tSource reduction\t\t+\t\t+\t\t+\t+\nHabitat manipulation\t\t\t\t\t\t+\t+\t\t+\nIrrigation management and design\t\t\t\t+\t\t\t+\t+\t+\nProximity of livestock\t\t\t\t+\t"},{"doc_id":3438308329834736600,"text":"[\n](https://bio.libretexts.org \"Biology LibreTexts\")"},{"doc_id":8203640509633569000,"text":" ** ^ ** Brusca, R.C.; Brusca, G.J. (1990). [ _Invertebrates_ ](https://archive.org/details/invertebrates0000brus). Sinauer Associates. p. 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[ \"Epigenetics of Ciliates\" ](https://cshperspectives.cshlp.org/content/5/12/a01"},{"doc_id":4392893669107679700,"text":"nlm.nih.gov/pmc/articles/PMC1694820). _Eukaryotic Cell_. **5** (12): 2138–46. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1128/EC.00258-06 ](https://doi.org/10.1128%2FEC.00258-06). [ PMC ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)"},{"doc_id":643743736695998500,"text":"* [ Wikimedia Commons ](https://commons.wikimedia.org/wiki/Category:Blood_agents)"},{"doc_id":9133070788862096000,"text":"General mechanism[edit]\nTransmissible cancers require a specific combination of related circumstances to occur. These conditions involve both the host species and the tumors being transferred. These typically include, low genetic diversity among individuals, effective physical and environmental transport system, effective dose of infective material and ideal micro-environments. The cancers reproduce faster in larger quantities with different means of reproduction tend to be favored for transmission if host conditions are met. Transmissible cancers follow the general pattern of cancer spread, starting with the growth of primary cancer cells at tumor sites followed by invasion of surrounding tissue and subsequent spread throughout the organism. The main hurdles for surviving cells of a successful spread to a new host are histocompatibility barriers. The cancers have to bypass the self recognition system, survive the difference in nutrients and induce the correct response in the new hosts to begin the cycle anew.\nTransmissible cancers behave as true parasites, relying primarily on transport systems like direct contact, environmental transport and vectors, rather than hematogenous and lymphatic carriers to spread between organisms. The amount of shredded cancer cells from initial host has to be high enough to increase survival probability. Direct contact transmissions through sexual or general contact such as in DFTD and CVTD ensures a higher potential for transmission. Population factors also play an important role. A dense population of available and uninfected potential hosts is ideal for the tumors given the complexity and difficulty of the overall process, hence its virulence and potency must be adequately controlled."},{"doc_id":5718975602497375000,"text":"* [ ](http://www.nature.com/scitable/blog/creature-cast)"},{"doc_id":2448606410365659600,"text":"doi.org/10.1073%2Fpnas.0502893102). [ PMC ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 1093311 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1093311). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 15888555 ](https://pubmed.ncbi.nlm.nih.gov"},{"doc_id":2807127579404374500,"text":" contaminated food or water is ingested. It can also occur when aerosols containing viruses are inhaled or by insect vectors such as when infected mosquitoes penetrate the skin of a host. Most types of viruses are restricted to just one or two of these mechanisms and they are referred to as \"respiratory viruses\" or \"enteric viruses\" and so forth. The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i.e., those not immune), the quality of healthcare and the weather.\nEpidemiology is used to break the"},{"doc_id":3262831976891495000,"text":"tera\". In Naumann, I. D.; Carne, P. B.; et al. (eds.). _The Insects of Australia. Volume 1_ (2 ed.). Melbourne University Press. pp. 405–409.\n15. ** ^ ** Stanislav N Gorb. \"Biological attachment devices: exploring nature's diversity for biomimetics Phil. _Trans. R. Soc. 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(2022).\\n[ https://www.who.int/news-room/fact-sheets/detail/malaria\\n](https://www.who.int/news-room/fact-sheets/detail/malaria)\"\n },\n {\n \"doc_id\": 4924544718779407000,\n \"text\": \"These issues come into focus when you consider the devastating malaria\\nepidemic that kills one to three million people worldwide every year. The\\npathogens that cause malaria are protozoan parasites from the genus\\n_Plasmodium_ that are transmitted through the bite of mosquitoes of the genus\\n_Anopheles_. Both of these animal genera contain hundreds of species,\\nalthough only a few are involved in transmitting malaria in humans. Recent\\ngenetic studies of the symbiotic bacteria in the midgut of the _Anopheles\\nstephensi_ mosquitoes have yielded\"\n },\n {\n \"doc_id\": 7414764089986002000,\n \"text\": \"### Plasmodium\"\n },\n {\n \"doc_id\": 482585183638768960,\n \"text\": \"Box 3\\nHosts, vectors, and vehicles involved in the life cycle of infectious agents transmitted indirectly\\nDefinitive host: A host in which a parasite reproduces sexually. Humans are definitive hosts for roundworms. By strict definition, mosquitoes are the definitive host of malaria as they are the organism in which sexual reproduction of the agent protozoa, Plasmodium spp., occurs.\"\n },\n {\n \"doc_id\": 5861845573567613000,\n \"text\": \" Evans D.B., Jha P., Mills A., Musgrove P., editors. Disease Control Priorities in Developing Countries. The International Bank for Reconstruction and Development/The World Bank Group; Washington, DC: 2006. [Google Scholar]\\nLanghorne J., Ndungu F.M., Sponaas A.M., Marsh K. 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Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.\\nTransmission[edit]\\n\\nMain article: Transmission (medicine)\\nA southern house mosquito (Culex quinquefasciatus) is a vector that transmits the pathogens that cause West Nile fever and avian malaria among others.\\nFor infecting organisms to survive and repeat the infection cycle in other hosts, they\"\n },\n {\n \"doc_id\": 1320637855125465900,\n \"text\": \"thropods\\\". _Insect Molecular Biology_. **19** (Suppl 1): 165–180. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1111/j.1365-2583.2009.00948.x ](https://doi.org/10.1111%2Fj.1365-2583.2009.00948.x). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [\"\n },\n {\n \"doc_id\": 1362592987580663000,\n \"text\": \"Biological transmission occurs when multiplication and/or development of a pathogenic agent within a vector (e.g., biological vector or intermediate host) is required for the agent to become infectious to humans. The time that is necessary for these events to occur is known as the extrinsic incubation period; in contrast to the intrinsic incubation period which is the time required for an exposed human host to become infectious. Indirect transmission by mosquito vectors is the primary mode of transmission of a large number of viruses (arthropod-borne viruses or arboviruses) of public health concern (\"\n },\n {\n \"doc_id\": 6707359235402285000,\n \"text\": \"Malaria\\nJapanese encephalitis\\nLyme\\nMalaria\\nHendra\\tSteinmann et al. (2006)\"\n },\n {\n \"doc_id\": 6899342711634635000,\n \"text\": \" PMID 14634648. 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PMID 21427751. ^ Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\"\n },\n {\n \"doc_id\": 4883073565850421000,\n \"text\": \"ifier\\\\)\\\") : [ 10.1042/BST0311095 ](https://doi.org/10.1042%2FBST0311095). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 14641005 ](https://pubmed.ncbi.nlm.nih.gov/14641005).\\n21. ^ _**a** _ _**b** _ _**c** _ Lodish, H.; Berk, A.; Matsudair\"\n },\n {\n \"doc_id\": 2429127711217836500,\n \"text\": \" mating.\\nSource: Modification of table from WHO, 2012. Handbook for Integrated Vector Management. WHO Press, France.\\nDiarrheal diseases primarily result from oral contact with water, food, or other vehicles contaminated with pathogenic agents originating from human or animal feces. Most (∼88%) of diarrhea-associated deaths are attributable to unsafe drinking water, inadequate sanitation, and insufficient hygiene (Black et al., 2003; Prüss-Üstün et al., 2008). Interruption of fecal–oral transmission through provision of safe water and adequate sanitation, and promotion\"\n },\n {\n \"doc_id\": 8184988300445025000,\n \"text\": \"(https://pure.rug.nl/ws/files/6771142/2011MolMicrobiolAjon.pdf) (PDF) . _Molecular Microbiology_. **82** (4). Wiley: 807–817. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1111/j.1365-2958.2011.07861.x ](https://doi.org/10.1111%2Fj.1365-2958\"\n },\n {\n \"doc_id\": 6686516171026289000,\n \"text\": \"* [ ](http://www.nature.com/scitable/blog/viruses101)\"\n },\n {\n \"doc_id\": 7056037267678778000,\n \"text\": \" promising results: _Enterobacter\\nagglomerans_ bacteria were genetically engineered to display two anti-\\n_Plasmodium_ effector molecules that kill the parasite before it is\\ntransmitted to humans (Riehle _et al._, 2007). Now consider the genetic and\\nphysiological differences between the wild-type and genetically modified\\n_Anopheles stephensi_ mosquitoes: they are still the same species by all major\\nstandards of species definition, yet what a difference it would make for\\nhumankind if the _Plasmodium-_ resistant\"\n },\n {\n \"doc_id\": 2052100685612085200,\n \"text\": \"Mechanical transmission does not require pathogen multiplication or development within a living organism. It occurs when an infectious agent is physically transferred by a live entity (mechanical vector) or inanimate object (vehicle) to a susceptible host. Classic examples of diseases spread by mechanical vector transmission are shigellosis (transmission of Shigella spp. on the appendages of flies) and plague (transmission of Yersinia pestis by fleas). Many diarrheal diseases are transmitted by the fecal-oral route with food and water often acting as vehicles (Figure 7 ).\"\n },\n {\n \"doc_id\": 7456314755400268000,\n \"text\": \" example, many hemipteran bugs have piercing and sucking mouthparts, adapted for feeding on plant sap, while species in groups such as fleas, lice, and mosquitoes are hematophagous, feeding on the blood of animals.\\nA parasitoid wasp ovipositing into an aphid\\n\\n\\n\\nPlant parasite or micropredator: a coreid bug sucking plant sap\\n\\n\\n\\nHuman head-lice are directly transmitted obligate ectoparasites.\"\n },\n {\n \"doc_id\": 2111678933010816500,\n \"text\": \"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H\"\n },\n {\n \"doc_id\": 7906178860567034000,\n \"text\": \"* [ Reference & Cite ](https://bio.libretexts.org/?refmodal)\"\n },\n {\n \"doc_id\": 2820692476841372700,\n \"text\": \" 20167026 ](https://pubmed.ncbi.nlm.nih.gov/20167026). [ S2CID ](/wiki/S2CID_\\\\(identifier\\\\) \\\"S2CID \\\\(identifier\\\\)\\\") [ 11617147 ](https://api.semanticscholar.org/CorpusID:11617147).\\n30. ** ^ ** Bernstein, H.; Hopf, F. A.; Michod, R. E. (1987). \\\"The molecular basis of the evolution of sex\\\". _\"\n },\n {\n \"doc_id\": 4050011035651363000,\n \"text\": \"[\\n](/articles/10.1186/s12931-022-02141-3/figures/2)\"\n },\n {\n \"doc_id\": 1663613504998063900,\n \"text\": \"372978). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 7968924 ](https://pubmed.ncbi.nlm.nih.gov/7968924).\\n50. ** ^ ** Lodé, T. (2012). [ \\\"Have Sex or Not? Lessons from Bacteria\\\" ](https://doi.org/10.1159%2F000342879). _Sexual Development_. **6** (6): 325–328. [ doi\"\n },\n {\n \"doc_id\": 622762634218896600,\n \"text\": \"[ Viruses101 ](http://www.nature.com/scitable/blog/viruses101)\"\n },\n {\n \"doc_id\": 1055589053584834800,\n \"text\": \"Kebede A., Mccann J.C., Kiszewski A.E., Ye-Ebiyo Y. New evidence of the effects of agro-ecologic change on malaria transmission. Am. J. Trop. Med. Hyg. 2005;73:676–680. [PubMed] [Google Scholar]\\nKeiser J., De Castro M.C., Maltese M.F., Bos R., Tanner M., Singer B.H., Utzinger J. Effect of irrigation and large dams on the burden of malaria on a global and regional scale\"\n },\n {\n \"doc_id\": 8543636506476261000,\n \"text\": \"[\\n](/articles/10.1186/s12931-022-02141-3/figures/1)\"\n },\n {\n \"doc_id\": 1976914264601072400,\n \"text\": \"mb.2006.07.045 ](https://doi.org/10.1016%2Fj.jmb.2006.07.045). [ PMC ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 3662476 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3662476). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 16934836 ](https\"\n },\n {\n \"doc_id\": 7291930789071169000,\n \"text\": \"[\\n](/articles/10.1186/s40101-022-00317-w/figures/1)\"\n },\n {\n \"doc_id\": 6723334600733621000,\n \"text\": \"Targeting Indirectly Transmitted Infectious Diseases VBDs comprise approximately 17% of the global burden of infectious diseases (Townson et al., 2005). For some diseases (e.g., dengue, Zika, Chagas), chemoprophylaxis and immunoprophylaxis are not prevention and control options, leaving vector control as the primary means of preventing disease transmission. Integrated vector management is defined by the WHO as, “a rational decision-making process to optimize the use of resources for vector control” (WHO, 2012). There are four major categories of\"\n },\n {\n \"doc_id\": 5437687107991960000,\n \"text\": \" of insect metamorphosis\\\" ](http://www.insecta.ufv.br/Entomologia/ent/disciplina/ban%20160/AULAT/aula8/truman.pdf) (PDF) . _[ Nature ](/wiki/Nature_\\\\(journal\\\\) \\\"Nature \\\\(journal\\\\)\\\") _. **401** (6752): 447–452. [ Bibcode ](/wiki/Bibcode_\\\\(identifier\\\\) \\\"Bibcode \\\\(identifier\\\\)\\\") : [ 1999Natur\"\n },\n {\n \"doc_id\": 3122746632115011600,\n \"text\": \" can also be quite different. For example, outbreaks of Ebola arise following an initial bush animal-to-human transmission event, and subsequent human-to-human transmission is often limited (Feldmann and Geisbert, 2011). In contrast, the four dengue viruses originally emerged from a sylvatic cycle between non-human primates and mosquitoes, and are now sustained by a continuous human-mosquito-human cycle of transmission with outbreaks occurring as a result of infected individuals entering into naïve populations (Vasilakis et al., 2011). Thus, while Ebola outbreak prevention efforts would\"\n },\n {\n \"doc_id\": 7451854849775404000,\n \"text\": \"Weatherall DJ (May 2008). \\\"Genetic variation and susceptibility to infection: the red cell and malaria\\\". British Journal of Haematology. 141 (3): 276–286. doi : 10.1111/j.1365-2141.2008.07085.x. PMID 18410566. S2CID 28191911.\\n\\nShelomi M, Zeuss D (5 April 2017). \\\"Bergmann's and Allen's Rules in Native European and Mediterranean Phasmatodea\\\". Frontiers in Ecology and Evolution. 5.\"\n },\n {\n \"doc_id\": 2013480756726217000,\n \"text\": \" via aerobic respiration when the host ATP pool is depleted [158].\"\n },\n {\n \"doc_id\": 2130377825308731600,\n \"text\": \" females ova. The ova develop into eggs that have a\\ncovering called the [ chorion ](/wiki/Chorion \\\"Chorion\\\"), which forms before\\ninternal fertilization. Insects have very diverse mating and reproductive\\nstrategies most often resulting in the male depositing a [ spermatophore\\n](/wiki/Spermatophore \\\"Spermatophore\\\") within the female, which she stores\\nuntil she is ready for egg fertilization. After fertilization, and the\\nformation of a zygote, and\"\n },\n {\n \"doc_id\": 4269165874862062000,\n \"text\": \"[\\n](/articles/10.1186/s12931-022-02141-3/figures/5)\"\n },\n {\n \"doc_id\": 7251020604647786000,\n \"text\": \"[\\n](/articles/10.1186/s12931-022-02141-3/figures/3)\"\n },\n {\n \"doc_id\": 7023781858110910000,\n \"text\": \"[\\n](/articles/10.1186/s12931-022-02141-3/figures/4)\"\n },\n {\n \"doc_id\": 6763275328965136000,\n \"text\": \" pili formation\\\" ](https://doi.org/10.1111%2Fj.1365-2958.2008.06459.x). _Molecular Microbiology_. **70** (4). Wiley: 938–952. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1111/j.1365-2958.2008.06459.x ](https://doi.org/10.1111%2Fj.1365-2958\"\n },\n {\n \"doc_id\": 712000520560809500,\n \"text\": \"identifier\\\\)\\\") : [ 10.1016/j.cub.2006.08.024 ](https://doi.org/10.1016%2Fj.cub.2006.08.024). 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[ PMID ](/wiki/PMID_\\\\(identifier\\\\)\"\n },\n {\n \"doc_id\": 110430267741693570,\n \"text\": \"Mechanical vector: A vector (often arthropod) that transmits an infectious organism from one host to another but is not essential to the life cycle of the organism. The house fly is a mechanical vector in the diarrheal disease shigellosis as it carries feces contaminated with the Shigella spp. bacterium to a susceptible person.\"\n },\n {\n \"doc_id\": 1262575473485306000,\n \"text\": \"/S2CID_\\\\(identifier\\\\) \\\"S2CID \\\\(identifier\\\\)\\\") [ 42880145 ](https://api.semanticscholar.org/CorpusID:42880145). [ Archived ](https://web.archive.org/web/20211010101112/https://pure.rug.nl/ws/files/6771142/2011MolMicrobiolAjon.pdf) (PDF) from the original on 10 October 2021 . Retrieved 13 December 2019 .\"\n },\n {\n \"doc_id\": 1910979479660340500,\n \"text\": \".16. PMC 3182497. 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The infectious agent of the helminthic NTDs, schistosomiasis, and dracunculiasis are transmitted indirectly via intermediate freshwater snail and copepod hosts, respectively.\"\n },\n {\n \"doc_id\": 7124345023361765000,\n \"text\": \"[ Microbe Matters ](http://www.nature.com/scitable/blog/microbe-matters)\"\n },\n {\n \"doc_id\": 6109593316207363000,\n \"text\": \"othecae.\\nFor males, the reproductive system consists of one or two testes, suspended in the body cavity by tracheae. The testes contain sperm tubes or follicles in a membranous sac. These connect to a duct that leads to the outside. The terminal portion of the duct may be sclerotized to form the intromittent organ, the aedeagus.\\nRespiratory\\n\\nMain article: Respiratory system of insects\\nThe tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body,\"\n },\n {\n \"doc_id\": 8895542693010381000,\n \"text\": \"Category\\tMethod\\tChagas disease\\tDengue\\tTrypanosomiasis\\tJapanese encephalitis\\tLeishmaniasis\\tLymphatic filariasis\\tMalaria\\tOnchocerciasis\\tSchistosomiasis\\tTrachoma\\nEnvironmental\\tSource reduction\\t\\t+\\t\\t+\\t\\t+\\t+\\nHabitat manipulation\\t\\t\\t\\t\\t\\t+\\t+\\t\\t+\\nIrrigation management and design\\t\\t\\t\\t+\\t\\t\\t+\\t+\\t+\\nProximity of livestock\\t\\t\\t\\t+\\t\"\n },\n {\n \"doc_id\": 3438308329834736600,\n \"text\": \"[\\n](https://bio.libretexts.org \\\"Biology LibreTexts\\\")\"\n },\n {\n \"doc_id\": 8203640509633569000,\n \"text\": \" ** ^ ** Brusca, R.C.; Brusca, G.J. (1990). 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[ 16.4 The Cytoskeleton ](https://slcc.pressbooks.pub/collegebiology1/chapter/the-cytoskeleton/)\"\n },\n {\n \"doc_id\": 8649311838822557000,\n \"text\": \"[ Biology ](/r/explainlikeimfive/?f=flair_name%3A%22Biology%22)\"\n },\n {\n \"doc_id\": 3071740065489387500,\n \"text\": \"1. ** ^ ** John Maynard Smith & Eörz Szathmáry, The Major Transitions in Evolution, [ W. H. Freeman and Company ](/wiki/W._H._Freeman_and_Company \\\"W. H. Freeman and Company\\\"), 1995, p 149\\n2. ^ _**a** _ _**b** _ Chalker, Douglas (2013). [ \\\"Epigenetics of Ciliates\\\" ](https://cshperspectives.cshlp.org/content/5/12/a01\"\n },\n {\n \"doc_id\": 4392893669107679700,\n \"text\": \"nlm.nih.gov/pmc/articles/PMC1694820). _Eukaryotic Cell_. **5** (12): 2138–46. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1128/EC.00258-06 ](https://doi.org/10.1128%2FEC.00258-06). [ PMC ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\"\n },\n {\n \"doc_id\": 643743736695998500,\n \"text\": \"* [ Wikimedia Commons ](https://commons.wikimedia.org/wiki/Category:Blood_agents)\"\n },\n {\n \"doc_id\": 9133070788862096000,\n \"text\": \"General mechanism[edit]\\nTransmissible cancers require a specific combination of related circumstances to occur. These conditions involve both the host species and the tumors being transferred. These typically include, low genetic diversity among individuals, effective physical and environmental transport system, effective dose of infective material and ideal micro-environments. The cancers reproduce faster in larger quantities with different means of reproduction tend to be favored for transmission if host conditions are met. Transmissible cancers follow the general pattern of cancer spread, starting with the growth of primary cancer cells at tumor sites followed by invasion of surrounding tissue and subsequent spread throughout the organism. The main hurdles for surviving cells of a successful spread to a new host are histocompatibility barriers. The cancers have to bypass the self recognition system, survive the difference in nutrients and induce the correct response in the new hosts to begin the cycle anew.\\nTransmissible cancers behave as true parasites, relying primarily on transport systems like direct contact, environmental transport and vectors, rather than hematogenous and lymphatic carriers to spread between organisms. The amount of shredded cancer cells from initial host has to be high enough to increase survival probability. Direct contact transmissions through sexual or general contact such as in DFTD and CVTD ensures a higher potential for transmission. Population factors also play an important role. A dense population of available and uninfected potential hosts is ideal for the tumors given the complexity and difficulty of the overall process, hence its virulence and potency must be adequately controlled.\"\n },\n {\n \"doc_id\": 5718975602497375000,\n \"text\": \"* [ ](http://www.nature.com/scitable/blog/creature-cast)\"\n },\n {\n \"doc_id\": 2448606410365659600,\n \"text\": \"doi.org/10.1073%2Fpnas.0502893102). [ PMC ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 1093311 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1093311). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 15888555 ](https://pubmed.ncbi.nlm.nih.gov\"\n },\n {\n \"doc_id\": 2807127579404374500,\n \"text\": \" contaminated food or water is ingested. It can also occur when aerosols containing viruses are inhaled or by insect vectors such as when infected mosquitoes penetrate the skin of a host. Most types of viruses are restricted to just one or two of these mechanisms and they are referred to as \\\"respiratory viruses\\\" or \\\"enteric viruses\\\" and so forth. The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i.e., those not immune), the quality of healthcare and the weather.\\nEpidemiology is used to break the\"\n },\n {\n \"doc_id\": 3262831976891495000,\n \"text\": \"tera\\\". In Naumann, I. D.; Carne, P. B.; et al. (eds.). _The Insects of Australia. Volume 1_ (2 ed.). Melbourne University Press. pp. 405–409.\\n15. ** ^ ** Stanislav N Gorb. \\\"Biological attachment devices: exploring nature's diversity for biomimetics Phil. _Trans. R. Soc. A_ 2008; 366(1870): 1557-1574 doi:10.1098/rsta.2007.2172 1471-2962\\n16.\"\n },\n {\n \"doc_id\": 4918094197942873000,\n \"text\": \"\"\n },\n {\n \"doc_id\": 4963878315018557000,\n \"text\": \"Ljk1LS4wOS0xLjQxLS4yNy0uNDYtLjE5LS44Ni0uNDEtMS4yLS42OHoiIGZpbGw9IiM1MzUzNTMiLz48L2c+PC9zdmc+)\\n](https://crossmark.crossref.org/dialog/?doi=10.1186/s40101-022-00317-w)\"\n },\n {\n \"doc_id\": 5477165122432237000,\n \"text\": \"[\\n](/articles/s41467-018-05547-0/figures/3)\"\n },\n {\n \"doc_id\": 403550371367205500,\n \"text\": \"[\\n](/articles/s41467-018-05547-0/figures/1)\"\n }\n]","isConversational":false}}},{"rowIdx":93,"cells":{"query":{"kind":"string","value":"I'm reading about the evolution of senescence and came across the paper by George C. Williams from 1957 titled \"Pleiotropy, Natural Selection, and the Evolution of Senescence.\" Williams discusses how pleiotropic genes, which have multiple effects on different traits, can influence the aging process through natural selection. Could someone elaborate on how pleiotropy contributes to the evolution of senescence and provide examples of pleiotropic genes that have been identified in this context? Additionally, how does this theory integrate with modern understanding of cancer cell biology and angiogenesis, particularly in the context of internal medicine?"},"positive_passages":{"kind":"list like","value":[{"doc_id":7033329867091455000,"text":"Adaptivity and senescence[edit]\nAn antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in their old age. An example of this is testosterone levels in male humans. Higher levels of this hormone lead to increased fitness in early life, while causing decreased fitness in later life due to a higher risk for prostate cancer. This is an example of antagonistic pleiotropy being an explanation for senescence. Senescence is the act of ageing in individuals; it's the failure over time of the individual's life processes by natural causes. Williams's theory has been the motivation for many of the experimental studies on the reasons for aging in the last 25 years. However, there is more than one theory out there for aging. The competing model to explain senescence is Medawar's \"mutation accumulation\" hypothesis, saying that \"over evolutionary time, late-acting mutations will accumulate at a much faster rate than early-acting mutation. These late-acting mutations will thus lead to declining viability and/or fertility as an organism ages.\" Medawar's theory is based around the older concept of selection shadow that had been discussed throughout the early 1900s and led to Medawar's theory after discussions with J. B. S. Haldane in the 1940s."},{"doc_id":8383252946560158000,"text":"The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls more than one phenotypic trait in an organism. A gene is considered to possess antagonistic pleiotropy if it controls more than one trait, where at least one of these traits is beneficial to the organism's fitness early on in life and at least one is detrimental to the organism's fitness later on due to a decline in the force of natural selection. The theme of G. C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs."},{"doc_id":3233800475697562600,"text":"Potential Examples[edit]\nDNA repair[edit]\nDNA Damage Theory of Aging[edit]\nA prominent explanation for aging at the molecular level is the DNA damage theory of aging. It has been proposed that genetic elements that regulate DNA repair in somatic cells may constitute an important example of age-dependent pleiotropic \"genes\". As pointed out by Vijg, genome repair and maintenance is beneficial early in life by swiftly eliminating DNA damage or damaged cells. However, studies of DNA repair in the brain and in muscle indicate that the transition from mitotic cell division to the post-mitotic condition that occurs early in life is accompanied by a reduction in DNA repair. The reduced expression of DNA repair is presumably part of an evolutionary adaptation for diverting the resources of the cell that were previously used for DNA repair, as well as for replication and cell division, to more essential neuronal and muscular functions.\nThe harmful effect of this genetically controlled reduction in expression is to allow increased accumulation of DNA damage. Reduced DNA repair causes increased impairment of transcription and progressive loss of cell and tissue function. However, these harmful effects of DNA damage are cumulative and most severe in chronologically older individuals whose numbers diminish with time (by causes of death that can be independent of senescence). As a consequence, the beneficial effects of the genetic elements that control the reduction of DNA repair early in life would predominate. Thus regulatory genetic elements that reduce expression of DNA repair genes in post-mitotic cells appear to be important examples of the postulated pleiotropic \"genes\" that are beneficial in youth but deleterious at an older age.\nTelomere Theory[edit]\nAnother example related to aging is the Telomere theory. Telomere theory proposes that telomeres shorten with repeated cell division which attribute to cell senescence and tissue damage. The end replication problem explains the mechanism behind the inability of DNA polymerase to commence the RNA primer to perform its function in completing the lagging strand due to the shortening of DNA. Telomere shortening is common in somatic cells. However, germ line and stem cells prevent the end replication problem with the help of telomerase. Telomerase elongates the 3’ end that is then formed into a t-loop to prevent the cell from entering the G0 phase and cell senescence.\nInflammation and damage to tissue are the underlying problems due to increased senescent cells. In several studies shortened telomeres have been associated with age related sarcopenia, atherosclerotic cardiovascular disease, and cancer. However, there is still the question whether telomere length causes these diseases or if the diseases cause shortened telomeres. Hence, the shortening of telomeres complies with antagonistic pleiotropy theory. The trade-off exists as the cell benefits from telomerase which prevents permanent growth arrest but telomere shortening is associated with functional loss.\nFree Radical Theory[edit]\nAnother example related to aging is the Free Radical theory. Free Radical Theory suggests that the free radicals, which are being produced by aerobic respiration, are causing oxidative stress to be put on the body. This oxidative stress will result in aging and lead to death. Oxygen centered radicals are very reactive and can cause the accumulation of damage on lipids, nucleic acids as well as proteins within the body. This accumulation of damage on the biological molecules changes the framework and leads to a reduction in the molecules' activity levels. Lipid peroxides accumulate in the membrane phospholipids, which in turn diminishes the mitochondrial membrane's effectiveness as a barrier. The process of DNA transcription and translation also acquires oxidative damage. The result is alterations in the base pairings of the DNA sequence. Research has found that DNA mutations from free radical damage are highly uncommon but would still lead to the build up of damaged proteins as well as decreased biological activity.\nHowever, a large body of research has shown that antioxidants (compounds that defuse free radicals) do not slow aging or age-related diseases, putting the popular free radical theory into doubt."},{"doc_id":8660851524417328000,"text":"Role in disease[edit]\nThe survival of many serious genetic disorders in our long evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are defined by the existence of deleterious alleles, then natural selection acting over evolutionary time would result in a lower frequency of mutations than are currently observed. In a recent article, Carter and Nguyen identify several genetic disorders, arguing that far from being a rare phenomenon, antagonistic pleiotropy might be a fundamental mechanism for the survival of these non-optimal alleles.\nIn one of these studies, 99 individuals with Laron syndrome (a rare form of dwarfism) were monitored alongside their non-dwarf kin for a period of ten years. Patients with Laron syndrome possess one of three genotypes for the growth hormone receptor gene (GHR). Most patients have an A->G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin. This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.\nAnother instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. Incidentally, the high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.\nAdditionally, it was found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. Yet, it was hypothesized that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.\nSickle cell anemia, Beta-thalassemia, and cystic fibrosis are some other examples of the role antagonistic pleiotropy may play in genetic disorders."},{"doc_id":5835331850275548000,"text":"Mechanisms[edit]\nThe IGF-1 pathway, which represses FOXO, thus preventing gene expression of longevity-inducing proteins\nGrowth and somatic maintenance[edit]\nMain article: Mechanistic target of rapamycin § Aging\nMuch research has been done on the antagonistic effects of increased growth on lifespan. Specifically, the hormone insulin-like growth factor 1 (IGF-1), binds to a cell receptor, leading to a phosphorylation cascade. This cascade results in kinases phosphorylating forkhead transcription factor (FOXO), deactivating it. Deactivation of FOXO results in an inability to express genes involved in responding to oxidative stress response, such as antioxidants, chaperones, and heat-shock proteins.\nAdditionally, uptake of IGF-1 stimulates the mTOR pathway, which activates protein synthesis (and therefore growth) through upregulation of the translation-promoting S6K1, and also inhibits autophagy, a process necessary for recycling of damaged cellular products. Decline of autophagy causes neurodegeneration, protein aggregation and premature aging. Lastly, studies have also indicated that the mTOR pathway also alters immune responses and stimulates cyclin-dependent kinase (CDK) inhibitors such as p16 and p21. This leads to alteration of the stem-cell niche and results in stem cell exhaustion, another theorized mechanism of aging.\nReproduction and somatic maintenance[edit]\nWhile reproduction inhibits lifespan with regard to multicellular organisms, the precise mechanism responsible for this effect remains unclear. Although many models do illustrate an inverse relationship, and the theory makes sense from an evolutionary perspective, the cellular mechanisms have yet to be explored. However, with regards to cellular replication, the progressive shortening of telomeres is a mechanism which limits the amount of generations of a single cell may undergo. Furthermore, in unicellular organisms like Saccharomyces cerevisiae, the formation of extrachromosomal rDNA circles (ERCs) in mother cells (but not daughter cells) upon every subsequent division is an identifiable type of DNA damage that is associated with replication. These ERCs accumulate over time and eventually trigger replicative senescence and death of the mother cell."},{"doc_id":2404171571940034000,"text":"Mechanism of action[edit]\n(a) The survival rate within a population decreases with age, while the reproduction rate remains constant. (b) The reproduction probability peaks early in life, at sexual maturity, and then steadily decreases as an individual ages, with the remaining share of the population decreasing with age as they enter the selection shadow.\nAmongst almost all populations, the likelihood that an individual will reproduce is related directly to their age. Starting at 0 at birth, the probability increases to its maximum in young adulthood once sexual maturity has been reached, before gradually decreasing with age. This decrease is caused by the increasing likelihood of death due to external pressures such as predation or illness, as well as the internal pressures inherent to organisms that experience senescence. In such cases deleterious mutations which are expressed early on are strongly selected against due to their major impact on the number of offspring produced by that individual. Mutations that present later in life, by contrast, are relatively unaffected by selective pressure, as their carriers have already passed on their genes, assuming they survive long enough for the mutation to be expressed at all. The result, as predicted by Medawar, is that deleterious late-life mutations will accumulate and result in the evolution of aging as it is known colloquially. This concept is portrayed graphically by Medawar through the concept of a \"selection shadow\". The shaded region represents the 'shadow' of time during which selective pressure has no effect. Mutations that are expressed within this selection shadow will remain as long as reproductive probability within that age range remains low."},{"doc_id":3685274842462513000,"text":"Benefits and costs[edit]\nAntagonistic pleiotropy has several negative consequences. It results in delayed adaptation, an altered path of evolution, and reduced adaptation of other traits. In addition, the overall benefit of alleles is cut down significantly (by about half) by pleiotropy. Still, antagonistic pleiotropy has some evolutionary benefits. In fact, the conservation of genes is directly related to the pleiotropic character of an organism. This implies that genes that control for multiple traits, even if the traits have different implications for the organism's fitness, have more staying power in an evolutionary context."},{"doc_id":221831125176784060,"text":"As a constraint on perfection[edit]\nAntagonistic pleiotropy is one of the several reasons evolutionary biologists give for organisms never being able to reach perfection through natural selection. Antagonistically pleiotropic genes are the explanation for fitness trade-offs. This means that genes that are pleiotropic control for some beneficial traits and some detrimental traits; thus, if they happen to persist through natural selection, this will prevent organisms from reaching perfection because if they possess the benefits of the gene, they must also possess the imperfections or faults. An example of this would be female rodents that live in a nest with other females and may end up feeding young that are not theirs due to their intense parental drive. This strong parental drive will be selected for, but the organisms will still make the mistake of feeding young that are not theirs and misallocating their resources."},{"doc_id":1633464797414410200,"text":" traits. Some traits that increase fitness early in life may also have negative effects later in life. But, because many more individuals are alive at young ages than at old ages, even small positive effects early can be strongly selected for, and large negative effects later may be very weakly selected against. Williams suggested the following example: Perhaps a gene codes for calcium deposition in bones, which promotes juvenile survival and will therefore be favored by natural selection; however, this same gene promotes calcium deposition in the arteries, causing negative atherosclerotic effects in old age. Thus, harmful biological changes in old age may"}],"string":"[\n {\n \"doc_id\": 7033329867091455000,\n \"text\": \"Adaptivity and senescence[edit]\\nAn antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in their old age. An example of this is testosterone levels in male humans. Higher levels of this hormone lead to increased fitness in early life, while causing decreased fitness in later life due to a higher risk for prostate cancer. This is an example of antagonistic pleiotropy being an explanation for senescence. Senescence is the act of ageing in individuals; it's the failure over time of the individual's life processes by natural causes. Williams's theory has been the motivation for many of the experimental studies on the reasons for aging in the last 25 years. However, there is more than one theory out there for aging. The competing model to explain senescence is Medawar's \\\"mutation accumulation\\\" hypothesis, saying that \\\"over evolutionary time, late-acting mutations will accumulate at a much faster rate than early-acting mutation. These late-acting mutations will thus lead to declining viability and/or fertility as an organism ages.\\\" Medawar's theory is based around the older concept of selection shadow that had been discussed throughout the early 1900s and led to Medawar's theory after discussions with J. B. S. Haldane in the 1940s.\"\n },\n {\n \"doc_id\": 8383252946560158000,\n \"text\": \"The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls more than one phenotypic trait in an organism. A gene is considered to possess antagonistic pleiotropy if it controls more than one trait, where at least one of these traits is beneficial to the organism's fitness early on in life and at least one is detrimental to the organism's fitness later on due to a decline in the force of natural selection. The theme of G. C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs.\"\n },\n {\n \"doc_id\": 3233800475697562600,\n \"text\": \"Potential Examples[edit]\\nDNA repair[edit]\\nDNA Damage Theory of Aging[edit]\\nA prominent explanation for aging at the molecular level is the DNA damage theory of aging. It has been proposed that genetic elements that regulate DNA repair in somatic cells may constitute an important example of age-dependent pleiotropic \\\"genes\\\". As pointed out by Vijg, genome repair and maintenance is beneficial early in life by swiftly eliminating DNA damage or damaged cells. However, studies of DNA repair in the brain and in muscle indicate that the transition from mitotic cell division to the post-mitotic condition that occurs early in life is accompanied by a reduction in DNA repair. The reduced expression of DNA repair is presumably part of an evolutionary adaptation for diverting the resources of the cell that were previously used for DNA repair, as well as for replication and cell division, to more essential neuronal and muscular functions.\\nThe harmful effect of this genetically controlled reduction in expression is to allow increased accumulation of DNA damage. Reduced DNA repair causes increased impairment of transcription and progressive loss of cell and tissue function. However, these harmful effects of DNA damage are cumulative and most severe in chronologically older individuals whose numbers diminish with time (by causes of death that can be independent of senescence). As a consequence, the beneficial effects of the genetic elements that control the reduction of DNA repair early in life would predominate. Thus regulatory genetic elements that reduce expression of DNA repair genes in post-mitotic cells appear to be important examples of the postulated pleiotropic \\\"genes\\\" that are beneficial in youth but deleterious at an older age.\\nTelomere Theory[edit]\\nAnother example related to aging is the Telomere theory. Telomere theory proposes that telomeres shorten with repeated cell division which attribute to cell senescence and tissue damage. The end replication problem explains the mechanism behind the inability of DNA polymerase to commence the RNA primer to perform its function in completing the lagging strand due to the shortening of DNA. Telomere shortening is common in somatic cells. However, germ line and stem cells prevent the end replication problem with the help of telomerase. Telomerase elongates the 3’ end that is then formed into a t-loop to prevent the cell from entering the G0 phase and cell senescence.\\nInflammation and damage to tissue are the underlying problems due to increased senescent cells. In several studies shortened telomeres have been associated with age related sarcopenia, atherosclerotic cardiovascular disease, and cancer. However, there is still the question whether telomere length causes these diseases or if the diseases cause shortened telomeres. Hence, the shortening of telomeres complies with antagonistic pleiotropy theory. The trade-off exists as the cell benefits from telomerase which prevents permanent growth arrest but telomere shortening is associated with functional loss.\\nFree Radical Theory[edit]\\nAnother example related to aging is the Free Radical theory. Free Radical Theory suggests that the free radicals, which are being produced by aerobic respiration, are causing oxidative stress to be put on the body. This oxidative stress will result in aging and lead to death. Oxygen centered radicals are very reactive and can cause the accumulation of damage on lipids, nucleic acids as well as proteins within the body. This accumulation of damage on the biological molecules changes the framework and leads to a reduction in the molecules' activity levels. Lipid peroxides accumulate in the membrane phospholipids, which in turn diminishes the mitochondrial membrane's effectiveness as a barrier. The process of DNA transcription and translation also acquires oxidative damage. The result is alterations in the base pairings of the DNA sequence. Research has found that DNA mutations from free radical damage are highly uncommon but would still lead to the build up of damaged proteins as well as decreased biological activity.\\nHowever, a large body of research has shown that antioxidants (compounds that defuse free radicals) do not slow aging or age-related diseases, putting the popular free radical theory into doubt.\"\n },\n {\n \"doc_id\": 8660851524417328000,\n \"text\": \"Role in disease[edit]\\nThe survival of many serious genetic disorders in our long evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are defined by the existence of deleterious alleles, then natural selection acting over evolutionary time would result in a lower frequency of mutations than are currently observed. In a recent article, Carter and Nguyen identify several genetic disorders, arguing that far from being a rare phenomenon, antagonistic pleiotropy might be a fundamental mechanism for the survival of these non-optimal alleles.\\nIn one of these studies, 99 individuals with Laron syndrome (a rare form of dwarfism) were monitored alongside their non-dwarf kin for a period of ten years. Patients with Laron syndrome possess one of three genotypes for the growth hormone receptor gene (GHR). Most patients have an A->G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin. This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.\\nAnother instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. Incidentally, the high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.\\nAdditionally, it was found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. Yet, it was hypothesized that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.\\nSickle cell anemia, Beta-thalassemia, and cystic fibrosis are some other examples of the role antagonistic pleiotropy may play in genetic disorders.\"\n },\n {\n \"doc_id\": 5835331850275548000,\n \"text\": \"Mechanisms[edit]\\nThe IGF-1 pathway, which represses FOXO, thus preventing gene expression of longevity-inducing proteins\\nGrowth and somatic maintenance[edit]\\nMain article: Mechanistic target of rapamycin § Aging\\nMuch research has been done on the antagonistic effects of increased growth on lifespan. Specifically, the hormone insulin-like growth factor 1 (IGF-1), binds to a cell receptor, leading to a phosphorylation cascade. This cascade results in kinases phosphorylating forkhead transcription factor (FOXO), deactivating it. Deactivation of FOXO results in an inability to express genes involved in responding to oxidative stress response, such as antioxidants, chaperones, and heat-shock proteins.\\nAdditionally, uptake of IGF-1 stimulates the mTOR pathway, which activates protein synthesis (and therefore growth) through upregulation of the translation-promoting S6K1, and also inhibits autophagy, a process necessary for recycling of damaged cellular products. Decline of autophagy causes neurodegeneration, protein aggregation and premature aging. Lastly, studies have also indicated that the mTOR pathway also alters immune responses and stimulates cyclin-dependent kinase (CDK) inhibitors such as p16 and p21. This leads to alteration of the stem-cell niche and results in stem cell exhaustion, another theorized mechanism of aging.\\nReproduction and somatic maintenance[edit]\\nWhile reproduction inhibits lifespan with regard to multicellular organisms, the precise mechanism responsible for this effect remains unclear. Although many models do illustrate an inverse relationship, and the theory makes sense from an evolutionary perspective, the cellular mechanisms have yet to be explored. However, with regards to cellular replication, the progressive shortening of telomeres is a mechanism which limits the amount of generations of a single cell may undergo. Furthermore, in unicellular organisms like Saccharomyces cerevisiae, the formation of extrachromosomal rDNA circles (ERCs) in mother cells (but not daughter cells) upon every subsequent division is an identifiable type of DNA damage that is associated with replication. These ERCs accumulate over time and eventually trigger replicative senescence and death of the mother cell.\"\n },\n {\n \"doc_id\": 2404171571940034000,\n \"text\": \"Mechanism of action[edit]\\n(a) The survival rate within a population decreases with age, while the reproduction rate remains constant. (b) The reproduction probability peaks early in life, at sexual maturity, and then steadily decreases as an individual ages, with the remaining share of the population decreasing with age as they enter the selection shadow.\\nAmongst almost all populations, the likelihood that an individual will reproduce is related directly to their age. Starting at 0 at birth, the probability increases to its maximum in young adulthood once sexual maturity has been reached, before gradually decreasing with age. This decrease is caused by the increasing likelihood of death due to external pressures such as predation or illness, as well as the internal pressures inherent to organisms that experience senescence. In such cases deleterious mutations which are expressed early on are strongly selected against due to their major impact on the number of offspring produced by that individual. Mutations that present later in life, by contrast, are relatively unaffected by selective pressure, as their carriers have already passed on their genes, assuming they survive long enough for the mutation to be expressed at all. The result, as predicted by Medawar, is that deleterious late-life mutations will accumulate and result in the evolution of aging as it is known colloquially. This concept is portrayed graphically by Medawar through the concept of a \\\"selection shadow\\\". The shaded region represents the 'shadow' of time during which selective pressure has no effect. Mutations that are expressed within this selection shadow will remain as long as reproductive probability within that age range remains low.\"\n },\n {\n \"doc_id\": 3685274842462513000,\n \"text\": \"Benefits and costs[edit]\\nAntagonistic pleiotropy has several negative consequences. It results in delayed adaptation, an altered path of evolution, and reduced adaptation of other traits. In addition, the overall benefit of alleles is cut down significantly (by about half) by pleiotropy. Still, antagonistic pleiotropy has some evolutionary benefits. In fact, the conservation of genes is directly related to the pleiotropic character of an organism. This implies that genes that control for multiple traits, even if the traits have different implications for the organism's fitness, have more staying power in an evolutionary context.\"\n },\n {\n \"doc_id\": 221831125176784060,\n \"text\": \"As a constraint on perfection[edit]\\nAntagonistic pleiotropy is one of the several reasons evolutionary biologists give for organisms never being able to reach perfection through natural selection. Antagonistically pleiotropic genes are the explanation for fitness trade-offs. This means that genes that are pleiotropic control for some beneficial traits and some detrimental traits; thus, if they happen to persist through natural selection, this will prevent organisms from reaching perfection because if they possess the benefits of the gene, they must also possess the imperfections or faults. An example of this would be female rodents that live in a nest with other females and may end up feeding young that are not theirs due to their intense parental drive. This strong parental drive will be selected for, but the organisms will still make the mistake of feeding young that are not theirs and misallocating their resources.\"\n },\n {\n \"doc_id\": 1633464797414410200,\n \"text\": \" traits. Some traits that increase fitness early in life may also have negative effects later in life. But, because many more individuals are alive at young ages than at old ages, even small positive effects early can be strongly selected for, and large negative effects later may be very weakly selected against. Williams suggested the following example: Perhaps a gene codes for calcium deposition in bones, which promotes juvenile survival and will therefore be favored by natural selection; however, this same gene promotes calcium deposition in the arteries, causing negative atherosclerotic effects in old age. Thus, harmful biological changes in old age may\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":488661444248667840,"text":" result from selection for pleiotropic genes that are beneficial early in life but harmful later on. In this case, selection pressure is relatively high when Fisher's reproductive value is high and relatively low when Fisher's reproductive value is low.\nCancer versus cellular senescence tradeoff theory of aging[edit]\n\nMain article: Immunosenescence\nSenescent cells within a multicellular organism can be purged by competition between cells, but this increases the risk of cancer. This leads to an inescapable dilemma between two possibilities—the accumulation of physiologically useless senescent cells"},{"doc_id":8263022581067022000,"text":" Williams describes his theory of antagonistic pleiotropy.\n\n^ Kirkwood, T.B.L. 1977. Evolution of aging. Nature, 270: 301–304. [1] Origin of the disposable soma theory.\n\n^ Medvedev, Zhores A. (1981). \"On the immortality of the germ line: Genetic and biochemical mechanisms. A review\". Mechanisms of Ageing and Development. 17 (4): 331–359. doi:10.1016/0047-6374(81)90052-X. ISSN 0047-"},{"doc_id":4857876600457936000,"text":" neuroendocrinal control mechanisms. (See homeostasis). Cellular theory of ageing can be categorized as telomere theory, free radical theory (free-radical theory of aging) and apoptosis. The stem cell theory of aging is also a sub-category of cellular theories.\nEvolutionary aging theories[edit]\n\nMain article: Evolution of ageing\nAntagonistic pleiotropy[edit]\n\nMain article: Antagonistic pleiotropy hypothesis\nOne theory was proposed by George C. Williams and involves antagonistic pleiotropy. A single gene may affect multiple"},{"doc_id":499041569660853000,"text":" are evolutionary theories, molecular theories, system theories and cellular theories. The evolutionary theory of ageing was first proposed in the late 1940s and can be explained briefly by the accumulation of mutations (evolution of ageing), disposable soma and antagonistic pleiotropy hypothesis. The molecular theories of ageing include phenomena such as gene regulation (gene expression), codon restriction, error catastrophe, somatic mutation, accumulation of genetic material (DNA) damage (DNA damage theory of aging) and dysdifferentiation. The system theories include the immunologic approach to ageing, rate-of-living and the alterations in"},{"doc_id":1858491888670639400,"text":"Background and history[edit]\nDespite Charles Darwin's completion of his theory of biological evolution in the 19th century, the modern logical framework for evolutionary theories of aging wouldn't emerge until almost a century later. Though August Weismann did propose his theory of programmed death, it was met with criticism and never gained mainstream attention. It wasn't until 1930 that Ronald Fisher first noted the conceptual insight which prompted the development of modern aging theories. This concept, namely that the force of natural selection on an individual decreases with age, was analysed further by J. B. S. Haldane, who suggested it as an explanation for the relatively high prevalence of Huntington's disease despite the autosomal dominant nature of the mutation. Specifically, as Huntington's only presents after the age of 30, the force of natural selection against it would have been relatively low in pre-modern societies. It was based on the ideas of Fisher and Haldane that Peter Medawar was able to work out the first complete model explaining why aging occurs, which he presented in a lecture in 1951 and then published in 1952"},{"doc_id":2657511928684241000,"text":"Evidence[edit]\nGrowth and aging[edit]\nThere is a large body of evidence indicating the negative effects of growth on longevity across many species. As a general rule, individuals of a smaller size generally live longer than larger individuals of the same species.\nAnimal models[edit]\nIn dwarf models of mice, such Snell or Ames mice, mutations have arisen, either rendering them incapable of producing IGF-1 or unable to have adequate receptors for IGF-1 uptake. Furthermore, mice injected with growth hormone have been shown to have progressive weight loss, roughing of the coat, curvature of the spine, enlargement of the organs, kidney lesions and increased cancer risk. This effect is also seen in different breeds of dogs, where smaller breeds of dogs typically live significantly longer compared to their larger counterparts. Selectively bred for their small size, smaller dog breeds like the Chihuahua (average lifespan of 15–20 years) have the B/B genotype for the IGF-1 haplotype, reducing the amount of IGF-1 produced. Conversely, large dogs like the Great Dane (average lifespan of 6–8 years) are homozygous for the IGF-1 I allele, which increases the amount of IGF-1 production.\nHuman models[edit]\nInitially, it was believed that growth hormone actually prolonged lifespan due to a 1990 study that indicated that injection of growth hormone to men over 60 years of age appeared to reverse various biomarkers implicated in aging, such as decreased muscle mass, bone density, skin thickness, and increased adipose tissue. However, a 1999 study found that administering growth hormone also significantly increased mortality rate. Recent genomic studies have confirmed that the genes involved in growth hormone uptake and signaling are largely conserved across a plethora of species, such as yeast, nematodes, fruit flies, mice and humans. These studies have also shown that individuals with Laron syndrome, an autosomal recessive disorder resulting in dwarfism due to defects in growth hormone receptors, have increased lifespan. Additionally, these individuals have much lower incidences of age-related diseases such as type 2 diabetes and cancer. Lastly, human centenarians around the world are disproportionately of short stature, and have low levels of IGF-1.\nReproduction and aging[edit]\nNumerous studies have found that lifespan is inversely correlated with both the total amount of offspring birthed, as well as the age at which females first gives birth, also known as primiparity. Additionally, it has been found that reproduction is a costly mechanism that alters the metabolism of fat. Lipids invested in reproduction would be unable to be allocated to support mechanisms involved in somatic maintenance.\nAnimal models[edit]\nThe disposable soma theory has been consistent with the majority of animal models. It was found in numerous animal studies that castration or genetic deformities of reproduction organs was correlated with increased lifespan. Moreover, in red squirrels, it was found that females with an early primiparity achieved the highest immediate and lifetime reproductive success. However, it was also found that these same individuals had a decreased median and maximum lifespan. Specifically squirrels who mated earlier had a 22.4% rate of mortality until two years of age compared to a 16.5% rate of mortality in late breeders. In addition, these squirrels had an average maximum lifespan of 1035 days compared to an average maximum lifespan of 1245 days for squirrels that bred later.\nIn another study, researchers selectively bred fruit flies over three years to develop two different strains, an early-reproducing strain and a late-reproducing strain. The late-reproducing line had a significantly longer lifespan than the early-reproducing line. Even more telling was that when the researchers introduced a mutation in the ovarian-associated gene ovoD1, resulting in defective oogenesis, the differences in lifespan between the two lines disappeared. The researchers in this case concluded that \"aging has evolved primarily because of the damaging effects of reproduction earlier in life\".\nProminent aging researcher Steven Austad also performed a large-scale ecological study on the coast of Georgia in 1993. Austad isolated two opossum populations, one from the predator-infested mainland and one from the predator-absent nearby island of Sapelo. According to the disposable soma theory, a genetically isolated population subject to low environmentally-induced mortality would evolve delayed reproduction and aging. This is because without the pressure of predation, it would be evolutionarily advantageous to allocate more resources to somatic maintenance than reproduction, as early offspring mortality would be low. As predicted, even after controlling for predation, the isolated population had a longer lifespan, delayed primiparity, and reduced aging biomarkers such as tail collagen cross-linking.\nHuman models[edit]\nIn general, only a few studies exist in human models. It was found that castrated men live longer than their fertile counterparts. Further studies found that in British women, primiparity was earliest in women who died early and latest in women who died at the oldest ages. Furthermore, increased number of children birthed was associated with a decreased lifespan. A final study found that female centenarians were more likely to have children in later life compared average, especially past the age of 40. The researchers discovered that 19.2% of female centenarians had their first child after the age of 40, compared to 5.5% of the rest of the female population.\nRelationship between cell damage and aging[edit]\n\nMain article: Free radical theory\n\nMain article: DNA damage theory of aging\nThe naked mole rat has a disproportionately long life of 30 years through efficient cellular repair mechanisms.\nThere are numerous studies that support cellular damage, often due to a lack of somatic maintenance mechanisms, as a primary determinant for aging, and these studies have given rise to the free radical theory of aging and the DNA damage theory of aging. One study found that the cells of short-living rodents in vitro show much greater mutation rates and a general lack of genome surveillance compared to human cells and are far more susceptible to oxidative stress.\nOther studies have been conducted on the naked mole rat, a rodent species with remarkable longevity (30 years), capable of outliving the brown rat (3 years) by ten-fold. Additionally, almost no incidence of cancer has ever been detected in naked mole rats. Nearly all of the differences found between these two organisms, which are otherwise rather genetically similar, was in somatic maintenance. Naked mole rats were found to have higher levels of superoxide dismutase, a reactive oxygen species clearing antioxidant. In addition, naked mole rats had higher levels of base excision repair, DNA damage response signaling, homologous recombination repair, mismatch repair, nucleotide excision repair, and non-homologous end joining. In fact, many of these processes were near or exceeded human levels. Proteins from naked mole rats were also more resistant to oxidation, misfolding, ubiquitination, and had increased translational fidelity.\nFurther studies have been conducted on patients with Hutchinson-Gilford Progeria Syndrome (HGPS), a condition that leads to premature aging. Patients with HGPS typically age about seven times faster than average and usually succumb to the disease in their early teens. Patients with HGPS have cellular defects, specifically in the lamin proteins, which regulate the organization of the lamina and nuclear envelope for mitosis. A-type lamins promote genetic stability by maintaining levels of proteins that have key roles in the repair processes of non-homologous end joining and homologous recombination. Mouse cells deficient for maturation of prelamin A show increased DNA damage and chromosome aberrations and have increased sensitivity to DNA damaging agents.\nLastly, as mentioned previously, it has been found that the suppression of autophagy is associated with reduced lifespan, while stimulation is associated with extended lifespan. Activated in times of caloric restriction, autophagy is a process that prevents cellular damage through clearance and recycling of damaged proteins and organelles."},{"doc_id":234030867318073300,"text":" to a young systemic environment.\nNature, 433, 760–764.\n14. Conboy, I. M., & Rando, T. A. (2005). Aging, stem cells and\ntissue regeneration: Lessons from muscle. Cell Cycle, 4, 407–410.\n15. d’Adda di Fagagna, F., Reaper, P. M., Clay-Farrace, L., Fiegler,\nH., Carr, P., Von Zglinicki, T., et al. (2003). A DNA damage\ncheck"},{"doc_id":1552312367596461600,"text":"The mutation accumulation theory of aging was first proposed by Peter Medawar in 1952 as an evolutionary explanation for biological aging and the associated decline in fitness that accompanies it. Medawar used the term 'senescence' to refer to this process. The theory explains that, in the case where harmful mutations are only expressed later in life, when reproduction has ceased and future survival is increasingly unlikely, then these mutations are likely to be unknowingly passed on to future generations. In this situation the force of natural selection will be weak, and so insufficient to consistently eliminate these mutations. Medawar posited that over time these mutations would accumulate due to genetic drift and lead to the evolution of what is now referred to as aging."},{"doc_id":2111678933010816500,"text":"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H"},{"doc_id":5651287262479473000,"text":"). Essential role of limiting telomeres in the\npathogenesis of Werner syndrome. Nature Genetics, 36, 877–882.\n11. Chin, L., Artandi, S. E., Shen, Q., Tam, A., Lee, S. L., Gottlieb, G. J.,\net al. (1999). p53 Deficiency rescues the adverse effects of telomere\nloss and cooperates with telomere dysfunction to accelerate\ncarcinogenesis. Cell, 97, 527–538.\n12. Commo, S., Gaillard"},{"doc_id":8833130470710907000,"text":"., & Weissman, I. L. (2007). Hematopoietic\nstem cell aging: Mechanism and consequence. Experimental\nGerontology, 42, 385–390.\n38. Rudolph, K. L., Chang, S., Lee, H. W., Blasco, M., Gottlieb, G. J.,\nGreider, C., et al. (1999). Longevity, stress response, and cancer in\naging telomerase-deficient mice. Cell, 96, 701–712.\n39. Sharpless, N."},{"doc_id":4091536527034847700,"text":", so less effort is invested in repair and maintenance of somatic cells, compared to germline cells, in order to focus on reproduction and species survival.\nProgrammed aging theories[edit]\nProgrammed theories of aging posit that aging is adaptive, normally invoking selection for evolvability or group selection.\nThe reproductive-cell cycle theory suggests that aging is regulated by changes in hormonal signaling over the lifespan.\nDamage accumulation theories[edit]\nThe free radical theory of aging[edit]\n\nMain article: Free-radical theory of aging\nOne of the most prominent theories of"},{"doc_id":4714964573793987000,"text":"Evidence supporting the mutation accumulation theory[edit]\nPredation and Delayed Senescence[edit]\nIn populations where extrinsic mortality is low, the drop in reproductive probability after maturity is less severe than in other cases. The mutation accumulation theory therefore predicts that such populations would evolve delayed senescence. One such example of this scenario can be seen when comparing birds to organisms of equivalent size. It has been suggested that their ability to fly, and therefore lower relative risk of predation, is the cause of their longer than expected life span. The implication that flight, and therefore lower predation, increases lifespan is further born out by the fact that bats live on average 3 times longer than similarly sized mammals with comparable metabolic rates. Providing further evidence, insect populations are known to experience very high rates of extrinsic mortality, and as such would be expected to experience rapid senescence and short life spans. The exception to this rule, however, is found in the longevity of eusocial insect queens. As expected when applying the mutation accumulation theory, established queens are at almost no risk of predation or other forms of extrinsic mortality, and consequently age far more slowly than others of their species.\nAge-specific reproductive success of Drosophila Melanogaster[edit]\nIn the interest of finding specific evidence for the mutation accumulation theory, separate from that which also supports the similar antagonistic pleiotropy hypothesis, an experiment was conducted involving the breeding of successive generations of Drosophila Melanogaster. Genetic models predict that, in the case of mutation accumulation, elements of fitness, such as reproductive success and survival, will show age-related increases in dominance, homozygous genetic variance and additive variance. Inbreeding depression will also increase with age. This is because these variables are proportional to the equilibrium frequencies of deleterious alleles, which are expected to increase with age under mutation accumulation but not under the antagonistic pleiotropy hypothesis. This was tested experimentally by measuring age specific reproductive success in 100 different genotypes of Drosophila Melanogaster, with findings ultimately supporting the mutation accumulation theory of aging."},{"doc_id":4719393388878590000,"text":" at older ages and their contribution to the next generation was therefore small relative to the large cohorts of younger age groups, the force of selection against such late-acting deleterious mutations was correspondingly small. Therefore, a genetic load of late-acting deleterious mutations could be substantial at mutation–selection balance. This concept came to be known as the selection shadow.\nPeter Medawar formalised this observation in his mutation accumulation theory of aging. \"The force of natural selection weakens with increasing age—even in a theoretically immortal population, provided only that it is exposed to real hazards of mortality"},{"doc_id":1862574801358712800,"text":"Criticism[edit]\nOne of the main weaknesses of the disposable soma theory is that it does not postulate any specific cellular mechanisms to which an organism shifts energy to somatic repair over reproduction. Instead, it only offers an evolutionary perspective on why aging may occur due to reproduction. Therefore, parts of it are rather limited outside of the field of evolutionary biology.\nCaloric restriction[edit]\n\nMain article: Calorie restriction § Sirtuin-mediated mechanism\nSchematic showing the reallocation of energy investment towards self-repair during caloric restriction\nCritics have pointed out the supposed inconsistencies of the disposable soma theory due to the observed effects of caloric restriction, which is correlated with increased lifespan. Although it activates autophagy, according to classical disposable soma principles, with less caloric intake, there would less total energy to be distributed towards somatic maintenance, and decreased lifespan would be observed (or at least the positive autophagic effects would be balanced out). However, Kirkwood, alongside his collaborator Darryl P. Shanley, assert that caloric restriction triggers an adaptive mechanism which causes the organism to shift a higher proportion of resources to somatic maintenance, away from reproduction. This theory is supported by multiple studies, which show that caloric restriction typically results in impaired fertility, but leave an otherwise healthy organism. Evolutionarily, an organism would want to delay reproduction to when resources were more plentiful. During a resource-barren period, it would evolutionarily unwise to invest resources in progeny that would be unlikely to survive in famine. Mechanistically, the NAD-dependent deacetylase Sirtuin 1 (SIRT-1) is upregulated during low-nutrient periods. SIRT-1 increases insulin sensitivity, decreases the amount of inflammatory cytokines, stimulates autophagy, and activates FOXO, the aforementioned protein involved in activating stress response genes. SIRT-1 is also found to result in decreased fertility.\nIn additional to differential partitioning of energy allocation during caloric restriction, less caloric intake would result in less metabolic waste in the forms of free radicals like hydrogen peroxide, superoxide and hydroxyl radicals, which damage important cellular components, particularly mitochondria. Elevated levels of free radicals in mice has been correlated with neurodegeneration, myocardial injury, severe anemia, and premature death.\nNo changes were observed in the spontaneous chromosomal mutation frequency of dietary restricted mice (aged 6 and 12 months) compared to ad libitum fed control mice. Thus dietary restriction appears to have no appreciable effect on spontaneous mutation in chromosomal DNA, and the increased longevity of dietary restricted mice apparently is not attributable to reduced chromosomal mutation frequency.\nThe grandmother hypothesis[edit]\n\nMain article: Menopause § Evolutionary rationale\nAnother primary criticism of the disposable soma theory is that it fails to account for why women tend to live longer than their male counterparts. Afterall, females invest significantly more resources into reproduction and according to the classical disposable soma principles, this would compromise energy diverted to somatic maintenance. However, this can be reconciled with the grandmother hypothesis. The Grandmother Hypothesis states that menopause comes about into older women in order to restrict the time of reproduction as a protective mechanism. This would allow women to live longer and increase the amount of care they could provide to their grandchildren, increasing their evolutionary fitness. And so, although women do invest a greater proportion of resources into reproduction during their fertile years, their overall reproductive period is significantly shorter than men, who are able of reproduction during and even beyond middle age. Additionally, males invest more resources into growth compare to females, which is correlated with decreased lifespan. Other variables such as increased testosterone levels in males are not accounted for. Increased testosterone is often associated with reckless behaviour, which may lead to a high accidental death rate.\nContradicting models[edit]\nA few contradicting animal models weaken the validity of the disposable soma theory. This includes studies done on the aforementioned naked mole rats. In these studies, it was found that reproductive naked mole rats actually show significantly increased lifespans compared to non-reproductive individuals, which contradicts the principles of disposable soma. However, although these naked mole rats are mammalian, they are highly atypical in terms of aging studies and may not serve as the best model for humans. For example, naked mole rats have a disproportionately high longevity quotient and live in eusocial societies, where breeding is usually designated to a queen.\nSex biases and environment[edit]\nThe disposable soma theory is tested disproportionately on female organisms for the relationship between reproduction and aging, as females carry a greater burden in reproduction. Additionally, for the relationship between growth and aging, studies are disproportionately conducted on males, to minimize the hormonal fluctuations that occur with menstrual cycling. Lastly, genetic and environmental factors, rather than reproductive patterns, may explain the variations in human lifespan. For example, studies have shown that poorer individuals, to whom nutritious food and medical care is less accessible, typically have higher birth rates and earlier primiparity."},{"doc_id":473002211056177540,"text":"point response in telomere-initiated senescence. Nature, 426,\n194–198.\n16. Globerson, A. (1999). Hematopoietic stem cells and aging.\nExperimental Gerontology, 34, 137–146.\n17. Gosain, A., & DiPietro, L. A. (2004). Aging and wound healing.\nWorld Journal of Surgery, 28, 321–326.\n18. Hemesath, T. J., Steingrimsson, E., McGill, G., Hansen,"},{"doc_id":6318142403895506000,"text":"ophagy\nchronic inflammation\ndysbiosis\nThe environment induces damage at various levels, e.g. damage to DNA, and damage to tissues and cells by oxygen radicals (widely known as free radicals), and some of this damage is not repaired and thus accumulates with time. Cloning from somatic cells rather than germ cells may begin life with a higher initial load of damage. Dolly the sheep died young from a contagious lung disease, but data on an entire population of cloned individuals would be necessary to measure mortality rates and quantify aging.\nThe evolutionary theorist George Williams"},{"doc_id":5883227412128785000,"text":" symptoms of aging.\nMutation accumulation[edit]\n\nMain article: Mutation accumulation theory\nNatural selection can support lethal and harmful alleles, if their effects are felt after reproduction. The geneticist J. B. S. Haldane wondered why the dominant mutation that causes Huntington's disease remained in the population, and why natural selection had not eliminated it. The onset of this neurological disease is (on average) at age 45 and is invariably fatal within 10–20 years. Haldane assumed that, in human prehistory, few survived until age 45. Since few were alive"},{"doc_id":8654772478664844000,"text":" an intrinsic age-related process of loss of viability and increase in vulnerability.\"\nIn 2013, a group of scientists defined nine hallmarks of aging that are common between organisms with emphasis on mammals:\ngenomic instability,\ntelomere attrition,\nepigenetic alterations,\nloss of proteostasis,\nderegulated nutrient sensing,\nmitochondrial dysfunction,\ncellular senescence,\nstem cell exhaustion,\naltered intercellular communication\nIn a decadal update, three hallmarks have been added, totaling 12 proposed hallmarks:\ndisabled macroaut"},{"doc_id":2167913692640285200,"text":"Senescence (/sɪˈnɛsəns/) or biological aging is the gradual deterioration of functional characteristics in living organisms. The word senescence can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence involves an increase in death rates and/or a decrease in fecundity with increasing age, at least in the later part of an organism's life cycle. However, the resulting effects of senescence can be delayed. The 1934 discovery that calorie restriction can extend lifespans by 50% in rats, the existence of species having negligible senescence, and the existence of potentially immortal organisms such as members of the genus Hydra have motivated research into delaying senescence and thus age-related diseases. Rare human mutations can cause accelerated aging diseases.\nEnvironmental factors may affect aging – for example, overexposure to ultraviolet radiation accelerates skin aging. Different parts of the body may age at different rates and distinctly, including the brain, the cardiovascular system, and muscle. Similarly, functions may distinctly decline with aging, including movement control and memory. Two organisms of the same species can also age at different rates, making biological aging and chronological aging distinct concepts."},{"doc_id":7799898147696638000,"text":". If a genetic disaster... happens late enough in individual life, its consequences may be completely unimportant\". Age-independent hazards such as predation, disease, and accidents, called 'extrinsic mortality', mean that even a population with negligible senescence will have fewer individuals alive in older age groups.\nOther damage[edit]\nA study concluded that retroviruses in the human genomes can become awakened from dormant states and contribute to aging which can be blocked by neutralizing antibodies, alleviating \"cellular senescence and tissue degeneration and, to some extent, organismal"},{"doc_id":2472103435295607300,"text":" genes in yeast. Individual cells, which are genetically identical, nonetheless can have substantially different responses to outside stimuli, and markedly different lifespans, indicating the epigenetic factors play an important role in gene expression and aging as well as genetic factors. There is research into epigenetics of aging.\nThe ability to repair DNA double-strand breaks declines with aging in mice and humans.\nA set of rare hereditary (genetics) disorders, each called progeria, has been known for some time. Sufferers exhibit symptoms resembling accelerated aging, including wrinkled skin. The cause of Hutchinson–Gil"},{"doc_id":3616913741624993000,"text":"., et al. (2006). Towards a “free radical theory of\ngraying”: Melanocyte apoptosis in the aging human hair follicle is\nan indicator of oxidative stress induced tissue damage. FASEB\nJournal, 20, 1567–1569.\n3. Bandyopadhyay, D., & Medrano, E. E. (2003). The emerging role\nof epigenetics in cellular and organismal aging. Experimental\nGerontology, 38, 1299–1307.\n4. Barsh, G. S. ("},{"doc_id":7785026333663044000,"text":"Principles[edit]\nThe disposable soma theory of aging posits that there is a trade-off in resource allocation between somatic maintenance and reproductive investment. Too low an investment in self-repair would be evolutionarily unsound, as the organism would likely die before reproductive age. However, too high an investment in self-repair would also be evolutionarily unsound due to the fact that one's offspring would likely die before reproductive age. Therefore, there is a compromise and resources are partitioned accordingly. However, this compromise is thought to damage somatic repair systems, which can lead to progressive cellular damage and senescence. Repair costs can be categorized into three groups: (1) the costs of increased durability of nonrenewable parts; (2) the costs of maintenance involving cell renewal, and (3) the costs of intracellular maintenance. In a nutshell, aging and decline is essentially a trade-off for increased reproductive robustness in youth."},{"doc_id":4045537731215091000,"text":" niche. Development, 132, 5589–5599.\n35. Quevedo, W. C., Szabo, G., & Virks, J. (1969). Influence of age\nand UV on the populations of dopa-positive melanocytes in\nhuman skin. Journal of Investigative Dermatology, 52, 287–290.\n36. Rando, T. A. (2006). Stem cells, ageing and the quest for\nimmortality. Nature, 441, 1080–1086.\n37. Rossi, D. J., Bryder, D"},{"doc_id":1973072004158489300,"text":", and cancer—both of which lead to increasing rates of mortality with age.\nDisposable soma[edit]\n\nMain article: Disposable soma theory of aging\nThe disposable soma theory of aging was proposed by Thomas Kirkwood in 1977. The theory suggests that aging occurs due to a strategy in which an individual only invests in maintenance of the soma for as long as it has a realistic chance of survival. A species that uses resources more efficiently will live longer, and therefore be able to pass on genetic information to the next generation. The demands of reproduction are high"},{"doc_id":5208197970163825000,"text":" theories of aging primarily explain why aging happens, but do not concern themselves with the molecular mechanism(s) that drive the process. All evolutionary theories of aging rest on the basic mechanisms that the force of natural selection declines with age. Mechanistic theories of aging can be divided into theories that propose aging is programmed, and damage accumulation theories, i.e. those that propose aging to be caused by specific molecular changes occurring over time.\nThis section is an excerpt from Stem cell theory of aging § Other theories of aging.[edit]\nThe aging process can be explained with different theories. These"},{"doc_id":2673916972512251400,"text":"See also[edit]\nEvolution of ageing"},{"doc_id":8422091187445539000,"text":"%2F978-1-4020-2666-9). [ ISBN ](/wiki/ISBN_\\(identifier\\) \"ISBN \\(identifier\\)\") [ 978-1-4020-2665-2 ](/wiki/Special:BookSources/978-1-4020-2665-2 \"Special:BookSources/978-1-4020-2665-2\").\n25. ** ^ ** Timiras, Paola S. (2007). [ _Physiological Basis of Aging and Ger"},{"doc_id":1669846751109624000,"text":"ford progeria syndrome was reported in the journal Nature in May 2003.\nThis report suggests that DNA damage, not oxidative stress, is the cause of this form of accelerated aging.\nA study indicates that aging may shift activity toward short genes or shorter transcript length and that this can be countered by interventions."},{"doc_id":8530136533271727000,"text":"ig4 deficiency in the mouse causes a progressive loss of HSCs during aging. These findings suggest that NHEJ is a key determinant of the ability of HSCs to maintain themselves over time.\nHematopoietic stem cell diversity aging\nA study showed that the clonal diversity of stem cells that produce blood cells gets drastically reduced around age 70 to a faster-growing few, substantiating a novel theory of ageing which could enable healthy aging.\nHematopoietic mosaic loss of chromosome Y\nA 2022 study showed that blood cells' loss of"},{"doc_id":4879645114437644000,"text":"Criticisms of the mutation accumulation theory[edit]\nUnder most assumptions, the mutation accumulation theory predicts that mortality rates will reach close to 100% shortly after reaching post-reproductive age. Experimental populations of Drosophila Melanogaster, and other organisms, however, exhibit age-specific mortality rates that plateau well before reaching 100%, making mutation accumulation alone an insufficient explanation. It is suggested instead that mutation accumulation is only one factor among many, which together form the cause of aging. In particular, the mutation accumulation theory, the antagonistic pleiotropy hypothesis and the disposable soma theory of aging are all believed to contribute in some way to senescence."},{"doc_id":5148695143890330000,"text":"1996). The genetics of pigmentation: From fancy\ngenes to complex traits. Trends in Genetics, 12, 299–305.\n5. Blackburn, E. H. (2001). Switching and signaling at the telomere.\nCell, 106, 661–673.\n6. Blanpain, C., Horsley, V., & Fuchs, E. (2007). Epithelial stem\ncells: Turning over new leaves. Cell, 128, 445–458.\n7. Blanpain, C., Lowry, W. E., Geog"},{"doc_id":161608011506090560,"text":"ch, G., Greco, V., Blanpain, C., Lowry, W. E.,\nRendl, M., et al. (2004). Defining the epithelial stem cell niche in\nskin. Science, 303, 359–363.\n47. Van Zant, G., & Liang, Y. (2003). The role of stem cells in aging.\nExperimental Hematology, 31, 659–672.\n48. Veis, D. J., Sorenson, C. M., Shutter, J. R., & K"},{"doc_id":2871123610651248600,"text":" proposed in a 2021 review to be the underlying cause of aging because of the mechanistic link of DNA damage to nearly every aspect of the aging phenotype. DNA damage-induced epigenetic alterations, such as DNA methylation and many histone modifications, appear to be of particular importance to the aging process. Evidence for the theory that DNA damage is the fundamental cause of aging was first reviewed in 1981.\nIt is believed that the impact of alcohol on aging can be partly explained by alcohol's activation of the HPA axis, which stimulates glucocorticoid secretion, long-term exposure to which produces"},{"doc_id":8639138980743491000,"text":"Aging, Graying and Loss of Melanocyte Stem Cells\nKavita Y. Sarin & Steven E. Artandi\nPublished online: 18 August 2007\n# Humana Press Inc. 2007\nAbstract Hair graying is one of the prototypical signs of\nhuman aging. Maintenance of hair pigmentation is dependent on the presence and functionality of melanocytes,\nneural crest derived cells which synthesize pigment for\ngrowing hair. The melanocytes, themselves, are maintained\nby a small number of stem cells which reside in the bulge\nregion of the hair follicle. The recent characterization of the\nmelanocyte lineage during aging has significantly accelerated our understanding of how age-related changes in the\nmelanocyte stem cell compartment contribute to hair\ngraying. This review will discuss our current understanding\nof hair graying, drawing on evidence from human and\nmouse studies, and consider the contribution of melanocyte\nstem cells to this process. Furthermore, using the melanocyte lineage as an example, it will discuss common theories\nof tissue and stem cell aging.\nKeywords Stem cells . Aging . Pigmentation . Graying .\nMelanocytes. Telomeres. Bcl2 . Vitiligo . Light mutation\nStem Cells and Aging\nNormal tissue homeostasis requires continual cellular\nreplenishment as existing cells are lost due to injury or\nphysiologic turnover. In order to support this critical demand\nfor renewal, many mammalian tissues harbor adult stem\ncells—specialized multipotent cells with the capacity to both\nself-renew and give rise to differentiated progeny in response\nto appropriate extrinsic signals. Stem cells have been\nidentified in a number of tissues including the skin, bone\nmarrow, intestine, brain, and skeletal muscle and serve as a\nreservoir for the production of new cells in these organ\nsystems. In this way, stem cells play a key role in regenerating\nand maintaining adult tissues over time.\nPhysiological aging is characterized by a gradual decline\nin the ability of tissues to maintain homeostasis and regenerate new tissue after injury. For example, the capacity of\nthe hematopoetic system to mobilize under stress becomes\nincreasingly impaired with chronological aging and differentiation of hematopoeitic progenitors becomes altered to\nfavor myeloid rather than lymphoid fates [16, 37]. A\nreduction in regenerative potential has also been demonstrated in skeletal muscle [14] and skin [17]. Although this\nphenomenon has been well described in multiple tissues,\nthe factors contributing to this impaired regenerative\ncapacity remain largely unknown. In particular, the extent\nto which changes in stem cell number or function contribute to this impairment remains poorly understood."},{"doc_id":2709192573423168500,"text":"References\n1. Allsopp, R. C., Morin, G. B., DePinho, R., Harley, C. B., &\nWeissman, I. L. (2003). Telomerase is required to slow telomere\nshortening and extend replicative lifespan of HSCs during serial\ntransplantation. Blood, 102, 517–520.\n2. Arck, P. C., Overall, R., Spatz, K., Liezman, C., Handjiski, B.,\nKlapp, B. F"},{"doc_id":7947817844724409000,"text":" Agley, C. C., Thompson, A. J., Gonzalez, G. A. et al. (2019). Niche stiffness underlies the ageing of central nervous system progenitor cells. Nature 573, 130-134. https://doi.org/10.1038/s41586-019-1484-9\nGoogle ScholarCrossref\nShemer, A., Grozovski, J., Tay, T. L., Tao, J., Volaski, A., Süß, P., Ardura-Fabregat, A.,"},{"doc_id":5627648949841832000,"text":" aging was first proposed by Harman in 1956. It posits that free radicals produced by dissolved oxygen, radiation, cellular respiration and other sources cause damage to the molecular machines in the cell and gradually wear them down. This is also known as oxidative stress.\nThere is substantial evidence to back up this theory. Old animals have larger amounts of oxidized proteins, DNA and lipids than their younger counterparts.\nChemical damage[edit]\n\nThis section may be too long to read and navigate comfortably. Please consider splitting content into sub-articles, condensing it, or adding subhead"},{"doc_id":853742459037597000,"text":"[ Article ](https://doi.org/10.1111%2Fage.12466) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=27453069)\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5082551) [\nGoogle Scholar ](http://scholar.google.com/scholar"},{"doc_id":5812084939865406000,"text":" aging\".\nStem cell theories of aging[edit]\nThis section is an excerpt from Stem cell theory of aging.[edit]\nThe stem cell theory of aging postulates that the aging process is the result of the inability of various types of stem cells to continue to replenish the tissues of an organism with functional differentiated cells capable of maintaining that tissue's (or organ's) original function. Damage and error accumulation in genetic material is always a problem for systems regardless of the age. The number of stem cells in young people is very much higher than older people and thus creates a better and"},{"doc_id":5931796567179650000,"text":"See also[edit]\nAging brain\nAmerican Academy of Anti-Aging Medicine\nCalico (company)\nCryptobiosis\nDNA damage theory of aging\nMaximum life span\nMethuselah Foundation\nReliability theory of aging and longevity\nRejuvenation (aging)\nStrategies for engineered negligible senescence (SENS)\nTelomerase in cancer cell\nTimeline of senescence research"},{"doc_id":6728271864373516000,"text":"ings. Please discuss this issue on the article's talk page. (March 2023)\nElderly Klamath woman photographed by Edward S. Curtis in 1924\n\nSee also: DNA damage theory of aging\nOne of the earliest aging theories was the Rate of Living Hypothesis described by Raymond Pearl in 1928 (based on earlier work by Max Rubner), which states that fast basal metabolic rate corresponds to short maximum life span.\nWhile there may be some validity to the idea that for various types of specific damage detailed below that are by-products of metabolism, all other things being"},{"doc_id":7614560587774558000,"text":"http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6500750) [\nGoogle Scholar\n](http://scholar.google.com/scholar_lookup?&title=Analysis%20of%20the%20wnt1%20regulatory%20chromosomal%20landscape&journal=Dev.%20Genes%20Evol.&doi=10.1007%2Fs00427-019-00629-5&volume=229&pages=43-52&publication_"},{"doc_id":7280379357968796000,"text":" E., & DePinho, R. A. (2004). Telomeres, stem cells,\nsenescence, and cancer. Journal of Clinical Investigation, 113,\n160–168.\n40. Slominski, A., & Paus, R. (1993). Melanogenesis is coupled to\nmurine anagen: Toward new concepts for the role of melanocytes\nand the regulation of melanogenesis in hair growth. Journal of\nInvestigative Dermatology, 101, 90S–97S.\n41. Spradling, A., Drum"},{"doc_id":5144017881462403000,"text":"The disposable soma theory of aging states that organisms age due to an evolutionary trade-off between growth, reproduction, and DNA repair maintenance. Formulated by Thomas Kirkwood, the disposable soma theory explains that an organism only has a limited amount of resources that it can allocate to its various cellular processes. Therefore, a greater investment in growth and reproduction would result in reduced investment in DNA repair maintenance, leading to increased cellular damage, shortened telomeres, accumulation of mutations, compromised stem cells, and ultimately, senescence. Although many models, both animal and human, have appeared to support this theory, parts of it are still controversial.\nSpecifically, while the evolutionary trade-off between growth and aging has been well established,\nthe relationship between reproduction and aging is still without scientific consensus, and the cellular mechanisms largely undiscovered."},{"doc_id":279991935367356260,"text":"mond-Barbosa, D., & Kai, T. (2001). Stem\ncells find their niche. Nature, 414, 98–104.\n42. Steingrimsson, E., Copeland, N. G., & Jenkins, N. A. (2005).\nMelanocyte stem cell maintenance and hair graying. Cell, 121,\n9–12.\n43. Takai, H., Smogorzewska, A., & de Lange, T. (2003). DNA\ndamage foci at dysfunctional telomeres. Current Biology,"},{"doc_id":6378792526111520000,"text":"Definition and characteristics[edit]\nOrganismal senescence is the aging of whole organisms. Actuarial senescence can be defined as an increase in mortality and/or a decrease in fecundity with age. The Gompertz–Makeham law of mortality says that the age-dependent component of the mortality rate increases exponentially with age.\nAging is characterized by the declining ability to respond to stress, increased homeostatic imbalance, and increased risk of aging-associated diseases including cancer and heart disease. Aging has been defined as \"a progressive deterioration of physiological function,"},{"doc_id":1008014595189329500,"text":"Finally, changes in cellular survival signals could contribute\nto the loss of melanocyte stem cells seen during hair\ngraying. Mice deficient for the anti-apoptotic protein Bcl2\ndemonstrate sudden onset of hair graying after the first hair\ncycle [48]. Recent characterization of melanocyte stem cells\nin Bcl2-deficient mice demonstrated that Bcl2 plays a\ncritical role in the survival of melanocyte stem cells during\nthe first hair follicle anagen cycle since the loss of\nmelanocyte stem cells at p8.5 precedes the disappearance\nof the melanocyte lineage and hair graying [32]. Another\nstudy suggested that dependence on Bcl2 for survival may\nbe restricted to melanoblasts in the c-kit-dependent stage\n[28]. Regardless, both studies demonstrate that changes in\ncellular survival signals such as Bcl2 can alter melanocyte\nsurvival and may contribute to the loss of melanocyte stem\ncells seen with aging.\nComparison to Aging in other Organs: is Hair Graying\nUnique among Aging Phenotypes?\nThe progenitor cell organization seen in the melanocyte\nlineage is architecturally similar to that seen in other compartments, consisting of three major types of cells: quiescent, self-renewing stem cells, highly proliferative transient\namplifying cells, and differentiated cells [6]. The changes\nseen in the melanocyte lineage—markedly reduced numbers of differentiated melanocytes and diminished numbers\nof melanocyte stem cells—are consistent with a primary\nmelanocyte stem cell problem leading to loss of the entire\nlineage. However, it is not clear to what extent the changes in\nmelanocytes are representative of those in other aging\ntissues. In contrast to melanocyte stem cells, hematopoeitic\nstem cells and skeletal muscle stem cells do not significantly\ndecline in number with aging [36]. Yet these organ systems\ndemonstrate either a reduced ability to regenerate or abnormal differentiation with advancing age, suggesting that\nchanges in stem cell function may explain the altered\nregenerative capacity of aging [47]. Evidence for systemic\nfactors that contribute to age-related decline come from\nstudies in which exposure to systemic factors restored\nregenerative potential of skeletal muscle [13]. Given the\nmounting evidence that altered stem cell function or reduced\nstem cell number may contribute to aging, an improved\nunderstanding of how to regulate tissue stem cells may have\nsignificant effects on our ability to prevent the eventual\ndecline in aging tissues.\nStem Cell Rev (2007) 3:212–217 215\nSignificant progress has been made in understanding\nwhat causes age-related hair graying. These data so far\nsuggest that hair graying occurs primarily due to a gradual\nloss of the melanocyte lineage rather than due to impaired\nmelanocyte function. However, the factors that lead to loss\nof the melanocyte lineage remain poorly understood. It\nremains possible that age-related changes in melanocyte\nsensitivity to oxidative species produced during melanogenesis could contribute to an increase in melanocyte\napoptosis. In support of this idea, one study found increased\napoptosis in the differentiated melanocytes of aging hair\nfollicles [2]. However, while this may contribute to loss of\ndifferentiated melanocytes, it does not explain the corresponding decrease in bulge melanocytes seen in aging hair\nfollicles, suggesting that other factors must also contribute\nto loss of melanocyte stem cells. Age-related epigenetic\nmodifications could trigger the growth arrest of melanocyte\nstem cells leading to their eventual depletion or alter to\nniche rendering it incapable of supporting self-renewal [3].\nAnother possibility is that melanocyte stem cells themselves may have an intrinsically determined lifespan and\ncan only proliferate for a limited number of cell divisions\n[39]. Finally, there may be changes in proliferation,\nsurvival, or differentiation signals in the melanocyte stem\ncell population that occur with human aging. A more\ndetailed molecular and genetic analysis of hair graying will\nclearly facilitate a deeper understanding of aging in other\ntissues and cell lineages."},{"doc_id":4826778915141533000,"text":"See also[edit]\nAnti-aging movement\nAntimuscarinics\nDementia\nDNA repair\nGeriatrics\nGerontology\nHeavy metals\nHomeostatic capacity\nImmortality\nIndex of topics related to life extension\nMitohormesis\nOld age\nPhenoptosis\nPlant senescence\nProgrammed cell death\nStrategies for engineered negligible senescence (SENS)\nSub-lethal damage\nTransgenerational design\nTimeline of senescence research"},{"doc_id":2791557587399947300,"text":", O., & Bernard, B. A. (2004). Human hair\ngreying is linked to a specific depletion of hair follicle\nmelanocytes affecting both the bulb and the outer root sheath.\nBritish Journal of Dermatology, 150, 435–443.\n13. Conboy, I. M., Conboy, M. J., Wagers, A. J., Girma, E. R.,\nWeissman, I. L., & Rando, T. A. (2005). Rejuvenation of aged\nprogenitor cells by exposure"},{"doc_id":7443190492468470000,"text":"Background and history[edit]\nBritish biologist Thomas Kirkwood first proposed the disposable soma theory of aging in a 1977 Nature review article. The theory was inspired by Leslie Orgel's Error Catastrophe Theory of Aging, which was published fourteen years earlier, in 1963. Orgel believed that the process of aging arose due to mutations acquired during the replication process, and Kirkwood developed the disposable soma theory in order to mediate Orgel's work with evolutionary genetics."},{"doc_id":4951364018294375000,"text":"©2009 Macmillan Publishers Limited. All rights reserved\n5. Kussell, E. & Leibler, S. Phenotypic diversity, population growth, and information in\nfluctuating environments. Science 309, 2075–2078 (2005).\n6. Acar, M., Mettetal, J. T. & van Oudenaarden, A. Stochastic switching as a survival\nstrategy in fluctuating environments. Nature Genet. 40, 471–475 (2008).\n7. Balaban, N. Q. et al"},{"doc_id":5546822385480442000,"text":"slcc.pressbooks.pub/collegebiology1/chapter/regulator-molecules-of-the-cell-cycle/#chapter-912-section-3)\n3. [ 17.3 Cancer ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer/)"},{"doc_id":4444909518440718000,"text":"Genetic determinants of aging[edit]\n\nMain article: Genetics of aging\nA number of genetic components of aging have been identified using model organisms, ranging from the simple budding yeast Saccharomyces cerevisiae to worms such as Caenorhabditis elegans and fruit flies (Drosophila melanogaster). Study of these organisms has revealed the presence of at least two conserved aging pathways.\nGene expression is imperfectly controlled, and it is possible that random fluctuations in the expression levels of many genes contribute to the aging process as suggested by a study of such"},{"doc_id":8192270329965011000,"text":"Mouse Models of Hair Graying\nDespite the recent evidence that hair graying is caused at\nleast in part by a decline in the number of melanocyte stem\ncells, it is unclear what factors cause the decline in stem\ncells seen with hair graying. Studies in mice have evoked a\nnumber of factors that could contribute to the loss of the\nmelanocyte lineage.\nAre melanocytes disappearing secondary to oxidative\ndamage to the melanocytic compartment? Do stem cells\nhave a limited lifespan and undergo replicative senescence\nafter a certain number of divisions? Are there age related\nchanges in cell survival signals that contribute to the\ndecline in stem cells? Are there changes in the specialized\nniche environment that occur with aging such that the niche\ncan no longer can support stem cells? Or do changes in the\ndelicate balance of self-renewal and differentiation occur\nwith aging, which leads to the depletion of the melanocytic\nstem cells? While the answers to these questions remain\nunknown, there have been a number of mouse model\nsystems suggesting that many of these factors may contribute to age-related hair graying.\nLight Mice: Evidence for Oxidative Damage\nto Melanocytes\nOne cause of the decline in the melanocyte lineage during\nhair graying may be due to cell death caused by oxidative\ntoxicity associated with melanin biosynthesis [19, 48]. In\nsupport of this, H2O2 has been shown to induce senescence\nin cultured fibroblasts in vitro. Furthermore, in vivo\nevidence for the role of reactive oxidative species in hair\ngraying came from mice harboring a dominant mutant\nallele of TRP-1, which results in the loss of the melanocyte\nlineage and pigmentation in C57/Bl6 mice [19]. Interestingly, melanocyte loss was not detected in albino mice\n214 Stem Cell Rev (2007) 3:212–217\ncarrying the same mutation. These data suggested that the\nloss of melanocytes required melanin synthesis to occur and\nwas likely caused by the generation of toxic oxidative\nproducts that occurred during melanin synthesis.\nTelomerase-Deficient Mice: Telomere Shortening Enhances\nGraying at Advanced Age\nTelomeres represent the nucleoprotein structures that\nprotect the ends of linear chromosomes. Because of the\ninability of DNA polymerase to replicate fully the ends of\nchromosomes, telomeres shorten in the absence of sufficient telomerase, the enzyme that adds DNA repeats to\nchromosome ends [5]. Telomeres shorten as many human\ncells divide in culture and also shorten in human tissues\nwith advancing age. Telomere shortening can induce\ncellular senescence or programmed cell death, as the\nprotective cap that constitutes the telomere is lost leading\nto a DNA damage response at the telomere end [15, 43] and\nactivation of the tumor suppressor protein p53 [11, 20].\nAnalysis of telomerase knockout mice has revealed that\ncritical telomere shortening impairs the function of proliferative tissues and blocks self-renewal of hematopoietic\nstem cells [1, 25]. There is evidence that telomere shortening causes premature aging in telomerase-deficient mice,\nwhich includes increased hair graying compared to controls\n[10, 38]. These data indicate that telomere length could play\na limiting role in the ability of cells in the melanocyte\nlineage cells to regenerate. It is not yet known if hair\ngraying in the context of shortened telomeres is due to\ndiminished melanocytes, and if so, whether melanocyte\nstem cell loss or stem cell dysfunction contributes to\ngraying in this context.\nVitiligo Mutant Mice: Altered Differentiation\nof Melanocyte Progenitors\nGradual hair pigmentation loss has also been demonstrated\nin mice that harbor a hypomorphic point mutation in the\nDNA binding region of the essential melanocyte lineage\ntranscription factor, MITF. The Mitfvit/vit mutant mice\nundergo gradual progressive hair graying due to a reduction\nmelanocyte number and become completely depigmented at\n6–12 months postnatally [18, 23, 24, 26]. While the exact\netiology is unclear, MITF has been shown to regulate a\nnumber of enzymes needed for pigmentation such as\ntyrosinase, TRP1, Dct, Pmel 17 and the anti-apoptotic\nfactor Bcl2 [29]. In addition, ectopic pigmentation was\ndetected in the melanocytic niche in these mutant mice\nsuggesting that the decline in melanocyte stem cells could\noccur due to premature differentiation [32]. Mutations in\nMITF have also been detected in premature graying\nconditions in humans such as Waardenberg Syndrome, an\ninherited disorder characterized by loss of skin and hair\npigmentation, as well as hearing loss.\nBcl2−/− Mice: Bcl2 is Required for Survival of Melanocyte\nProgenitors"},{"doc_id":8782227544835548000,"text":"/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1101/cshperspect.a016154 ](https://doi.org/10.1101%2Fcshperspect.a016154). [ PMC ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 3949356 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3949356). [ PMID ](/wiki"},{"doc_id":9209714122028345000,"text":"Theories of aging[edit]\nThis section needs expansion. You can help by adding to it. (March 2023)\nMore than 300 different theories have been posited to explain the nature (mechanisms) and causes (reasons for natural emergence or factors) of aging. Good theories would both explain past observations and predict the results of future experiments. Some of the theories may complement each other, overlap, contradict, or may not preclude various other theories.\nTheories of aging fall into two broad categories, evolutionary theories of aging and mechanistic theories of aging. Evolutionary"},{"doc_id":1285922490615968500,"text":"alia S. (April 2010). \"Demographic Consequences of Defeating Aging\". Rejuvenation Research. 13 (2–3): 329–334. doi:10.1089/rej.2009.0977. ISSN 1549-1684. PMC 3192186. PMID 20426616.\n\n^ \"Scientists' Open Letter on Aging\". Retrieved 20 April 2015.\n\n^ \"A Single-Issue Political Party for Longevity Science\". Fight Aging!. 27 July 2012. Retrieved 20 April 2015.\n\n^ Pent"},{"doc_id":3895641590567795700,"text":" ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3839606). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 24296171 ](https://pubmed.ncbi.nlm.nih.gov/24296171). [ Archived ](https://web.archive.org/web/20220913054521/https://cshperspectives.cshlp.org/content/5/12/"},{"doc_id":4976934889088200000,"text":"[ PubMed Abstract\n](http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=21549330)\n| [ CrossRef Full Text ](https://doi.org/10.1016/j.stem.2011.03.010) | [\nGoogle Scholar ](http://scholar.google.com/scholar_lookup?&title=Division-\ncoupled+astrocytic+differentiation+and+age-\nrelated+"},{"doc_id":7639397133823105000,"text":"Search PMC Full-Text Archive\nSearch PMC Full-Text Archive\nSearch in PMC\nAdvanced Search User Guide\nJournal List PLoS Genet v.12(7); 2016 Jul PMC4956268\nAs a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health.\nLearn more: PMC Disclaimer | PMC Copyright Notice\nLogo of plosgen\nPLoS Genet. 2016 Jul; 12"},{"doc_id":5412326387684365000,"text":"Ubiquity[edit]\nAlthough there are so many negative effects related to genes that are antagonistically pleiotropic, it is still present among most forms of life. Indeed, pleiotropy is one of the most common traits possessed by genes overall. In addition to that, pleiotropy is under strong stabilizing selection. In one experiment with mice and the morphology of the mandible, 1/5 of the loci had effects of pleiotropy for the entire mandible. One other example was in the Russian biologist Dmitry K. Belyaev's study on the domestication of the fox. In Dmitry K. Belyaev's farm-fox experiment, wild foxes were bred for docile behavior alone. After 40 generations, other physiological changes had surfaced including shortened tails, floppy ears, a white star in the forehead, rolled tails, shorter legs. Since the only thing being selected for was behavior, this leads scientists to believe that these secondary characteristics were controlled by the same gene or genes as docile behavior."},{"doc_id":1648645431169788400,"text":" Jülicher, F. (2021). Theory of time delayed genetic oscillations with external noisy regulation. New J. Phys. 23, 033030. https://doi.org/10.1088/1367-2630/abd80b\nGoogle ScholarCrossref\nNett, I. R. E., Mulas, C., Gatto, L., Lilley, K. S. and Smith, A. (2018). Negative feedback via RSK modulates Erk-dependent progression from naïve pluripotency. EMBO Rep. 19,"},{"doc_id":7555524859195046000,"text":" T cells have been used to give good predictions of the expected lifespan of middle-aged mice.\nAging clocks[edit]\n\nThis section needs expansion. You can help by adding to it. (March 2023)\nThere is interest in an epigenetic clock as a biomarker of aging, based on its ability to predict human chronological age. Basic blood biochemistry and cell counts can also be used to accurately predict the chronological age. It is also possible to predict the human chronological age using the transcriptomic aging clocks.\nThere is research and development of further biomarkers, detection"},{"doc_id":7566684989972600000,"text":" may cause exhaustion of stem cells.\nHematopoietic stem cell aging\nHematopoietic stem cells (HSCs) regenerate the blood system throughout life and maintain homeostasis. DNA strand breaks accumulate in long term HSCs during aging. This accumulation is associated with a broad attenuation of DNA repair and response pathways that depends on HSC quiescence. DNA ligase 4 (Lig4) has a highly specific role in the repair of double-strand breaks by non-homologous end joining (NHEJ). L"},{"doc_id":410698984248633150,"text":"Peccei JS (2001). \"Menopause: Adaptation or epiphenomenon?\". Evolutionary Anthropology. 10 (2): 43–57. doi : 10.1002/evan.1013. S2CID 1665503.\n\nMarziali C (7 December 2010). \"Reaching Toward the Fountain of Youth\". USC Trojan Family Magazine. Archived from the original on 13 December 2010. Retrieved 7 December 2010."},{"doc_id":5886191137619035000,"text":"Hair graying, one of the prototypical signs of human\naging, is caused by a progressive loss of pigmentation from\ngrowing hair shafts. In normal aging, the onset of hair\ngraying occurs at 34±9.6 years of age in Caucasians and\n43.9±10.3 years in African Americans, a period in which\nhair follicles pass through approximately 7–15 complete\ncycles [45]. Hair graying represents an impaired ability of\nmelanocytes to maintain normal homeostasis and replenish\nmelanin, pigment for the newly growing hair. Until recently,\nit was largely unknown what contributes to the loss of hair\npigmentation seen during human aging. The recent identification of melanocyte stem cells and characterization of\nchanges in the melanocyte lineage during aging have\nprovided new insights into how alterations in stem cell\nfunction and number contribute to the hair graying seen\nwith physiological aging.\nStem Cell Rev (2007) 3:212–217\nDOI 10.1007/s12015-007-0028-0\nK. Y. Sarin : S. E. Artandi (*)\nDepartment of Medicine, Stanford University School of Medicine,\nStanford, CA 94305, USA\ne-mail: sartandi@stanford.edu\nIn this review, we will discuss the function of melanocyte\nstem cells in hair pigmentation and explore the relationship\nbetween the age-related changes in melanocyte stem cell\ncompartment and hair graying. Using the example of the\nmelanocyte lineage, we will consider common theories of\naging stem cells and suggest future avenues for investigation.\nMelanocyte Stem Cells\nMelanocytes are specialized neural crest-derived cells,\nwhich synthesize and transfer pigment to neighboring\nkeratinocytes. In humans, melanocytes are located throughout the skin, including the epidermis, dermis, and hair\nfollicle, as well as in other organ systems such as the uvea\nof the eye and the inner ear. In mouse skin, melanocytes are\nprimarily restricted to the hair follicle, although they are\nalso found in the epidermis and dermis of hairless regions\nsuch as murine ear, ventral paws, and tail.\nDuring embryogenesis, melanocyte precursor cells, called\nmelanoblasts, arise from neural crest and migrate through\nthe dermis and epidermis into newly developing hair\nplacodes. The migration and survival of these melanoblasts\nare dependent on c-kit and its ligand stem cell factor (SCF),\nas neonatal deletion of either SCF or c-kit results in an\nirreversible loss of coat pigmentation [9, 27, 30, 31, 49]."},{"doc_id":680403308465968500,"text":"1734.\")\nTuberous sclerosis proteins 1 and 2 (TSC1/2) inhibit mTOR, and HIF1A is\nstrongly upregulated in _Tsc1−/−_ neuroblasts, thereby increasing their\nsurvival. [ 77 ](/articles/cddis201764#ref-CR77 \"Feliciano DM, Zhang S, Quon\nJL, Bordey A. Hypoxia-inducible factor 1a is a Tsc1-regulated survival factor\nin newborn neurons in tuberous sclerosis complex."},{"doc_id":5870541505121028000,"text":"[ Article ](https://doi.org/10.1016%2Fj.ijdevneu.2014.04.004) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=24798024)\n[ Google Scholar\n](http://scholar.google.com/scholar_lookup?&title=The%20apoptotic%20perspective%20of%20"},{"doc_id":8504326372591973000,"text":"2E+Perspect%2E+Biol%2E&author=Ganz+J.&author=Brand+M.&publication_year=2016&volume=8&issue=a019018)"},{"doc_id":2275480560330257200,"text":", A., Müller, F.-J., Babtie, A., Chan, T. E., Stumpf, M. P. H., Please, C. P., Howison, S. D. et al. (2017). Stem cell differentiation as a non-markov stochastic process. Cell Syst. 5, 268-282.e7. https://doi.org/10.1016/j.cels.2017.08.009\nGoogle ScholarCrossref\nWagner, D. E. and Klein, A. M."},{"doc_id":7107188148454427000,"text":" more efficient replacement mechanism in the young contrary to the old. In other words, aging is not a matter of the increase in damage, but a matter of failure to replace it due to a decreased number of stem cells. Stem cells decrease in number and tend to lose the ability to differentiate into progenies or lymphoid lineages and myeloid lineages.\nMaintaining the dynamic balance of stem cell pools requires several conditions. Balancing proliferation and quiescence along with homing (See niche) and self-renewal of hematopoietic stem cells are"},{"doc_id":3990271081171909000,"text":"Biological immortality (sometimes referred to as bio-indefinite mortality) is a state in which the rate of mortality from senescence is stable or decreasing, thus decoupling it from chronological age. Various unicellular and multicellular species, including some vertebrates, achieve this state either throughout their existence or after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury, poison, disease, predation, lack of available resources, or changes to environment.\nThis definition of immortality has been challenged in the Handbook of the Biology of Aging, because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high.\nThe term is also used by biologists to describe cells that are not subject to the Hayflick limit on how many times they can divide."},{"doc_id":7686507863959127000,"text":"[\n](/cddis)"},{"doc_id":6198489821588383000,"text":"rez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=25409825)\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4405208) [\nGoogle Scholar\n](http://scholar.google.com/scholar_lookup?&title=Conservation%20of%20trans-\nacting%20circuitry%20during%20mammalian%20regulatory%20evolution&"},{"doc_id":678460758480640400,"text":"Variation among species[edit]\n\nFurther information: Longevity § Non-human biological longevity\nDifferent speeds with which mortality increases with age correspond to different maximum life span among species. For example, a mouse is elderly at 3 years, a human is elderly at 80 years, and ginkgo trees show little effect of age even at 667 years.\nAlmost all organisms senesce, including bacteria which have asymmetries between \"mother\" and \"daughter\" cells upon cell division, with the mother cell experiencing aging, while the daughter is rejuvenated. There is negligible senescence in some groups, such as the genus Hydra. Planarian flatworms have \"apparently limitless telomere regenerative capacity fueled by a population of highly proliferative adult stem cells.\" These planarians are not biologically immortal, but rather their death rate slowly increases with age. Organisms that are thought to be biologically immortal would, in one instance, be Turritopsis dohrnii, also known as the \"immortal jellyfish\", due to its ability to revert to its youth when it undergoes stress during adulthood. The reproductive system is observed to remain intact, and even the gonads of Turritopsis dohrnii are existing.\nSome species exhibit \"negative senescence\", in which reproduction capability increases or is stable, and mortality falls with age, resulting from the advantages of increased body size during aging."},{"doc_id":5902401109933815000,"text":"[ Article ](https://doi.org/10.1111%2Fage.12717) [ CAS ](/articles/cas-\nredirect/1:CAS:528:DC%2BC1cXitFyhtr%2FE) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=30311254)\n[ PubMed Central ](http://www.ncbi"},{"doc_id":1200511620083040300,"text":" systems and software systems to measure biological age of different tissues or systems or overall. For example, a deep learning (DL) software using anatomic magnetic resonance images estimated brain age with relatively high accuracy, including detecting early signs of Alzheimer's disease and varying neuroanatomical patterns of neurological aging, and a DL tool was reported as to calculate a person's inflammatory age based on patterns of systemic age-related inflammation.\nAging clocks have been used to evaluate impacts of interventions on humans, including combination therapies."},{"doc_id":1179232220599721500,"text":"1. [ Cancer and the Cell Cycle ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer-and-the-cell-cycle/#chapter-915-section-1)\n2. [ Proto-oncogenes ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer-and-the-cell-cycle/#chapter-915-section-2)\n3. [ Tumor Suppressor Genes ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer"},{"doc_id":1133625993065273900,"text":"4+ T cells and degeneration of the spiral ganglion in Samp1 mice, a murine model of human senescence. This technology remains years or even decades away from human application."},{"doc_id":8349157190851547000,"text":"ichia coli strain MG1655: Growth\ndefects and apparent cross-regulation of gene expression. J. Bacteriol. 185,\n5611–5626 (2003).\n14. Oxman, E., Alon, U. & Dekel, E. Defined order of evolutionary adaptations:\nexperimental evidence. Evolution 62, 1547–1554 (2008).\n15. Mortimer, R. K. Evolution and variation of the yeast (Saccharomyces) genome.\nGenome Res. 10, 403–409 (2000).\n16"},{"doc_id":962550441146100100,"text":", when modern statistical methods for correcting for the effects of body size and phylogeny are employed, metabolic rate does not correlate with longevity in mammals or birds.\nWith respect to specific types of chemical damage caused by metabolism, it is suggested that damage to long-lived biopolymers, such as structural proteins or DNA, caused by ubiquitous chemical agents in the body such as oxygen and sugars, are in part responsible for aging. The damage can include breakage of biopolymer chains, cross-linking of biopolymers, or chemical attachment of unnatural substituents (haptens) to"},{"doc_id":5995368045220096000,"text":"://www.ncbi.nlm.nih.gov/pmc/articles/PMC2898526) [\nGoogle Scholar\n](http://scholar.google.com/scholar_lookup?&title=Simple%20combinations%20of%20lineage-\ndetermining%20transcription%20factors%20prime%20cis-\nregulatory%20elements%20required%20for%20macrophage%20and%20B%20cell%20identities&journal=Mol.%20"},{"doc_id":4517178045387123000,"text":"] [CrossRef] [Google Scholar]\n29. Siklenka K, Erkek S, Godmann M, Lambrot R, McGraw S, et al. Disruption of histone methylation in developing sperm impairs offspring health transgenerationally. Science. 2015;350:aab2006 10.1126/science.aab2006 [PubMed] [CrossRef] [Google Scholar]\n30. Zordan R, Miller M, Galgoczy D, Tuch B, Johnson A. Interlocking transcriptional feedback loops control white-opaque"},{"doc_id":1646067418514023400,"text":"As each model has important implications for the underlying biology, it is necessary to test the extent to which the underlying assumptions are valid. For example, describing a biological process as a phase transition implies cells must go through a critical point, characterised by the appearance of power-law patterns (i.e. when one variable changes as a power of another, independent of the initial conditions) and sharp increases in variance/disorder, as shown recently in tissue remodelling (Petridou et al., 2021). Similarly, describing a transition as a Markov-chain process (Wheat et al.,"},{"doc_id":5742440580135641000,"text":"^ Walker, Peter (1991). International Search and Rescue Teams. League Discussion Paper. Geneva, CH: League of the Red Cross and Red Crescent Societies.\n\n^ \"What is Aging?\". Retrieved 6 November 2020.\n\n^ Lin Kah Wai (18 April 2004). \"Telomeres, Telomerase, and Tumorigenesis – A Review\". MedGenMed. 6 (3): 19. PMC 1435592. PMID 15520642.\n\n^ Wade, Nicholas (26 January 2017). \"New Prospects for Growing Human Replacement Or"},{"doc_id":3835360740815392300,"text":"age tracing on transcriptional landscapes links state to fate during differentiation. Science 367, eaaw3381. https://doi.org/10.1126/science.aaw3381\nGoogle ScholarCrossref\nWheat, J. C., Sella, Y., Willcockson, M., Skoultchi, A. I., Bergman, A., Singer, R. H. and Steidl, U. (2020). Single-molecule imaging of transcription dynamics in somatic stem cells. Nature 583, 431-436. https://doi"},{"doc_id":8675078867840300000,"text":" ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce. On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer.\nSuch cooperation within species may have evolved through the process of kin selection"},{"doc_id":3677536378871693000,"text":"., & de Lange, T.\n(1999). p53-and ATM-dependent apoptosis induced by telomeres\nlacking TRF2. Science, 283, 1321–1325.\n21. Kiger, A. A., Jones, D. L., Schulz, C., Rogers, M. B., & Fuller, M.\nT. (2001). Stem cell self-renewal specified by JAK-STAT\nactivation in response to a support cell cue. Science, 294, 2542–\n2545.\n22. Kunis"}],"string":"[\n {\n \"doc_id\": 488661444248667840,\n \"text\": \" result from selection for pleiotropic genes that are beneficial early in life but harmful later on. In this case, selection pressure is relatively high when Fisher's reproductive value is high and relatively low when Fisher's reproductive value is low.\\nCancer versus cellular senescence tradeoff theory of aging[edit]\\n\\nMain article: Immunosenescence\\nSenescent cells within a multicellular organism can be purged by competition between cells, but this increases the risk of cancer. This leads to an inescapable dilemma between two possibilities—the accumulation of physiologically useless senescent cells\"\n },\n {\n \"doc_id\": 8263022581067022000,\n \"text\": \" Williams describes his theory of antagonistic pleiotropy.\\n\\n^ Kirkwood, T.B.L. 1977. Evolution of aging. Nature, 270: 301–304. [1] Origin of the disposable soma theory.\\n\\n^ Medvedev, Zhores A. (1981). \\\"On the immortality of the germ line: Genetic and biochemical mechanisms. A review\\\". Mechanisms of Ageing and Development. 17 (4): 331–359. doi:10.1016/0047-6374(81)90052-X. ISSN 0047-\"\n },\n {\n \"doc_id\": 4857876600457936000,\n \"text\": \" neuroendocrinal control mechanisms. (See homeostasis). Cellular theory of ageing can be categorized as telomere theory, free radical theory (free-radical theory of aging) and apoptosis. The stem cell theory of aging is also a sub-category of cellular theories.\\nEvolutionary aging theories[edit]\\n\\nMain article: Evolution of ageing\\nAntagonistic pleiotropy[edit]\\n\\nMain article: Antagonistic pleiotropy hypothesis\\nOne theory was proposed by George C. Williams and involves antagonistic pleiotropy. A single gene may affect multiple\"\n },\n {\n \"doc_id\": 499041569660853000,\n \"text\": \" are evolutionary theories, molecular theories, system theories and cellular theories. The evolutionary theory of ageing was first proposed in the late 1940s and can be explained briefly by the accumulation of mutations (evolution of ageing), disposable soma and antagonistic pleiotropy hypothesis. The molecular theories of ageing include phenomena such as gene regulation (gene expression), codon restriction, error catastrophe, somatic mutation, accumulation of genetic material (DNA) damage (DNA damage theory of aging) and dysdifferentiation. The system theories include the immunologic approach to ageing, rate-of-living and the alterations in\"\n },\n {\n \"doc_id\": 1858491888670639400,\n \"text\": \"Background and history[edit]\\nDespite Charles Darwin's completion of his theory of biological evolution in the 19th century, the modern logical framework for evolutionary theories of aging wouldn't emerge until almost a century later. Though August Weismann did propose his theory of programmed death, it was met with criticism and never gained mainstream attention. It wasn't until 1930 that Ronald Fisher first noted the conceptual insight which prompted the development of modern aging theories. This concept, namely that the force of natural selection on an individual decreases with age, was analysed further by J. B. S. Haldane, who suggested it as an explanation for the relatively high prevalence of Huntington's disease despite the autosomal dominant nature of the mutation. Specifically, as Huntington's only presents after the age of 30, the force of natural selection against it would have been relatively low in pre-modern societies. It was based on the ideas of Fisher and Haldane that Peter Medawar was able to work out the first complete model explaining why aging occurs, which he presented in a lecture in 1951 and then published in 1952\"\n },\n {\n \"doc_id\": 2657511928684241000,\n \"text\": \"Evidence[edit]\\nGrowth and aging[edit]\\nThere is a large body of evidence indicating the negative effects of growth on longevity across many species. As a general rule, individuals of a smaller size generally live longer than larger individuals of the same species.\\nAnimal models[edit]\\nIn dwarf models of mice, such Snell or Ames mice, mutations have arisen, either rendering them incapable of producing IGF-1 or unable to have adequate receptors for IGF-1 uptake. Furthermore, mice injected with growth hormone have been shown to have progressive weight loss, roughing of the coat, curvature of the spine, enlargement of the organs, kidney lesions and increased cancer risk. This effect is also seen in different breeds of dogs, where smaller breeds of dogs typically live significantly longer compared to their larger counterparts. Selectively bred for their small size, smaller dog breeds like the Chihuahua (average lifespan of 15–20 years) have the B/B genotype for the IGF-1 haplotype, reducing the amount of IGF-1 produced. Conversely, large dogs like the Great Dane (average lifespan of 6–8 years) are homozygous for the IGF-1 I allele, which increases the amount of IGF-1 production.\\nHuman models[edit]\\nInitially, it was believed that growth hormone actually prolonged lifespan due to a 1990 study that indicated that injection of growth hormone to men over 60 years of age appeared to reverse various biomarkers implicated in aging, such as decreased muscle mass, bone density, skin thickness, and increased adipose tissue. However, a 1999 study found that administering growth hormone also significantly increased mortality rate. Recent genomic studies have confirmed that the genes involved in growth hormone uptake and signaling are largely conserved across a plethora of species, such as yeast, nematodes, fruit flies, mice and humans. These studies have also shown that individuals with Laron syndrome, an autosomal recessive disorder resulting in dwarfism due to defects in growth hormone receptors, have increased lifespan. Additionally, these individuals have much lower incidences of age-related diseases such as type 2 diabetes and cancer. Lastly, human centenarians around the world are disproportionately of short stature, and have low levels of IGF-1.\\nReproduction and aging[edit]\\nNumerous studies have found that lifespan is inversely correlated with both the total amount of offspring birthed, as well as the age at which females first gives birth, also known as primiparity. Additionally, it has been found that reproduction is a costly mechanism that alters the metabolism of fat. Lipids invested in reproduction would be unable to be allocated to support mechanisms involved in somatic maintenance.\\nAnimal models[edit]\\nThe disposable soma theory has been consistent with the majority of animal models. It was found in numerous animal studies that castration or genetic deformities of reproduction organs was correlated with increased lifespan. Moreover, in red squirrels, it was found that females with an early primiparity achieved the highest immediate and lifetime reproductive success. However, it was also found that these same individuals had a decreased median and maximum lifespan. Specifically squirrels who mated earlier had a 22.4% rate of mortality until two years of age compared to a 16.5% rate of mortality in late breeders. In addition, these squirrels had an average maximum lifespan of 1035 days compared to an average maximum lifespan of 1245 days for squirrels that bred later.\\nIn another study, researchers selectively bred fruit flies over three years to develop two different strains, an early-reproducing strain and a late-reproducing strain. The late-reproducing line had a significantly longer lifespan than the early-reproducing line. Even more telling was that when the researchers introduced a mutation in the ovarian-associated gene ovoD1, resulting in defective oogenesis, the differences in lifespan between the two lines disappeared. The researchers in this case concluded that \\\"aging has evolved primarily because of the damaging effects of reproduction earlier in life\\\".\\nProminent aging researcher Steven Austad also performed a large-scale ecological study on the coast of Georgia in 1993. Austad isolated two opossum populations, one from the predator-infested mainland and one from the predator-absent nearby island of Sapelo. According to the disposable soma theory, a genetically isolated population subject to low environmentally-induced mortality would evolve delayed reproduction and aging. This is because without the pressure of predation, it would be evolutionarily advantageous to allocate more resources to somatic maintenance than reproduction, as early offspring mortality would be low. As predicted, even after controlling for predation, the isolated population had a longer lifespan, delayed primiparity, and reduced aging biomarkers such as tail collagen cross-linking.\\nHuman models[edit]\\nIn general, only a few studies exist in human models. It was found that castrated men live longer than their fertile counterparts. Further studies found that in British women, primiparity was earliest in women who died early and latest in women who died at the oldest ages. Furthermore, increased number of children birthed was associated with a decreased lifespan. A final study found that female centenarians were more likely to have children in later life compared average, especially past the age of 40. The researchers discovered that 19.2% of female centenarians had their first child after the age of 40, compared to 5.5% of the rest of the female population.\\nRelationship between cell damage and aging[edit]\\n\\nMain article: Free radical theory\\n\\nMain article: DNA damage theory of aging\\nThe naked mole rat has a disproportionately long life of 30 years through efficient cellular repair mechanisms.\\nThere are numerous studies that support cellular damage, often due to a lack of somatic maintenance mechanisms, as a primary determinant for aging, and these studies have given rise to the free radical theory of aging and the DNA damage theory of aging. One study found that the cells of short-living rodents in vitro show much greater mutation rates and a general lack of genome surveillance compared to human cells and are far more susceptible to oxidative stress.\\nOther studies have been conducted on the naked mole rat, a rodent species with remarkable longevity (30 years), capable of outliving the brown rat (3 years) by ten-fold. Additionally, almost no incidence of cancer has ever been detected in naked mole rats. Nearly all of the differences found between these two organisms, which are otherwise rather genetically similar, was in somatic maintenance. Naked mole rats were found to have higher levels of superoxide dismutase, a reactive oxygen species clearing antioxidant. In addition, naked mole rats had higher levels of base excision repair, DNA damage response signaling, homologous recombination repair, mismatch repair, nucleotide excision repair, and non-homologous end joining. In fact, many of these processes were near or exceeded human levels. Proteins from naked mole rats were also more resistant to oxidation, misfolding, ubiquitination, and had increased translational fidelity.\\nFurther studies have been conducted on patients with Hutchinson-Gilford Progeria Syndrome (HGPS), a condition that leads to premature aging. Patients with HGPS typically age about seven times faster than average and usually succumb to the disease in their early teens. Patients with HGPS have cellular defects, specifically in the lamin proteins, which regulate the organization of the lamina and nuclear envelope for mitosis. A-type lamins promote genetic stability by maintaining levels of proteins that have key roles in the repair processes of non-homologous end joining and homologous recombination. Mouse cells deficient for maturation of prelamin A show increased DNA damage and chromosome aberrations and have increased sensitivity to DNA damaging agents.\\nLastly, as mentioned previously, it has been found that the suppression of autophagy is associated with reduced lifespan, while stimulation is associated with extended lifespan. Activated in times of caloric restriction, autophagy is a process that prevents cellular damage through clearance and recycling of damaged proteins and organelles.\"\n },\n {\n \"doc_id\": 234030867318073300,\n \"text\": \" to a young systemic environment.\\nNature, 433, 760–764.\\n14. Conboy, I. M., & Rando, T. A. (2005). Aging, stem cells and\\ntissue regeneration: Lessons from muscle. Cell Cycle, 4, 407–410.\\n15. d’Adda di Fagagna, F., Reaper, P. M., Clay-Farrace, L., Fiegler,\\nH., Carr, P., Von Zglinicki, T., et al. (2003). A DNA damage\\ncheck\"\n },\n {\n \"doc_id\": 1552312367596461600,\n \"text\": \"The mutation accumulation theory of aging was first proposed by Peter Medawar in 1952 as an evolutionary explanation for biological aging and the associated decline in fitness that accompanies it. Medawar used the term 'senescence' to refer to this process. The theory explains that, in the case where harmful mutations are only expressed later in life, when reproduction has ceased and future survival is increasingly unlikely, then these mutations are likely to be unknowingly passed on to future generations. In this situation the force of natural selection will be weak, and so insufficient to consistently eliminate these mutations. Medawar posited that over time these mutations would accumulate due to genetic drift and lead to the evolution of what is now referred to as aging.\"\n },\n {\n \"doc_id\": 2111678933010816500,\n \"text\": \"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H\"\n },\n {\n \"doc_id\": 5651287262479473000,\n \"text\": \"). Essential role of limiting telomeres in the\\npathogenesis of Werner syndrome. Nature Genetics, 36, 877–882.\\n11. Chin, L., Artandi, S. E., Shen, Q., Tam, A., Lee, S. L., Gottlieb, G. J.,\\net al. (1999). p53 Deficiency rescues the adverse effects of telomere\\nloss and cooperates with telomere dysfunction to accelerate\\ncarcinogenesis. Cell, 97, 527–538.\\n12. Commo, S., Gaillard\"\n },\n {\n \"doc_id\": 8833130470710907000,\n \"text\": \"., & Weissman, I. L. (2007). Hematopoietic\\nstem cell aging: Mechanism and consequence. Experimental\\nGerontology, 42, 385–390.\\n38. Rudolph, K. L., Chang, S., Lee, H. W., Blasco, M., Gottlieb, G. J.,\\nGreider, C., et al. (1999). Longevity, stress response, and cancer in\\naging telomerase-deficient mice. Cell, 96, 701–712.\\n39. Sharpless, N.\"\n },\n {\n \"doc_id\": 4091536527034847700,\n \"text\": \", so less effort is invested in repair and maintenance of somatic cells, compared to germline cells, in order to focus on reproduction and species survival.\\nProgrammed aging theories[edit]\\nProgrammed theories of aging posit that aging is adaptive, normally invoking selection for evolvability or group selection.\\nThe reproductive-cell cycle theory suggests that aging is regulated by changes in hormonal signaling over the lifespan.\\nDamage accumulation theories[edit]\\nThe free radical theory of aging[edit]\\n\\nMain article: Free-radical theory of aging\\nOne of the most prominent theories of\"\n },\n {\n \"doc_id\": 4714964573793987000,\n \"text\": \"Evidence supporting the mutation accumulation theory[edit]\\nPredation and Delayed Senescence[edit]\\nIn populations where extrinsic mortality is low, the drop in reproductive probability after maturity is less severe than in other cases. The mutation accumulation theory therefore predicts that such populations would evolve delayed senescence. One such example of this scenario can be seen when comparing birds to organisms of equivalent size. It has been suggested that their ability to fly, and therefore lower relative risk of predation, is the cause of their longer than expected life span. The implication that flight, and therefore lower predation, increases lifespan is further born out by the fact that bats live on average 3 times longer than similarly sized mammals with comparable metabolic rates. Providing further evidence, insect populations are known to experience very high rates of extrinsic mortality, and as such would be expected to experience rapid senescence and short life spans. The exception to this rule, however, is found in the longevity of eusocial insect queens. As expected when applying the mutation accumulation theory, established queens are at almost no risk of predation or other forms of extrinsic mortality, and consequently age far more slowly than others of their species.\\nAge-specific reproductive success of Drosophila Melanogaster[edit]\\nIn the interest of finding specific evidence for the mutation accumulation theory, separate from that which also supports the similar antagonistic pleiotropy hypothesis, an experiment was conducted involving the breeding of successive generations of Drosophila Melanogaster. Genetic models predict that, in the case of mutation accumulation, elements of fitness, such as reproductive success and survival, will show age-related increases in dominance, homozygous genetic variance and additive variance. Inbreeding depression will also increase with age. This is because these variables are proportional to the equilibrium frequencies of deleterious alleles, which are expected to increase with age under mutation accumulation but not under the antagonistic pleiotropy hypothesis. This was tested experimentally by measuring age specific reproductive success in 100 different genotypes of Drosophila Melanogaster, with findings ultimately supporting the mutation accumulation theory of aging.\"\n },\n {\n \"doc_id\": 4719393388878590000,\n \"text\": \" at older ages and their contribution to the next generation was therefore small relative to the large cohorts of younger age groups, the force of selection against such late-acting deleterious mutations was correspondingly small. Therefore, a genetic load of late-acting deleterious mutations could be substantial at mutation–selection balance. This concept came to be known as the selection shadow.\\nPeter Medawar formalised this observation in his mutation accumulation theory of aging. \\\"The force of natural selection weakens with increasing age—even in a theoretically immortal population, provided only that it is exposed to real hazards of mortality\"\n },\n {\n \"doc_id\": 1862574801358712800,\n \"text\": \"Criticism[edit]\\nOne of the main weaknesses of the disposable soma theory is that it does not postulate any specific cellular mechanisms to which an organism shifts energy to somatic repair over reproduction. Instead, it only offers an evolutionary perspective on why aging may occur due to reproduction. Therefore, parts of it are rather limited outside of the field of evolutionary biology.\\nCaloric restriction[edit]\\n\\nMain article: Calorie restriction § Sirtuin-mediated mechanism\\nSchematic showing the reallocation of energy investment towards self-repair during caloric restriction\\nCritics have pointed out the supposed inconsistencies of the disposable soma theory due to the observed effects of caloric restriction, which is correlated with increased lifespan. Although it activates autophagy, according to classical disposable soma principles, with less caloric intake, there would less total energy to be distributed towards somatic maintenance, and decreased lifespan would be observed (or at least the positive autophagic effects would be balanced out). However, Kirkwood, alongside his collaborator Darryl P. Shanley, assert that caloric restriction triggers an adaptive mechanism which causes the organism to shift a higher proportion of resources to somatic maintenance, away from reproduction. This theory is supported by multiple studies, which show that caloric restriction typically results in impaired fertility, but leave an otherwise healthy organism. Evolutionarily, an organism would want to delay reproduction to when resources were more plentiful. During a resource-barren period, it would evolutionarily unwise to invest resources in progeny that would be unlikely to survive in famine. Mechanistically, the NAD-dependent deacetylase Sirtuin 1 (SIRT-1) is upregulated during low-nutrient periods. SIRT-1 increases insulin sensitivity, decreases the amount of inflammatory cytokines, stimulates autophagy, and activates FOXO, the aforementioned protein involved in activating stress response genes. SIRT-1 is also found to result in decreased fertility.\\nIn additional to differential partitioning of energy allocation during caloric restriction, less caloric intake would result in less metabolic waste in the forms of free radicals like hydrogen peroxide, superoxide and hydroxyl radicals, which damage important cellular components, particularly mitochondria. Elevated levels of free radicals in mice has been correlated with neurodegeneration, myocardial injury, severe anemia, and premature death.\\nNo changes were observed in the spontaneous chromosomal mutation frequency of dietary restricted mice (aged 6 and 12 months) compared to ad libitum fed control mice. Thus dietary restriction appears to have no appreciable effect on spontaneous mutation in chromosomal DNA, and the increased longevity of dietary restricted mice apparently is not attributable to reduced chromosomal mutation frequency.\\nThe grandmother hypothesis[edit]\\n\\nMain article: Menopause § Evolutionary rationale\\nAnother primary criticism of the disposable soma theory is that it fails to account for why women tend to live longer than their male counterparts. Afterall, females invest significantly more resources into reproduction and according to the classical disposable soma principles, this would compromise energy diverted to somatic maintenance. However, this can be reconciled with the grandmother hypothesis. The Grandmother Hypothesis states that menopause comes about into older women in order to restrict the time of reproduction as a protective mechanism. This would allow women to live longer and increase the amount of care they could provide to their grandchildren, increasing their evolutionary fitness. And so, although women do invest a greater proportion of resources into reproduction during their fertile years, their overall reproductive period is significantly shorter than men, who are able of reproduction during and even beyond middle age. Additionally, males invest more resources into growth compare to females, which is correlated with decreased lifespan. Other variables such as increased testosterone levels in males are not accounted for. Increased testosterone is often associated with reckless behaviour, which may lead to a high accidental death rate.\\nContradicting models[edit]\\nA few contradicting animal models weaken the validity of the disposable soma theory. This includes studies done on the aforementioned naked mole rats. In these studies, it was found that reproductive naked mole rats actually show significantly increased lifespans compared to non-reproductive individuals, which contradicts the principles of disposable soma. However, although these naked mole rats are mammalian, they are highly atypical in terms of aging studies and may not serve as the best model for humans. For example, naked mole rats have a disproportionately high longevity quotient and live in eusocial societies, where breeding is usually designated to a queen.\\nSex biases and environment[edit]\\nThe disposable soma theory is tested disproportionately on female organisms for the relationship between reproduction and aging, as females carry a greater burden in reproduction. Additionally, for the relationship between growth and aging, studies are disproportionately conducted on males, to minimize the hormonal fluctuations that occur with menstrual cycling. Lastly, genetic and environmental factors, rather than reproductive patterns, may explain the variations in human lifespan. For example, studies have shown that poorer individuals, to whom nutritious food and medical care is less accessible, typically have higher birth rates and earlier primiparity.\"\n },\n {\n \"doc_id\": 473002211056177540,\n \"text\": \"point response in telomere-initiated senescence. Nature, 426,\\n194–198.\\n16. Globerson, A. (1999). Hematopoietic stem cells and aging.\\nExperimental Gerontology, 34, 137–146.\\n17. Gosain, A., & DiPietro, L. A. (2004). Aging and wound healing.\\nWorld Journal of Surgery, 28, 321–326.\\n18. Hemesath, T. J., Steingrimsson, E., McGill, G., Hansen,\"\n },\n {\n \"doc_id\": 6318142403895506000,\n \"text\": \"ophagy\\nchronic inflammation\\ndysbiosis\\nThe environment induces damage at various levels, e.g. damage to DNA, and damage to tissues and cells by oxygen radicals (widely known as free radicals), and some of this damage is not repaired and thus accumulates with time. Cloning from somatic cells rather than germ cells may begin life with a higher initial load of damage. Dolly the sheep died young from a contagious lung disease, but data on an entire population of cloned individuals would be necessary to measure mortality rates and quantify aging.\\nThe evolutionary theorist George Williams\"\n },\n {\n \"doc_id\": 5883227412128785000,\n \"text\": \" symptoms of aging.\\nMutation accumulation[edit]\\n\\nMain article: Mutation accumulation theory\\nNatural selection can support lethal and harmful alleles, if their effects are felt after reproduction. The geneticist J. B. S. Haldane wondered why the dominant mutation that causes Huntington's disease remained in the population, and why natural selection had not eliminated it. The onset of this neurological disease is (on average) at age 45 and is invariably fatal within 10–20 years. Haldane assumed that, in human prehistory, few survived until age 45. Since few were alive\"\n },\n {\n \"doc_id\": 8654772478664844000,\n \"text\": \" an intrinsic age-related process of loss of viability and increase in vulnerability.\\\"\\nIn 2013, a group of scientists defined nine hallmarks of aging that are common between organisms with emphasis on mammals:\\ngenomic instability,\\ntelomere attrition,\\nepigenetic alterations,\\nloss of proteostasis,\\nderegulated nutrient sensing,\\nmitochondrial dysfunction,\\ncellular senescence,\\nstem cell exhaustion,\\naltered intercellular communication\\nIn a decadal update, three hallmarks have been added, totaling 12 proposed hallmarks:\\ndisabled macroaut\"\n },\n {\n \"doc_id\": 2167913692640285200,\n \"text\": \"Senescence (/sɪˈnɛsəns/) or biological aging is the gradual deterioration of functional characteristics in living organisms. The word senescence can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence involves an increase in death rates and/or a decrease in fecundity with increasing age, at least in the later part of an organism's life cycle. However, the resulting effects of senescence can be delayed. The 1934 discovery that calorie restriction can extend lifespans by 50% in rats, the existence of species having negligible senescence, and the existence of potentially immortal organisms such as members of the genus Hydra have motivated research into delaying senescence and thus age-related diseases. Rare human mutations can cause accelerated aging diseases.\\nEnvironmental factors may affect aging – for example, overexposure to ultraviolet radiation accelerates skin aging. Different parts of the body may age at different rates and distinctly, including the brain, the cardiovascular system, and muscle. Similarly, functions may distinctly decline with aging, including movement control and memory. Two organisms of the same species can also age at different rates, making biological aging and chronological aging distinct concepts.\"\n },\n {\n \"doc_id\": 7799898147696638000,\n \"text\": \". If a genetic disaster... happens late enough in individual life, its consequences may be completely unimportant\\\". Age-independent hazards such as predation, disease, and accidents, called 'extrinsic mortality', mean that even a population with negligible senescence will have fewer individuals alive in older age groups.\\nOther damage[edit]\\nA study concluded that retroviruses in the human genomes can become awakened from dormant states and contribute to aging which can be blocked by neutralizing antibodies, alleviating \\\"cellular senescence and tissue degeneration and, to some extent, organismal\"\n },\n {\n \"doc_id\": 2472103435295607300,\n \"text\": \" genes in yeast. Individual cells, which are genetically identical, nonetheless can have substantially different responses to outside stimuli, and markedly different lifespans, indicating the epigenetic factors play an important role in gene expression and aging as well as genetic factors. There is research into epigenetics of aging.\\nThe ability to repair DNA double-strand breaks declines with aging in mice and humans.\\nA set of rare hereditary (genetics) disorders, each called progeria, has been known for some time. Sufferers exhibit symptoms resembling accelerated aging, including wrinkled skin. The cause of Hutchinson–Gil\"\n },\n {\n \"doc_id\": 3616913741624993000,\n \"text\": \"., et al. (2006). Towards a “free radical theory of\\ngraying”: Melanocyte apoptosis in the aging human hair follicle is\\nan indicator of oxidative stress induced tissue damage. FASEB\\nJournal, 20, 1567–1569.\\n3. Bandyopadhyay, D., & Medrano, E. E. (2003). The emerging role\\nof epigenetics in cellular and organismal aging. Experimental\\nGerontology, 38, 1299–1307.\\n4. Barsh, G. S. (\"\n },\n {\n \"doc_id\": 7785026333663044000,\n \"text\": \"Principles[edit]\\nThe disposable soma theory of aging posits that there is a trade-off in resource allocation between somatic maintenance and reproductive investment. Too low an investment in self-repair would be evolutionarily unsound, as the organism would likely die before reproductive age. However, too high an investment in self-repair would also be evolutionarily unsound due to the fact that one's offspring would likely die before reproductive age. Therefore, there is a compromise and resources are partitioned accordingly. However, this compromise is thought to damage somatic repair systems, which can lead to progressive cellular damage and senescence. Repair costs can be categorized into three groups: (1) the costs of increased durability of nonrenewable parts; (2) the costs of maintenance involving cell renewal, and (3) the costs of intracellular maintenance. In a nutshell, aging and decline is essentially a trade-off for increased reproductive robustness in youth.\"\n },\n {\n \"doc_id\": 4045537731215091000,\n \"text\": \" niche. Development, 132, 5589–5599.\\n35. Quevedo, W. C., Szabo, G., & Virks, J. (1969). Influence of age\\nand UV on the populations of dopa-positive melanocytes in\\nhuman skin. Journal of Investigative Dermatology, 52, 287–290.\\n36. Rando, T. A. (2006). Stem cells, ageing and the quest for\\nimmortality. Nature, 441, 1080–1086.\\n37. Rossi, D. J., Bryder, D\"\n },\n {\n \"doc_id\": 1973072004158489300,\n \"text\": \", and cancer—both of which lead to increasing rates of mortality with age.\\nDisposable soma[edit]\\n\\nMain article: Disposable soma theory of aging\\nThe disposable soma theory of aging was proposed by Thomas Kirkwood in 1977. The theory suggests that aging occurs due to a strategy in which an individual only invests in maintenance of the soma for as long as it has a realistic chance of survival. A species that uses resources more efficiently will live longer, and therefore be able to pass on genetic information to the next generation. The demands of reproduction are high\"\n },\n {\n \"doc_id\": 5208197970163825000,\n \"text\": \" theories of aging primarily explain why aging happens, but do not concern themselves with the molecular mechanism(s) that drive the process. All evolutionary theories of aging rest on the basic mechanisms that the force of natural selection declines with age. Mechanistic theories of aging can be divided into theories that propose aging is programmed, and damage accumulation theories, i.e. those that propose aging to be caused by specific molecular changes occurring over time.\\nThis section is an excerpt from Stem cell theory of aging § Other theories of aging.[edit]\\nThe aging process can be explained with different theories. These\"\n },\n {\n \"doc_id\": 2673916972512251400,\n \"text\": \"See also[edit]\\nEvolution of ageing\"\n },\n {\n \"doc_id\": 8422091187445539000,\n \"text\": \"%2F978-1-4020-2666-9). [ ISBN ](/wiki/ISBN_\\\\(identifier\\\\) \\\"ISBN \\\\(identifier\\\\)\\\") [ 978-1-4020-2665-2 ](/wiki/Special:BookSources/978-1-4020-2665-2 \\\"Special:BookSources/978-1-4020-2665-2\\\").\\n25. ** ^ ** Timiras, Paola S. (2007). [ _Physiological Basis of Aging and Ger\"\n },\n {\n \"doc_id\": 1669846751109624000,\n \"text\": \"ford progeria syndrome was reported in the journal Nature in May 2003.\\nThis report suggests that DNA damage, not oxidative stress, is the cause of this form of accelerated aging.\\nA study indicates that aging may shift activity toward short genes or shorter transcript length and that this can be countered by interventions.\"\n },\n {\n \"doc_id\": 8530136533271727000,\n \"text\": \"ig4 deficiency in the mouse causes a progressive loss of HSCs during aging. These findings suggest that NHEJ is a key determinant of the ability of HSCs to maintain themselves over time.\\nHematopoietic stem cell diversity aging\\nA study showed that the clonal diversity of stem cells that produce blood cells gets drastically reduced around age 70 to a faster-growing few, substantiating a novel theory of ageing which could enable healthy aging.\\nHematopoietic mosaic loss of chromosome Y\\nA 2022 study showed that blood cells' loss of\"\n },\n {\n \"doc_id\": 4879645114437644000,\n \"text\": \"Criticisms of the mutation accumulation theory[edit]\\nUnder most assumptions, the mutation accumulation theory predicts that mortality rates will reach close to 100% shortly after reaching post-reproductive age. Experimental populations of Drosophila Melanogaster, and other organisms, however, exhibit age-specific mortality rates that plateau well before reaching 100%, making mutation accumulation alone an insufficient explanation. It is suggested instead that mutation accumulation is only one factor among many, which together form the cause of aging. In particular, the mutation accumulation theory, the antagonistic pleiotropy hypothesis and the disposable soma theory of aging are all believed to contribute in some way to senescence.\"\n },\n {\n \"doc_id\": 5148695143890330000,\n \"text\": \"1996). The genetics of pigmentation: From fancy\\ngenes to complex traits. Trends in Genetics, 12, 299–305.\\n5. Blackburn, E. H. (2001). Switching and signaling at the telomere.\\nCell, 106, 661–673.\\n6. Blanpain, C., Horsley, V., & Fuchs, E. (2007). Epithelial stem\\ncells: Turning over new leaves. Cell, 128, 445–458.\\n7. Blanpain, C., Lowry, W. E., Geog\"\n },\n {\n \"doc_id\": 161608011506090560,\n \"text\": \"ch, G., Greco, V., Blanpain, C., Lowry, W. E.,\\nRendl, M., et al. (2004). Defining the epithelial stem cell niche in\\nskin. Science, 303, 359–363.\\n47. Van Zant, G., & Liang, Y. (2003). The role of stem cells in aging.\\nExperimental Hematology, 31, 659–672.\\n48. Veis, D. J., Sorenson, C. M., Shutter, J. R., & K\"\n },\n {\n \"doc_id\": 2871123610651248600,\n \"text\": \" proposed in a 2021 review to be the underlying cause of aging because of the mechanistic link of DNA damage to nearly every aspect of the aging phenotype. DNA damage-induced epigenetic alterations, such as DNA methylation and many histone modifications, appear to be of particular importance to the aging process. Evidence for the theory that DNA damage is the fundamental cause of aging was first reviewed in 1981.\\nIt is believed that the impact of alcohol on aging can be partly explained by alcohol's activation of the HPA axis, which stimulates glucocorticoid secretion, long-term exposure to which produces\"\n },\n {\n \"doc_id\": 8639138980743491000,\n \"text\": \"Aging, Graying and Loss of Melanocyte Stem Cells\\nKavita Y. Sarin & Steven E. Artandi\\nPublished online: 18 August 2007\\n# Humana Press Inc. 2007\\nAbstract Hair graying is one of the prototypical signs of\\nhuman aging. Maintenance of hair pigmentation is dependent on the presence and functionality of melanocytes,\\nneural crest derived cells which synthesize pigment for\\ngrowing hair. The melanocytes, themselves, are maintained\\nby a small number of stem cells which reside in the bulge\\nregion of the hair follicle. The recent characterization of the\\nmelanocyte lineage during aging has significantly accelerated our understanding of how age-related changes in the\\nmelanocyte stem cell compartment contribute to hair\\ngraying. This review will discuss our current understanding\\nof hair graying, drawing on evidence from human and\\nmouse studies, and consider the contribution of melanocyte\\nstem cells to this process. Furthermore, using the melanocyte lineage as an example, it will discuss common theories\\nof tissue and stem cell aging.\\nKeywords Stem cells . Aging . Pigmentation . Graying .\\nMelanocytes. Telomeres. Bcl2 . Vitiligo . Light mutation\\nStem Cells and Aging\\nNormal tissue homeostasis requires continual cellular\\nreplenishment as existing cells are lost due to injury or\\nphysiologic turnover. In order to support this critical demand\\nfor renewal, many mammalian tissues harbor adult stem\\ncells—specialized multipotent cells with the capacity to both\\nself-renew and give rise to differentiated progeny in response\\nto appropriate extrinsic signals. Stem cells have been\\nidentified in a number of tissues including the skin, bone\\nmarrow, intestine, brain, and skeletal muscle and serve as a\\nreservoir for the production of new cells in these organ\\nsystems. In this way, stem cells play a key role in regenerating\\nand maintaining adult tissues over time.\\nPhysiological aging is characterized by a gradual decline\\nin the ability of tissues to maintain homeostasis and regenerate new tissue after injury. For example, the capacity of\\nthe hematopoetic system to mobilize under stress becomes\\nincreasingly impaired with chronological aging and differentiation of hematopoeitic progenitors becomes altered to\\nfavor myeloid rather than lymphoid fates [16, 37]. A\\nreduction in regenerative potential has also been demonstrated in skeletal muscle [14] and skin [17]. Although this\\nphenomenon has been well described in multiple tissues,\\nthe factors contributing to this impaired regenerative\\ncapacity remain largely unknown. In particular, the extent\\nto which changes in stem cell number or function contribute to this impairment remains poorly understood.\"\n },\n {\n \"doc_id\": 2709192573423168500,\n \"text\": \"References\\n1. Allsopp, R. C., Morin, G. B., DePinho, R., Harley, C. B., &\\nWeissman, I. L. (2003). Telomerase is required to slow telomere\\nshortening and extend replicative lifespan of HSCs during serial\\ntransplantation. Blood, 102, 517–520.\\n2. Arck, P. C., Overall, R., Spatz, K., Liezman, C., Handjiski, B.,\\nKlapp, B. F\"\n },\n {\n \"doc_id\": 7947817844724409000,\n \"text\": \" Agley, C. C., Thompson, A. J., Gonzalez, G. A. et al. (2019). Niche stiffness underlies the ageing of central nervous system progenitor cells. Nature 573, 130-134. https://doi.org/10.1038/s41586-019-1484-9\\nGoogle ScholarCrossref\\nShemer, A., Grozovski, J., Tay, T. L., Tao, J., Volaski, A., Süß, P., Ardura-Fabregat, A.,\"\n },\n {\n \"doc_id\": 5627648949841832000,\n \"text\": \" aging was first proposed by Harman in 1956. It posits that free radicals produced by dissolved oxygen, radiation, cellular respiration and other sources cause damage to the molecular machines in the cell and gradually wear them down. This is also known as oxidative stress.\\nThere is substantial evidence to back up this theory. Old animals have larger amounts of oxidized proteins, DNA and lipids than their younger counterparts.\\nChemical damage[edit]\\n\\nThis section may be too long to read and navigate comfortably. Please consider splitting content into sub-articles, condensing it, or adding subhead\"\n },\n {\n \"doc_id\": 853742459037597000,\n \"text\": \"[ Article ](https://doi.org/10.1111%2Fage.12466) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=27453069)\\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5082551) [\\nGoogle Scholar ](http://scholar.google.com/scholar\"\n },\n {\n \"doc_id\": 5812084939865406000,\n \"text\": \" aging\\\".\\nStem cell theories of aging[edit]\\nThis section is an excerpt from Stem cell theory of aging.[edit]\\nThe stem cell theory of aging postulates that the aging process is the result of the inability of various types of stem cells to continue to replenish the tissues of an organism with functional differentiated cells capable of maintaining that tissue's (or organ's) original function. Damage and error accumulation in genetic material is always a problem for systems regardless of the age. The number of stem cells in young people is very much higher than older people and thus creates a better and\"\n },\n {\n \"doc_id\": 5931796567179650000,\n \"text\": \"See also[edit]\\nAging brain\\nAmerican Academy of Anti-Aging Medicine\\nCalico (company)\\nCryptobiosis\\nDNA damage theory of aging\\nMaximum life span\\nMethuselah Foundation\\nReliability theory of aging and longevity\\nRejuvenation (aging)\\nStrategies for engineered negligible senescence (SENS)\\nTelomerase in cancer cell\\nTimeline of senescence research\"\n },\n {\n \"doc_id\": 6728271864373516000,\n \"text\": \"ings. Please discuss this issue on the article's talk page. (March 2023)\\nElderly Klamath woman photographed by Edward S. Curtis in 1924\\n\\nSee also: DNA damage theory of aging\\nOne of the earliest aging theories was the Rate of Living Hypothesis described by Raymond Pearl in 1928 (based on earlier work by Max Rubner), which states that fast basal metabolic rate corresponds to short maximum life span.\\nWhile there may be some validity to the idea that for various types of specific damage detailed below that are by-products of metabolism, all other things being\"\n },\n {\n \"doc_id\": 7614560587774558000,\n \"text\": \"http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6500750) [\\nGoogle Scholar\\n](http://scholar.google.com/scholar_lookup?&title=Analysis%20of%20the%20wnt1%20regulatory%20chromosomal%20landscape&journal=Dev.%20Genes%20Evol.&doi=10.1007%2Fs00427-019-00629-5&volume=229&pages=43-52&publication_\"\n },\n {\n \"doc_id\": 7280379357968796000,\n \"text\": \" E., & DePinho, R. A. (2004). Telomeres, stem cells,\\nsenescence, and cancer. Journal of Clinical Investigation, 113,\\n160–168.\\n40. Slominski, A., & Paus, R. (1993). Melanogenesis is coupled to\\nmurine anagen: Toward new concepts for the role of melanocytes\\nand the regulation of melanogenesis in hair growth. Journal of\\nInvestigative Dermatology, 101, 90S–97S.\\n41. Spradling, A., Drum\"\n },\n {\n \"doc_id\": 5144017881462403000,\n \"text\": \"The disposable soma theory of aging states that organisms age due to an evolutionary trade-off between growth, reproduction, and DNA repair maintenance. Formulated by Thomas Kirkwood, the disposable soma theory explains that an organism only has a limited amount of resources that it can allocate to its various cellular processes. Therefore, a greater investment in growth and reproduction would result in reduced investment in DNA repair maintenance, leading to increased cellular damage, shortened telomeres, accumulation of mutations, compromised stem cells, and ultimately, senescence. Although many models, both animal and human, have appeared to support this theory, parts of it are still controversial.\\nSpecifically, while the evolutionary trade-off between growth and aging has been well established,\\nthe relationship between reproduction and aging is still without scientific consensus, and the cellular mechanisms largely undiscovered.\"\n },\n {\n \"doc_id\": 279991935367356260,\n \"text\": \"mond-Barbosa, D., & Kai, T. (2001). Stem\\ncells find their niche. Nature, 414, 98–104.\\n42. Steingrimsson, E., Copeland, N. G., & Jenkins, N. A. (2005).\\nMelanocyte stem cell maintenance and hair graying. Cell, 121,\\n9–12.\\n43. Takai, H., Smogorzewska, A., & de Lange, T. (2003). DNA\\ndamage foci at dysfunctional telomeres. Current Biology,\"\n },\n {\n \"doc_id\": 6378792526111520000,\n \"text\": \"Definition and characteristics[edit]\\nOrganismal senescence is the aging of whole organisms. Actuarial senescence can be defined as an increase in mortality and/or a decrease in fecundity with age. The Gompertz–Makeham law of mortality says that the age-dependent component of the mortality rate increases exponentially with age.\\nAging is characterized by the declining ability to respond to stress, increased homeostatic imbalance, and increased risk of aging-associated diseases including cancer and heart disease. Aging has been defined as \\\"a progressive deterioration of physiological function,\"\n },\n {\n \"doc_id\": 1008014595189329500,\n \"text\": \"Finally, changes in cellular survival signals could contribute\\nto the loss of melanocyte stem cells seen during hair\\ngraying. Mice deficient for the anti-apoptotic protein Bcl2\\ndemonstrate sudden onset of hair graying after the first hair\\ncycle [48]. Recent characterization of melanocyte stem cells\\nin Bcl2-deficient mice demonstrated that Bcl2 plays a\\ncritical role in the survival of melanocyte stem cells during\\nthe first hair follicle anagen cycle since the loss of\\nmelanocyte stem cells at p8.5 precedes the disappearance\\nof the melanocyte lineage and hair graying [32]. Another\\nstudy suggested that dependence on Bcl2 for survival may\\nbe restricted to melanoblasts in the c-kit-dependent stage\\n[28]. Regardless, both studies demonstrate that changes in\\ncellular survival signals such as Bcl2 can alter melanocyte\\nsurvival and may contribute to the loss of melanocyte stem\\ncells seen with aging.\\nComparison to Aging in other Organs: is Hair Graying\\nUnique among Aging Phenotypes?\\nThe progenitor cell organization seen in the melanocyte\\nlineage is architecturally similar to that seen in other compartments, consisting of three major types of cells: quiescent, self-renewing stem cells, highly proliferative transient\\namplifying cells, and differentiated cells [6]. The changes\\nseen in the melanocyte lineage—markedly reduced numbers of differentiated melanocytes and diminished numbers\\nof melanocyte stem cells—are consistent with a primary\\nmelanocyte stem cell problem leading to loss of the entire\\nlineage. However, it is not clear to what extent the changes in\\nmelanocytes are representative of those in other aging\\ntissues. In contrast to melanocyte stem cells, hematopoeitic\\nstem cells and skeletal muscle stem cells do not significantly\\ndecline in number with aging [36]. Yet these organ systems\\ndemonstrate either a reduced ability to regenerate or abnormal differentiation with advancing age, suggesting that\\nchanges in stem cell function may explain the altered\\nregenerative capacity of aging [47]. Evidence for systemic\\nfactors that contribute to age-related decline come from\\nstudies in which exposure to systemic factors restored\\nregenerative potential of skeletal muscle [13]. Given the\\nmounting evidence that altered stem cell function or reduced\\nstem cell number may contribute to aging, an improved\\nunderstanding of how to regulate tissue stem cells may have\\nsignificant effects on our ability to prevent the eventual\\ndecline in aging tissues.\\nStem Cell Rev (2007) 3:212–217 215\\nSignificant progress has been made in understanding\\nwhat causes age-related hair graying. These data so far\\nsuggest that hair graying occurs primarily due to a gradual\\nloss of the melanocyte lineage rather than due to impaired\\nmelanocyte function. However, the factors that lead to loss\\nof the melanocyte lineage remain poorly understood. It\\nremains possible that age-related changes in melanocyte\\nsensitivity to oxidative species produced during melanogenesis could contribute to an increase in melanocyte\\napoptosis. In support of this idea, one study found increased\\napoptosis in the differentiated melanocytes of aging hair\\nfollicles [2]. However, while this may contribute to loss of\\ndifferentiated melanocytes, it does not explain the corresponding decrease in bulge melanocytes seen in aging hair\\nfollicles, suggesting that other factors must also contribute\\nto loss of melanocyte stem cells. Age-related epigenetic\\nmodifications could trigger the growth arrest of melanocyte\\nstem cells leading to their eventual depletion or alter to\\nniche rendering it incapable of supporting self-renewal [3].\\nAnother possibility is that melanocyte stem cells themselves may have an intrinsically determined lifespan and\\ncan only proliferate for a limited number of cell divisions\\n[39]. Finally, there may be changes in proliferation,\\nsurvival, or differentiation signals in the melanocyte stem\\ncell population that occur with human aging. A more\\ndetailed molecular and genetic analysis of hair graying will\\nclearly facilitate a deeper understanding of aging in other\\ntissues and cell lineages.\"\n },\n {\n \"doc_id\": 4826778915141533000,\n \"text\": \"See also[edit]\\nAnti-aging movement\\nAntimuscarinics\\nDementia\\nDNA repair\\nGeriatrics\\nGerontology\\nHeavy metals\\nHomeostatic capacity\\nImmortality\\nIndex of topics related to life extension\\nMitohormesis\\nOld age\\nPhenoptosis\\nPlant senescence\\nProgrammed cell death\\nStrategies for engineered negligible senescence (SENS)\\nSub-lethal damage\\nTransgenerational design\\nTimeline of senescence research\"\n },\n {\n \"doc_id\": 2791557587399947300,\n \"text\": \", O., & Bernard, B. A. (2004). Human hair\\ngreying is linked to a specific depletion of hair follicle\\nmelanocytes affecting both the bulb and the outer root sheath.\\nBritish Journal of Dermatology, 150, 435–443.\\n13. Conboy, I. M., Conboy, M. J., Wagers, A. J., Girma, E. R.,\\nWeissman, I. L., & Rando, T. A. (2005). Rejuvenation of aged\\nprogenitor cells by exposure\"\n },\n {\n \"doc_id\": 7443190492468470000,\n \"text\": \"Background and history[edit]\\nBritish biologist Thomas Kirkwood first proposed the disposable soma theory of aging in a 1977 Nature review article. The theory was inspired by Leslie Orgel's Error Catastrophe Theory of Aging, which was published fourteen years earlier, in 1963. Orgel believed that the process of aging arose due to mutations acquired during the replication process, and Kirkwood developed the disposable soma theory in order to mediate Orgel's work with evolutionary genetics.\"\n },\n {\n \"doc_id\": 4951364018294375000,\n \"text\": \"©2009 Macmillan Publishers Limited. All rights reserved\\n5. Kussell, E. & Leibler, S. Phenotypic diversity, population growth, and information in\\nfluctuating environments. Science 309, 2075–2078 (2005).\\n6. Acar, M., Mettetal, J. T. & van Oudenaarden, A. Stochastic switching as a survival\\nstrategy in fluctuating environments. Nature Genet. 40, 471–475 (2008).\\n7. Balaban, N. Q. et al\"\n },\n {\n \"doc_id\": 5546822385480442000,\n \"text\": \"slcc.pressbooks.pub/collegebiology1/chapter/regulator-molecules-of-the-cell-cycle/#chapter-912-section-3)\\n3. [ 17.3 Cancer ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer/)\"\n },\n {\n \"doc_id\": 4444909518440718000,\n \"text\": \"Genetic determinants of aging[edit]\\n\\nMain article: Genetics of aging\\nA number of genetic components of aging have been identified using model organisms, ranging from the simple budding yeast Saccharomyces cerevisiae to worms such as Caenorhabditis elegans and fruit flies (Drosophila melanogaster). Study of these organisms has revealed the presence of at least two conserved aging pathways.\\nGene expression is imperfectly controlled, and it is possible that random fluctuations in the expression levels of many genes contribute to the aging process as suggested by a study of such\"\n },\n {\n \"doc_id\": 8192270329965011000,\n \"text\": \"Mouse Models of Hair Graying\\nDespite the recent evidence that hair graying is caused at\\nleast in part by a decline in the number of melanocyte stem\\ncells, it is unclear what factors cause the decline in stem\\ncells seen with hair graying. Studies in mice have evoked a\\nnumber of factors that could contribute to the loss of the\\nmelanocyte lineage.\\nAre melanocytes disappearing secondary to oxidative\\ndamage to the melanocytic compartment? Do stem cells\\nhave a limited lifespan and undergo replicative senescence\\nafter a certain number of divisions? Are there age related\\nchanges in cell survival signals that contribute to the\\ndecline in stem cells? Are there changes in the specialized\\nniche environment that occur with aging such that the niche\\ncan no longer can support stem cells? Or do changes in the\\ndelicate balance of self-renewal and differentiation occur\\nwith aging, which leads to the depletion of the melanocytic\\nstem cells? While the answers to these questions remain\\nunknown, there have been a number of mouse model\\nsystems suggesting that many of these factors may contribute to age-related hair graying.\\nLight Mice: Evidence for Oxidative Damage\\nto Melanocytes\\nOne cause of the decline in the melanocyte lineage during\\nhair graying may be due to cell death caused by oxidative\\ntoxicity associated with melanin biosynthesis [19, 48]. In\\nsupport of this, H2O2 has been shown to induce senescence\\nin cultured fibroblasts in vitro. Furthermore, in vivo\\nevidence for the role of reactive oxidative species in hair\\ngraying came from mice harboring a dominant mutant\\nallele of TRP-1, which results in the loss of the melanocyte\\nlineage and pigmentation in C57/Bl6 mice [19]. Interestingly, melanocyte loss was not detected in albino mice\\n214 Stem Cell Rev (2007) 3:212–217\\ncarrying the same mutation. These data suggested that the\\nloss of melanocytes required melanin synthesis to occur and\\nwas likely caused by the generation of toxic oxidative\\nproducts that occurred during melanin synthesis.\\nTelomerase-Deficient Mice: Telomere Shortening Enhances\\nGraying at Advanced Age\\nTelomeres represent the nucleoprotein structures that\\nprotect the ends of linear chromosomes. Because of the\\ninability of DNA polymerase to replicate fully the ends of\\nchromosomes, telomeres shorten in the absence of sufficient telomerase, the enzyme that adds DNA repeats to\\nchromosome ends [5]. Telomeres shorten as many human\\ncells divide in culture and also shorten in human tissues\\nwith advancing age. Telomere shortening can induce\\ncellular senescence or programmed cell death, as the\\nprotective cap that constitutes the telomere is lost leading\\nto a DNA damage response at the telomere end [15, 43] and\\nactivation of the tumor suppressor protein p53 [11, 20].\\nAnalysis of telomerase knockout mice has revealed that\\ncritical telomere shortening impairs the function of proliferative tissues and blocks self-renewal of hematopoietic\\nstem cells [1, 25]. There is evidence that telomere shortening causes premature aging in telomerase-deficient mice,\\nwhich includes increased hair graying compared to controls\\n[10, 38]. These data indicate that telomere length could play\\na limiting role in the ability of cells in the melanocyte\\nlineage cells to regenerate. It is not yet known if hair\\ngraying in the context of shortened telomeres is due to\\ndiminished melanocytes, and if so, whether melanocyte\\nstem cell loss or stem cell dysfunction contributes to\\ngraying in this context.\\nVitiligo Mutant Mice: Altered Differentiation\\nof Melanocyte Progenitors\\nGradual hair pigmentation loss has also been demonstrated\\nin mice that harbor a hypomorphic point mutation in the\\nDNA binding region of the essential melanocyte lineage\\ntranscription factor, MITF. The Mitfvit/vit mutant mice\\nundergo gradual progressive hair graying due to a reduction\\nmelanocyte number and become completely depigmented at\\n6–12 months postnatally [18, 23, 24, 26]. While the exact\\netiology is unclear, MITF has been shown to regulate a\\nnumber of enzymes needed for pigmentation such as\\ntyrosinase, TRP1, Dct, Pmel 17 and the anti-apoptotic\\nfactor Bcl2 [29]. In addition, ectopic pigmentation was\\ndetected in the melanocytic niche in these mutant mice\\nsuggesting that the decline in melanocyte stem cells could\\noccur due to premature differentiation [32]. Mutations in\\nMITF have also been detected in premature graying\\nconditions in humans such as Waardenberg Syndrome, an\\ninherited disorder characterized by loss of skin and hair\\npigmentation, as well as hearing loss.\\nBcl2−/− Mice: Bcl2 is Required for Survival of Melanocyte\\nProgenitors\"\n },\n {\n \"doc_id\": 8782227544835548000,\n \"text\": \"/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1101/cshperspect.a016154 ](https://doi.org/10.1101%2Fcshperspect.a016154). [ PMC ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 3949356 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3949356). [ PMID ](/wiki\"\n },\n {\n \"doc_id\": 9209714122028345000,\n \"text\": \"Theories of aging[edit]\\nThis section needs expansion. You can help by adding to it. (March 2023)\\nMore than 300 different theories have been posited to explain the nature (mechanisms) and causes (reasons for natural emergence or factors) of aging. Good theories would both explain past observations and predict the results of future experiments. Some of the theories may complement each other, overlap, contradict, or may not preclude various other theories.\\nTheories of aging fall into two broad categories, evolutionary theories of aging and mechanistic theories of aging. Evolutionary\"\n },\n {\n \"doc_id\": 1285922490615968500,\n \"text\": \"alia S. (April 2010). \\\"Demographic Consequences of Defeating Aging\\\". Rejuvenation Research. 13 (2–3): 329–334. doi:10.1089/rej.2009.0977. ISSN 1549-1684. PMC 3192186. PMID 20426616.\\n\\n^ \\\"Scientists' Open Letter on Aging\\\". Retrieved 20 April 2015.\\n\\n^ \\\"A Single-Issue Political Party for Longevity Science\\\". Fight Aging!. 27 July 2012. Retrieved 20 April 2015.\\n\\n^ Pent\"\n },\n {\n \"doc_id\": 3895641590567795700,\n \"text\": \" ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3839606). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 24296171 ](https://pubmed.ncbi.nlm.nih.gov/24296171). [ Archived ](https://web.archive.org/web/20220913054521/https://cshperspectives.cshlp.org/content/5/12/\"\n },\n {\n \"doc_id\": 4976934889088200000,\n \"text\": \"[ PubMed Abstract\\n](http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=21549330)\\n| [ CrossRef Full Text ](https://doi.org/10.1016/j.stem.2011.03.010) | [\\nGoogle Scholar ](http://scholar.google.com/scholar_lookup?&title=Division-\\ncoupled+astrocytic+differentiation+and+age-\\nrelated+\"\n },\n {\n \"doc_id\": 7639397133823105000,\n \"text\": \"Search PMC Full-Text Archive\\nSearch PMC Full-Text Archive\\nSearch in PMC\\nAdvanced Search User Guide\\nJournal List PLoS Genet v.12(7); 2016 Jul PMC4956268\\nAs a library, NLM provides access to scientific literature. Inclusion in an NLM database does not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of Health.\\nLearn more: PMC Disclaimer | PMC Copyright Notice\\nLogo of plosgen\\nPLoS Genet. 2016 Jul; 12\"\n },\n {\n \"doc_id\": 5412326387684365000,\n \"text\": \"Ubiquity[edit]\\nAlthough there are so many negative effects related to genes that are antagonistically pleiotropic, it is still present among most forms of life. Indeed, pleiotropy is one of the most common traits possessed by genes overall. In addition to that, pleiotropy is under strong stabilizing selection. In one experiment with mice and the morphology of the mandible, 1/5 of the loci had effects of pleiotropy for the entire mandible. One other example was in the Russian biologist Dmitry K. Belyaev's study on the domestication of the fox. In Dmitry K. Belyaev's farm-fox experiment, wild foxes were bred for docile behavior alone. After 40 generations, other physiological changes had surfaced including shortened tails, floppy ears, a white star in the forehead, rolled tails, shorter legs. Since the only thing being selected for was behavior, this leads scientists to believe that these secondary characteristics were controlled by the same gene or genes as docile behavior.\"\n },\n {\n \"doc_id\": 1648645431169788400,\n \"text\": \" Jülicher, F. (2021). Theory of time delayed genetic oscillations with external noisy regulation. New J. Phys. 23, 033030. https://doi.org/10.1088/1367-2630/abd80b\\nGoogle ScholarCrossref\\nNett, I. R. E., Mulas, C., Gatto, L., Lilley, K. S. and Smith, A. (2018). Negative feedback via RSK modulates Erk-dependent progression from naïve pluripotency. EMBO Rep. 19,\"\n },\n {\n \"doc_id\": 7555524859195046000,\n \"text\": \" T cells have been used to give good predictions of the expected lifespan of middle-aged mice.\\nAging clocks[edit]\\n\\nThis section needs expansion. You can help by adding to it. (March 2023)\\nThere is interest in an epigenetic clock as a biomarker of aging, based on its ability to predict human chronological age. Basic blood biochemistry and cell counts can also be used to accurately predict the chronological age. It is also possible to predict the human chronological age using the transcriptomic aging clocks.\\nThere is research and development of further biomarkers, detection\"\n },\n {\n \"doc_id\": 7566684989972600000,\n \"text\": \" may cause exhaustion of stem cells.\\nHematopoietic stem cell aging\\nHematopoietic stem cells (HSCs) regenerate the blood system throughout life and maintain homeostasis. DNA strand breaks accumulate in long term HSCs during aging. This accumulation is associated with a broad attenuation of DNA repair and response pathways that depends on HSC quiescence. DNA ligase 4 (Lig4) has a highly specific role in the repair of double-strand breaks by non-homologous end joining (NHEJ). L\"\n },\n {\n \"doc_id\": 410698984248633150,\n \"text\": \"Peccei JS (2001). \\\"Menopause: Adaptation or epiphenomenon?\\\". Evolutionary Anthropology. 10 (2): 43–57. doi : 10.1002/evan.1013. S2CID 1665503.\\n\\nMarziali C (7 December 2010). \\\"Reaching Toward the Fountain of Youth\\\". USC Trojan Family Magazine. Archived from the original on 13 December 2010. Retrieved 7 December 2010.\"\n },\n {\n \"doc_id\": 5886191137619035000,\n \"text\": \"Hair graying, one of the prototypical signs of human\\naging, is caused by a progressive loss of pigmentation from\\ngrowing hair shafts. In normal aging, the onset of hair\\ngraying occurs at 34±9.6 years of age in Caucasians and\\n43.9±10.3 years in African Americans, a period in which\\nhair follicles pass through approximately 7–15 complete\\ncycles [45]. Hair graying represents an impaired ability of\\nmelanocytes to maintain normal homeostasis and replenish\\nmelanin, pigment for the newly growing hair. Until recently,\\nit was largely unknown what contributes to the loss of hair\\npigmentation seen during human aging. The recent identification of melanocyte stem cells and characterization of\\nchanges in the melanocyte lineage during aging have\\nprovided new insights into how alterations in stem cell\\nfunction and number contribute to the hair graying seen\\nwith physiological aging.\\nStem Cell Rev (2007) 3:212–217\\nDOI 10.1007/s12015-007-0028-0\\nK. Y. Sarin : S. E. Artandi (*)\\nDepartment of Medicine, Stanford University School of Medicine,\\nStanford, CA 94305, USA\\ne-mail: sartandi@stanford.edu\\nIn this review, we will discuss the function of melanocyte\\nstem cells in hair pigmentation and explore the relationship\\nbetween the age-related changes in melanocyte stem cell\\ncompartment and hair graying. Using the example of the\\nmelanocyte lineage, we will consider common theories of\\naging stem cells and suggest future avenues for investigation.\\nMelanocyte Stem Cells\\nMelanocytes are specialized neural crest-derived cells,\\nwhich synthesize and transfer pigment to neighboring\\nkeratinocytes. In humans, melanocytes are located throughout the skin, including the epidermis, dermis, and hair\\nfollicle, as well as in other organ systems such as the uvea\\nof the eye and the inner ear. In mouse skin, melanocytes are\\nprimarily restricted to the hair follicle, although they are\\nalso found in the epidermis and dermis of hairless regions\\nsuch as murine ear, ventral paws, and tail.\\nDuring embryogenesis, melanocyte precursor cells, called\\nmelanoblasts, arise from neural crest and migrate through\\nthe dermis and epidermis into newly developing hair\\nplacodes. The migration and survival of these melanoblasts\\nare dependent on c-kit and its ligand stem cell factor (SCF),\\nas neonatal deletion of either SCF or c-kit results in an\\nirreversible loss of coat pigmentation [9, 27, 30, 31, 49].\"\n },\n {\n \"doc_id\": 680403308465968500,\n \"text\": \"1734.\\\")\\nTuberous sclerosis proteins 1 and 2 (TSC1/2) inhibit mTOR, and HIF1A is\\nstrongly upregulated in _Tsc1−/−_ neuroblasts, thereby increasing their\\nsurvival. [ 77 ](/articles/cddis201764#ref-CR77 \\\"Feliciano DM, Zhang S, Quon\\nJL, Bordey A. Hypoxia-inducible factor 1a is a Tsc1-regulated survival factor\\nin newborn neurons in tuberous sclerosis complex.\"\n },\n {\n \"doc_id\": 5870541505121028000,\n \"text\": \"[ Article ](https://doi.org/10.1016%2Fj.ijdevneu.2014.04.004) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=24798024)\\n[ Google Scholar\\n](http://scholar.google.com/scholar_lookup?&title=The%20apoptotic%20perspective%20of%20\"\n },\n {\n \"doc_id\": 8504326372591973000,\n \"text\": \"2E+Perspect%2E+Biol%2E&author=Ganz+J.&author=Brand+M.&publication_year=2016&volume=8&issue=a019018)\"\n },\n {\n \"doc_id\": 2275480560330257200,\n \"text\": \", A., Müller, F.-J., Babtie, A., Chan, T. E., Stumpf, M. P. H., Please, C. P., Howison, S. D. et al. (2017). Stem cell differentiation as a non-markov stochastic process. Cell Syst. 5, 268-282.e7. https://doi.org/10.1016/j.cels.2017.08.009\\nGoogle ScholarCrossref\\nWagner, D. E. and Klein, A. M.\"\n },\n {\n \"doc_id\": 7107188148454427000,\n \"text\": \" more efficient replacement mechanism in the young contrary to the old. In other words, aging is not a matter of the increase in damage, but a matter of failure to replace it due to a decreased number of stem cells. Stem cells decrease in number and tend to lose the ability to differentiate into progenies or lymphoid lineages and myeloid lineages.\\nMaintaining the dynamic balance of stem cell pools requires several conditions. Balancing proliferation and quiescence along with homing (See niche) and self-renewal of hematopoietic stem cells are\"\n },\n {\n \"doc_id\": 3990271081171909000,\n \"text\": \"Biological immortality (sometimes referred to as bio-indefinite mortality) is a state in which the rate of mortality from senescence is stable or decreasing, thus decoupling it from chronological age. Various unicellular and multicellular species, including some vertebrates, achieve this state either throughout their existence or after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury, poison, disease, predation, lack of available resources, or changes to environment.\\nThis definition of immortality has been challenged in the Handbook of the Biology of Aging, because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high.\\nThe term is also used by biologists to describe cells that are not subject to the Hayflick limit on how many times they can divide.\"\n },\n {\n \"doc_id\": 7686507863959127000,\n \"text\": \"[\\n](/cddis)\"\n },\n {\n \"doc_id\": 6198489821588383000,\n \"text\": \"rez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=25409825)\\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4405208) [\\nGoogle Scholar\\n](http://scholar.google.com/scholar_lookup?&title=Conservation%20of%20trans-\\nacting%20circuitry%20during%20mammalian%20regulatory%20evolution&\"\n },\n {\n \"doc_id\": 678460758480640400,\n \"text\": \"Variation among species[edit]\\n\\nFurther information: Longevity § Non-human biological longevity\\nDifferent speeds with which mortality increases with age correspond to different maximum life span among species. For example, a mouse is elderly at 3 years, a human is elderly at 80 years, and ginkgo trees show little effect of age even at 667 years.\\nAlmost all organisms senesce, including bacteria which have asymmetries between \\\"mother\\\" and \\\"daughter\\\" cells upon cell division, with the mother cell experiencing aging, while the daughter is rejuvenated. There is negligible senescence in some groups, such as the genus Hydra. Planarian flatworms have \\\"apparently limitless telomere regenerative capacity fueled by a population of highly proliferative adult stem cells.\\\" These planarians are not biologically immortal, but rather their death rate slowly increases with age. Organisms that are thought to be biologically immortal would, in one instance, be Turritopsis dohrnii, also known as the \\\"immortal jellyfish\\\", due to its ability to revert to its youth when it undergoes stress during adulthood. The reproductive system is observed to remain intact, and even the gonads of Turritopsis dohrnii are existing.\\nSome species exhibit \\\"negative senescence\\\", in which reproduction capability increases or is stable, and mortality falls with age, resulting from the advantages of increased body size during aging.\"\n },\n {\n \"doc_id\": 5902401109933815000,\n \"text\": \"[ Article ](https://doi.org/10.1111%2Fage.12717) [ CAS ](/articles/cas-\\nredirect/1:CAS:528:DC%2BC1cXitFyhtr%2FE) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=30311254)\\n[ PubMed Central ](http://www.ncbi\"\n },\n {\n \"doc_id\": 1200511620083040300,\n \"text\": \" systems and software systems to measure biological age of different tissues or systems or overall. For example, a deep learning (DL) software using anatomic magnetic resonance images estimated brain age with relatively high accuracy, including detecting early signs of Alzheimer's disease and varying neuroanatomical patterns of neurological aging, and a DL tool was reported as to calculate a person's inflammatory age based on patterns of systemic age-related inflammation.\\nAging clocks have been used to evaluate impacts of interventions on humans, including combination therapies.\"\n },\n {\n \"doc_id\": 1179232220599721500,\n \"text\": \"1. [ Cancer and the Cell Cycle ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer-and-the-cell-cycle/#chapter-915-section-1)\\n2. [ Proto-oncogenes ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer-and-the-cell-cycle/#chapter-915-section-2)\\n3. [ Tumor Suppressor Genes ](https://slcc.pressbooks.pub/collegebiology1/chapter/cancer\"\n },\n {\n \"doc_id\": 1133625993065273900,\n \"text\": \"4+ T cells and degeneration of the spiral ganglion in Samp1 mice, a murine model of human senescence. This technology remains years or even decades away from human application.\"\n },\n {\n \"doc_id\": 8349157190851547000,\n \"text\": \"ichia coli strain MG1655: Growth\\ndefects and apparent cross-regulation of gene expression. J. Bacteriol. 185,\\n5611–5626 (2003).\\n14. Oxman, E., Alon, U. & Dekel, E. Defined order of evolutionary adaptations:\\nexperimental evidence. Evolution 62, 1547–1554 (2008).\\n15. Mortimer, R. K. Evolution and variation of the yeast (Saccharomyces) genome.\\nGenome Res. 10, 403–409 (2000).\\n16\"\n },\n {\n \"doc_id\": 962550441146100100,\n \"text\": \", when modern statistical methods for correcting for the effects of body size and phylogeny are employed, metabolic rate does not correlate with longevity in mammals or birds.\\nWith respect to specific types of chemical damage caused by metabolism, it is suggested that damage to long-lived biopolymers, such as structural proteins or DNA, caused by ubiquitous chemical agents in the body such as oxygen and sugars, are in part responsible for aging. The damage can include breakage of biopolymer chains, cross-linking of biopolymers, or chemical attachment of unnatural substituents (haptens) to\"\n },\n {\n \"doc_id\": 5995368045220096000,\n \"text\": \"://www.ncbi.nlm.nih.gov/pmc/articles/PMC2898526) [\\nGoogle Scholar\\n](http://scholar.google.com/scholar_lookup?&title=Simple%20combinations%20of%20lineage-\\ndetermining%20transcription%20factors%20prime%20cis-\\nregulatory%20elements%20required%20for%20macrophage%20and%20B%20cell%20identities&journal=Mol.%20\"\n },\n {\n \"doc_id\": 4517178045387123000,\n \"text\": \"] [CrossRef] [Google Scholar]\\n29. Siklenka K, Erkek S, Godmann M, Lambrot R, McGraw S, et al. Disruption of histone methylation in developing sperm impairs offspring health transgenerationally. Science. 2015;350:aab2006 10.1126/science.aab2006 [PubMed] [CrossRef] [Google Scholar]\\n30. Zordan R, Miller M, Galgoczy D, Tuch B, Johnson A. Interlocking transcriptional feedback loops control white-opaque\"\n },\n {\n \"doc_id\": 1646067418514023400,\n \"text\": \"As each model has important implications for the underlying biology, it is necessary to test the extent to which the underlying assumptions are valid. For example, describing a biological process as a phase transition implies cells must go through a critical point, characterised by the appearance of power-law patterns (i.e. when one variable changes as a power of another, independent of the initial conditions) and sharp increases in variance/disorder, as shown recently in tissue remodelling (Petridou et al., 2021). Similarly, describing a transition as a Markov-chain process (Wheat et al.,\"\n },\n {\n \"doc_id\": 5742440580135641000,\n \"text\": \"^ Walker, Peter (1991). International Search and Rescue Teams. League Discussion Paper. Geneva, CH: League of the Red Cross and Red Crescent Societies.\\n\\n^ \\\"What is Aging?\\\". Retrieved 6 November 2020.\\n\\n^ Lin Kah Wai (18 April 2004). \\\"Telomeres, Telomerase, and Tumorigenesis – A Review\\\". MedGenMed. 6 (3): 19. PMC 1435592. PMID 15520642.\\n\\n^ Wade, Nicholas (26 January 2017). \\\"New Prospects for Growing Human Replacement Or\"\n },\n {\n \"doc_id\": 3835360740815392300,\n \"text\": \"age tracing on transcriptional landscapes links state to fate during differentiation. Science 367, eaaw3381. https://doi.org/10.1126/science.aaw3381\\nGoogle ScholarCrossref\\nWheat, J. C., Sella, Y., Willcockson, M., Skoultchi, A. I., Bergman, A., Singer, R. H. and Steidl, U. (2020). Single-molecule imaging of transcription dynamics in somatic stem cells. Nature 583, 431-436. https://doi\"\n },\n {\n \"doc_id\": 8675078867840300000,\n \"text\": \" ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce. On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer.\\nSuch cooperation within species may have evolved through the process of kin selection\"\n },\n {\n \"doc_id\": 3677536378871693000,\n \"text\": \"., & de Lange, T.\\n(1999). p53-and ATM-dependent apoptosis induced by telomeres\\nlacking TRF2. Science, 283, 1321–1325.\\n21. Kiger, A. A., Jones, D. L., Schulz, C., Rogers, M. B., & Fuller, M.\\nT. (2001). Stem cell self-renewal specified by JAK-STAT\\nactivation in response to a support cell cue. Science, 294, 2542–\\n2545.\\n22. Kunis\"\n }\n]","isConversational":false}}},{"rowIdx":94,"cells":{"query":{"kind":"string","value":"In the context of human vision, myopia, or nearsightedness, is a condition where the eye can see nearby objects clearly, but distant objects appear blurry. This is due to a refractive error where the focal point of the light entering the eye falls in front of the retina instead of directly on it. However, I'm curious about the mechanics behind this. Why does the focal point issue in myopia only affect distant objects and not close ones? Shouldn't the refractive error cause issues with focusing on objects at all distances? How does the eye adjust to focus on nearby objects despite the refractive error? I'm looking for a detailed explanation based on the principles of optics and the anatomy of the eye. Thanks!"},"positive_passages":{"kind":"list like","value":[{"doc_id":4660484379006952000,"text":"For a myope, the refraction power is already too high to focus distant object. As the object gets closer, there will be point where the (excess for distance) refraction is just right. Objects beyond this point will be out-of-focus. At & within this point, the eye"},{"doc_id":8388844947352058000,"text":"Profile photo for Brian Park\nBrian Park\nBS in Electrical Engineering (BSEE) & Physics, Optics, Mechanics, University of Akron (Graduated 1973)Author has 378 answers and 136.6K answer views\n·\n1y\nRelated\nWhy does the eye lense refraction error of myopia, issue of focal point being before the retina, only occur when objects are far away. Why shouldn't it occur regardless of whether an object is near or far?\nActually, for every different object distance a DIFFERENT refraction power is required. The minimum refraction"},{"doc_id":1815057626218135600,"text":"Profile photo for Bill Otto\nBill Otto\nTop Writer Eyesight & Optometry at QuoraAuthor has 8.2K answers and 66.2M answer views\n·\n1y\nRelated\nWhy does the eye lense refraction error of myopia, issue of focal point being before the retina, only occur when objects are far away. Why shouldn't it occur regardless of whether an object is near or far?\nImage distance is a function of object distance. Closer objects always focus further away from the lens. It is possible for an object to be close enough that it happens"},{"doc_id":997536635586971800,"text":"Myopia, or nearsightedness, is a vision condition in which close objects are\nseen clearly, but objects farther away appear blurred. Nearsightedness occurs\nif the eyeball is too long (axial myopia) or the cornea has too much curvature\n(refractive myopia). As a result, the light entering the eye is not focused\ncorrectly and distant objects look blurred.\n\nNearsightedness is a very common vision condition affecting nearly 30 percent\nof the US and EU population. Some research supports the theory that\nnearsight"},{"doc_id":1743566479811413800,"text":"Focusing mechanisms[edit]\nThe path of light through the eye calculated using four refractive indexes, cornea and lens curvatures approximating components of real eyes. Note objects in some size ranges and distances do not require the light path to bend noticeably to achieve focus.\nFocusing the light scattered by objects in a three dimensional environment into a two dimensional collection of individual bright points of light requires the light to be bent. To get a good image of these points of light on a defined area requires a precise systematic bending of light called refraction. The real image formed from millions of these points of light is what animals see using their retinas. Very even systematic curvature of parts of the cornea and lens produces this systematic bending of light onto the retina.\nVirtual eye showing the contribution to focus of different components.\nDue to the nature of optics the focused image on the retina is always inverted relative to the object.\nDifferent animals live in different environments having different refractive indexes involving water, air and often both. The eyes are therefor required to bend light different amounts leading to different mechanisms of focus being used in different environments. The air/cornea interface involves a larger difference in refractive index than hydrated structures within the eye. As a result, animals living in air have most of the bending of light achieved at the air/cornea interface with the lens being involved in finer focus of the image.\n3D reconstruction based on measurements taken from a 20 year old human male focusing from 26mm to infinity (4.85 dioptre change). Side & back views shown. Most image distortions near the center are due to model being limited to 512 faces to make up the lens. Peripheral distortions are also present in animal lenses though are typically covered by the iris (anatomy)\nThe previous video of the eye lens changing shape with focus is placed into context as the lens in this video is placed into the context of a virtual eye.\nGenerally mammals, birds and reptiles living in air vary their eyes' optical power by subtly and precisely changing the shape of the elastic lens using the ciliary body.\nThe small difference in refractive index between water and the hydrated cornea means fish and amphibians need to bend the light more using the internal structures of the eye. Therefore, eyes evolved in water have a mechanism involving changing the distance between a rigid rounder more refractive lens and the retina using less uniform muscles rather than subtly changing the shape of the lens itself using circularly arranged muscles.\nLand based animals and the shape changing lens[edit]\nVarying forms of direct experimental proof outlined in this article show that most non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of their lenses.\nWhat is less well understood is how the subtle, precise and very quick changes in lens shape are made. Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. When considering vertebrates, aspects of all models may play varying roles in lens focus. The models can be broadly divided into two camps. Those models that stress the importance of external forces acting on a more passively elastic lens and other models that include forces that may be generated by the lens internally.\nExternal forces[edit]\nThe model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Others such as Helmholtz and Huxley refined the model in the mid-1800s explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by Helmholtz in 1909. The model may be summarized like this. Normally the lens is held under tension by its suspending ligaments and capsule being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts, stretching the ciliary body and relieving some of the tension on the suspensory ligaments, allowing the lens to elastically round up a bit, increasing refractive power. Changing focus to an object at a greater distance requires a thinner less curved lens. This is achieved by relaxing some of the sphincter like ciliary muscles allowing the ciliarly body to spring back, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried none has come close enough to working using only the Helmholtz mechanisms.\nSchachar model of lens focus\nSchachar has proposed a model for land based vertebrates that was not well received. The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles, while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling.\nThe \"catenary\" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change its curve when the poles are moved closer together. This model requires precise fluid movement of the lens front only rather than trying to change the shape of the lens as a whole. While this concept may be involved in the focusing it has been shown by Scheimpflug photography that the rear of the lens also changes shape in the living eye.\nInternal forces[edit]\nTracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant\nWrinkled lens fibers in picture below compared to straight fibers above\nWhen Thomas Young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. This type of model is termed intracapsular accommodation as it relies on activity within the lens. In a 1911 Nobel lecture Allvar Gullstrand spoke on \"How I found the intracapsular mechanism of accommodation\" and this aspect of lens focusing continues to be investigated. Young spent time searching for the nerves that could stimulate the lens to contract without success. Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence. Pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. Membrane proteins such as aquaporins which allow water to flow into and out of cells are the most abundant membrane protein in the lens. Connexins which allow electrical coupling of cells are also prevalent. Electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens. As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.\nHuman eyes[edit]\nDuane's classical curves showing the amplitude or width of accommodation as changing with age. Mean (B) and approximate lower (A) and upper (C) standard deviations are shown.\nThe young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye. This dramatic change in focal power of the eye of approximately 15 dioptres (the reciprocal of focal length in metres) occurs as a consequence of a reduction in zonular tension induced by ciliary muscle contraction. This process can occur in as little as 224 ± 30 milliseconds in bright light. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude can decline so that the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are emmetropic (i.e., do not require optical correction for distance vision) will need an optical aid for near vision; those who are myopic (nearsighted and require an optical correction for distance or far vision), will find that they see better at near without their distance correction; and those who are hyperopic (farsighted) will find that they may need a correction for both distance and near vision. Note that these effects are most noticeable when the pupil is large; i.e. in dim light. The age-related decline in accommodation occurs almost universally to less than 2 dioptres by the time a person reaches 45 to 50 years, by which time most of the population will have noticed a decrease in their ability to focus on close objects and hence require glasses for reading or bifocal lenses. Accommodation decreases to about 1 dioptre at the age of 70 years. The dependency of accommodation amplitude on age is graphically summarized by Duane's classical curves.\nTheories on how humans focus[edit]\nHelmholtz—The most widely held theory of accommodation is that proposed by Hermann von Helmholtz in 1855. When viewing a far object, the circularly arranged ciliary muscle relaxes allowing the lens zonules and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera. When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.\nSchachar—Ronald A. Schachar has proposed in 1992 what has been called a \"rather bizarre geometric theory\" which claims that focus by the human lens is associated with increased tension on the lens via the equatorial zonules; that when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. The increased equatorial zonular tension keeps the lens stable and flattens the peripheral lens surface during accommodation. As a consequence, gravity does not affect the amplitude of accommodation and primary spherical aberration shifts in the negative direction during accommodation. The theory has not found much independent support.\nCatenary—D. Jackson Coleman proposes that the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape. It is in this lens shape that the mechanically reproducible state of a steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross section, of a catenary occurs. The anterior capsule and the zonule form a trampoline shape or hammock shaped surface that is totally reproducible depending on the circular dimensions, i.e. the diameter of the ciliary body (Müeller's muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need to support an equatorial traction force to flatten the lens.\nInduced effects of accommodation[edit]\nWhen humans accommodate to a near object, they also converge their eyes and constrict their pupils. The combination of these three movements (accommodation, convergence and miosis) is under the control of the Edinger-Westphal nucleus and is referred to as the near triad, or accommodation reflex. While it is well understood that proper convergence is necessary to prevent diplopia, the functional role of the pupillary constriction remains less clear. Arguably, it may increase the depth of field by reducing the aperture of the eye, and thus reduce the amount of accommodation needed to bring the image in focus on the retina.\nThere is a measurable ratio (Matthiessen's ratio) between how much convergence takes place because of accommodation (AC/A ratio, CA/C ratio). Abnormalities with this can lead to binocular vision problems.\nAnomalies of accommodation described in humans[edit]\nThere are many types of accommodation anomalies. It can be broadly classified into two, decreased accommodation and increased accommodation. Decreased accommodation may occur due to physiological (presbyopia), pharmacological (cycloplegia) or pathological. Excessive accommodation and spasm of accommodation are types of increased accommodation.\nPresbyopia[edit]\nPresbyopia, physiological insufficiency of accommodation due to age related changes in lens (decreased elasticity and increased hardness) and ciliary muscle power is the commonest form of accommodative dysfunction. It will cause gradual decrease in near vision.\nAccommodative insufficiency[edit]\nAccommodative insufficiency is the condition where amplitude of accommodation of a person is lesser compared to physiological limits for their age. Premature sclerosis of lens or ciliary muscle weaknesses due to systemic or local cases may cause accommodative insufficiency.\nAccommodative insufficiency is further categorised into different categories.\nIll-sustained accommodation[edit]\nIll-sustained accommodation is a condition similar to accommodative insufficiency. In this, range of accommodation will be normal, but after excessive near work accommodative power will decrease.\nParalysis of accommodation[edit]\nIn paralysis of accommodation, amplitude of accommodation is either markedly reduced or completely absent (cycloplegia). It may occur due to ciliary muscle paralysis or occulomotor nerve paralysis. Parasympatholytic drugs like atropine will also cause paralysis of accommodation.\nUnequal accommodation[edit]\nIf there is amplitude of accommodation between the eyes differ 0.5 dioptre or more, it is considered as unequal. Organic diseases, head trauma or functional amblyopia may be responsible for unequal accommodation.\nAccommodative infacility[edit]\nAccommodative infacility is also known as accommodative inertia. In this condition there will be difficulty in changing accommodation from one point to other. There may be difficulty in adjusting focus from distance from near. It is a comparatively rare condition.\nSpasm of accommodation[edit]\nSpasm of accommodation also known as ciliary spasm is a condition of abnormally excessive accommodation which is out of voluntary control of the person. Vision may be blurred due to induced pseudomyopia.\nAccommodative excess[edit]\nAccommodative excess occurs when an individual uses more than normal accommodation for performing certain near work. Modern definitions simply regard it as an inability to relax accommodation readily.\nAquatic animals[edit]\nDiving bird (Cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.\nBony fish eye. Note the harder more spherical lens than in land based animals and a none circular muscle to pull the lens backward\nAquatic animals include some that also thrive in the air so focusing mechanisms vary more than in those that are only land based. Some whales and seals are able to focus above and below water having two areas of retina with high numbers of rods and cones rather than one as in humans. Having two high resolution area of retina presumably allows two axis of vision one for above and one for below water. In reptiles and birds, the ciliary body which supports the lens via suspensory ligaments also touches the lens with a number of pads on its inner surface. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in mammals. With vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus.\nIn cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal. In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle.\nIn the simplest vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards.\nWhile not vertebrate, brief mention is made here of the convergent evolution of vertebrate and Molluscan eyes. The most complex Molluscan eye is the Cephalopod eye which is superficially similar structure and function to a vertebrate eye, including accommodation, while differing in basic ways such as having a two part lens and no cornea. The fundamental requirements of optics must be filled by all eyes with lenses using the tissues at their disposal so superficially eyes all tend to look similar. It is the way optical requirements are met using different cell types and structural mechanisms that varies among animals."},{"doc_id":4244157888613556700,"text":"holtz and Huxley refined the model in the mid 1800s explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by Helmholtz in 1909. The model may be summarized like this. Normally the lens is held under tension by its suspending ligaments being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts relieving some of the tension on the ligaments, allowing the lens to elastically round up a bit, increasing refractive power. Changing focus to an object at a greater distance requires a thinner"},{"doc_id":9053365922261031000,"text":"Changing the curvature of the lens is carried out by the [ ciliary muscles\n](/wiki/Ciliary_muscle \"Ciliary muscle\") surrounding the lens; this process is\nknown as \"accommodation\". Accommodation narrows the inner diameter of the\nciliary body, which actually relaxes the fibers of the suspensory ligament\nattached to the periphery of the lens, and also allows the lens to relax into\na more convex, or globular, shape. A more convex lens refracts light more\nstrongly and focuses"},{"doc_id":797130488406740100,"text":"Nearsightedness occurs when there is a mismatch between the focusing power of\nthe eye and the length of the eye. Light rays are focused in front of the\nretina, rather than directly on it. As a result, what you see is blurry. Most\nof the eye's focusing power comes from the cornea."},{"doc_id":758996407282252700,"text":" to the retina rather than changing lens shape. Accommodation is analogous to the focusing of a photographic camera via changing its lenses. In land vertebrates the lens is flatter on its anterior side than on its posterior side, while in fish the lens is often close to spherical.\nAccommodation in humans is well studied and allows artificial means of supplementing our focus such as glasses for correction of sight as we age. The refractive power of a younger human lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power of about 60 di"},{"doc_id":6052385935703241000,"text":"Normal vision occurs when light is focused directly on the retina rather than\nin front or behind it. A person with normal vision can see objects clearly\nnear and faraway. Nearsightedness results in blurred vision when the visual\nimage is focused in front of the retina, rather than directly on it. It occurs\nwhen the physical length of the eye is greater than the optical length. For\nthis reason, nearsightedness often develops in the rapidly growing school-aged\nchild or teenager, and progresses during the growth years, requiring frequent\nchanges in glasses or contact lenses. A"},{"doc_id":1509484961915717600,"text":"Myopia occurs if the eyeball is too long or the cornea (the clear front cover\nof the eye) is too curved. As a result, the light entering the eye isn't\nfocused correctly, and distant objects look blurred. Myopia affects nearly 30%\nof the U.S. population. While the exact cause of myopia is unknown, there is\nsignificant evidence that many people inherit myopia, or at least the tendency\nto develop myopia. If one or both parents are nearsighted, there is an\nincreased chance their children will be nearsighted."},{"doc_id":4249337666591342000,"text":"Function[edit]\nFocusing[edit]\nAn image that is partially in focus, but mostly out of focus in varying degrees.\nEye and detailed ray path including one intraocular lens layer\nIn humans the widely quoted Helmholtz mechanism of focusing, also called accommodation, is often referred to as a \"model\". Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. When considering all vertebrates aspects of all models may play varying roles in lens focus.\nThe shape changing lens of many land based vertebrates[edit]\n3D reconstruction of lens in a living 20 year old human male focusing from 0 dioptres (infinity) to 4.85 dioptres (26mm) side & back views\nExternal forces[edit]\nTwo horse lenses suspended on water by cling wrap with 4 approximately parallel lasers directed through them. The 1 cm spaced grid indicates an accommodated, i.e. relaxed, near focus, focal length of around 6cm\nThe model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Others such as Helmholtz and Huxley refined the model in the mid 1800s explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by Helmholtz in 1909. The model may be summarized like this. Normally the lens is held under tension by its suspending ligaments being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts relieving some of the tension on the ligaments, allowing the lens to elastically round up a bit, increasing refractive power. Changing focus to an object at a greater distance requires a thinner less curved lens. This is achieved by relaxing some of the sphincter like ciliary muscles. While not referenced this presumably allows the pressure in the eyeball to again expand it outwards, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried none has come close enough to working using only the Helmholtz mechanisms.\nSchachar model of lens focus\nSchachar has proposed a model for land based vertebrates that was not well received. The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling.\nThe \"catenary\" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change it's curve when the poles are moved closer together. This model requires fluid movement of the lens front only rather than trying to change the shape of the lens as a whole.\nInternal forces[edit]\nTracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant\nWrinkled lens fibers in picture below compared to straight fibers above\nWhen Thomas Young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. This type of model is termed intracapsular accommodation as it relies on activity within the lens. In a 1911 Nobel lecture Allvar Gullstrand spoke on \"How I found the intracapsular mechanism of accommodation\" and this aspect of lens focusing continues to be investigated. Young spent time searching for the nerves that could stimulate the lens to contract without success. Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence. Pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. Membrane proteins such as aquaporins which allow water to flow into and out of cells are the most abundant membrane protein in the lens. Connexins which allow electrical coupling of cells are also prevalent. Electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens. As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.\nLenses of birds, reptiles, amphibians, fish and others[edit]\nDiving bird (Cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.\nBony fish eye. Note the spherical lens and muscle to pull the lens backward\nIn reptiles and birds, the ciliary body which supports the lens via suspensory ligaments also touches the lens with a number of pads on its inner surface. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in mammals. With vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus.\nIn cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal. In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle.\nIn the simplest vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards.\nWhile not vertebrate, brief mention is made here of the convergent evolution of vertebrate and Molluscan eyes. The most complex Molluscan eye is the Cephalopod eye which is superficially similar structure and function to a vertebrate eye, including accommodation, while differing in basic ways such as having a two part lens and no cornea. The fundamental requirements of optics must be filled by all eyes with lenses using the tissues at their disposal so superficially eyes all tend to look similar. It is the way optical requirements are met using different cell types and structural mechanisms that varies among animals.\nCrystallins and transparency[edit]\nGraph showing optical density (OD) of the human crystalline lens for newborn, 30-year-old, and 65-year-old from wavelengths 300-1400 nm.\nCrystallins are water-soluble proteins that compose over 90% of the protein within the lens. The three main crystallin types found in the human eye are α-, β-, and γ-crystallins. Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in lens fibers, thus increasing the index of refraction of the lens while maintaining its transparency. β and γ crystallins are found primarily in the lens, while subunits of α -crystallin have been isolated from other parts of the eye and the body. α-crystallin proteins belong to a larger superfamily of molecular chaperone proteins, and so it is believed that the crystallin proteins were evolutionarily recruited from chaperone proteins for optical purposes. The chaperone functions of α-crystallin may also help maintain the lens proteins, which must last a human for their entire lifetime.\nAnother important factor in maintaining the transparency of the lens is the absence of light-scattering organelles such as the nucleus, endoplasmic reticulum, and mitochondria within the mature lens fibers. Lens fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the lens fibers; disruptions/mutations in certain cytoskeletal elements can lead to the loss of transparency.\nThe lens blocks most ultraviolet light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. The pigment responsible for blocking the light is 3-hydroxykynurenine glucoside, a product of tryptophan catabolism in the lens epithelium. High intensity ultraviolet light can harm the retina, and artificial intraocular lenses are therefore manufactured to also block ultraviolet light. People lacking a lens (a condition known as aphakia) perceive ultraviolet light as whitish blue or whitish-violet.\nNourishment[edit]\nThe lens is metabolically active and requires nourishment in order to maintain its growth and transparency. Compared to other tissues in the eye, however, the lens has considerably lower energy demands.\nBy nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery. Beginning in the fourth month of development, the hyaloid artery and its related vasculature begin to atrophy and completely disappear by birth. In the postnatal eye, Cloquet's canal marks the former location of the hyaloid artery.\nChannels regulate lens transport.\nAfter regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor. Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the lens and out of the equatorial regions, a dynamic that is maintained by the Na/K-ATPase pumps located in the equatorially positioned cells of the lens epithelium. The interaction of these pumps with water channels into cells called aquaporins, molecules less than 100 daltons in size among cells via gap junctions, and calcium using transporters/regulators (TRPV channels) results in a flow of nutrients throughout the lens.\nGlucose is the primary energy source for the lens. As mature lens fibers do not have mitochondria, approximately 80% of the glucose is metabolized via anaerobic metabolism. The remaining fraction of glucose is shunted primarily down the pentose phosphate pathway. The lack of aerobic respiration means that the lens consumes very little oxygen."},{"doc_id":6445900096251042000,"text":"A transparent biconvex body just lies at the back of the iris, called the\ncrystalline lens composed of transparent flexible material, so that the lens\ncan adjust its shape. It is fixed with the support of suspensory ligaments\nwhich are controlled by ciliary muscles. When these muscles contract or\nexpand, the curvature of the lens increases or decreases accordingly. The\nciliary muscles also act involuntary. We see different objects at different\ndistances at different moments. Without our knowing, immediately, the\ncurvature of the lens changes its shape"},{"doc_id":5192538267906782000,"text":"It’s not the “…elongated eye lens…” that causes myopia, but the elongated globe, or eye ball. The lens has some ability to fine tune the focus of the eye from near to far vision (which diminishes with age), but not enough to permanently correct for a globe that is too long axially (myopia), or one that is too short (hyperopia)."},{"doc_id":6787073046764652000,"text":" nearsighted person sees near objects\nclearly, while objects in the distance are blurred. Farsightedness is the\nresult of the visual image being focused behind the retina rather than\ndirectly on it. It may be caused by the eyeball being too small or the\nfocusing power being too weak. Farsightedness is often present from birth, but\nchildren can often tolerate moderate amounts without difficulty and most\noutgrow the condition. A farsighted person sees faraway objects clearly, while\nobjects that are near are blurred."},{"doc_id":1717641018414186000,"text":" lens focuses light at the\nback of the eye. About 9mm in diameter and 4 mm thick, the crystalline lens\nprovides perhaps 20% of the refracting power of the eye. The index ranges from\nabout 1.406 at the center to about 1.386 in outer layers making it a gradient\nindex lens. It is pliable, and changes shape for close focusing. This is known\nas accommodation."}],"string":"[\n {\n \"doc_id\": 4660484379006952000,\n \"text\": \"For a myope, the refraction power is already too high to focus distant object. As the object gets closer, there will be point where the (excess for distance) refraction is just right. Objects beyond this point will be out-of-focus. At & within this point, the eye\"\n },\n {\n \"doc_id\": 8388844947352058000,\n \"text\": \"Profile photo for Brian Park\\nBrian Park\\nBS in Electrical Engineering (BSEE) & Physics, Optics, Mechanics, University of Akron (Graduated 1973)Author has 378 answers and 136.6K answer views\\n·\\n1y\\nRelated\\nWhy does the eye lense refraction error of myopia, issue of focal point being before the retina, only occur when objects are far away. Why shouldn't it occur regardless of whether an object is near or far?\\nActually, for every different object distance a DIFFERENT refraction power is required. The minimum refraction\"\n },\n {\n \"doc_id\": 1815057626218135600,\n \"text\": \"Profile photo for Bill Otto\\nBill Otto\\nTop Writer Eyesight & Optometry at QuoraAuthor has 8.2K answers and 66.2M answer views\\n·\\n1y\\nRelated\\nWhy does the eye lense refraction error of myopia, issue of focal point being before the retina, only occur when objects are far away. Why shouldn't it occur regardless of whether an object is near or far?\\nImage distance is a function of object distance. Closer objects always focus further away from the lens. It is possible for an object to be close enough that it happens\"\n },\n {\n \"doc_id\": 997536635586971800,\n \"text\": \"Myopia, or nearsightedness, is a vision condition in which close objects are\\nseen clearly, but objects farther away appear blurred. Nearsightedness occurs\\nif the eyeball is too long (axial myopia) or the cornea has too much curvature\\n(refractive myopia). As a result, the light entering the eye is not focused\\ncorrectly and distant objects look blurred.\\n\\nNearsightedness is a very common vision condition affecting nearly 30 percent\\nof the US and EU population. Some research supports the theory that\\nnearsight\"\n },\n {\n \"doc_id\": 1743566479811413800,\n \"text\": \"Focusing mechanisms[edit]\\nThe path of light through the eye calculated using four refractive indexes, cornea and lens curvatures approximating components of real eyes. Note objects in some size ranges and distances do not require the light path to bend noticeably to achieve focus.\\nFocusing the light scattered by objects in a three dimensional environment into a two dimensional collection of individual bright points of light requires the light to be bent. To get a good image of these points of light on a defined area requires a precise systematic bending of light called refraction. The real image formed from millions of these points of light is what animals see using their retinas. Very even systematic curvature of parts of the cornea and lens produces this systematic bending of light onto the retina.\\nVirtual eye showing the contribution to focus of different components.\\nDue to the nature of optics the focused image on the retina is always inverted relative to the object.\\nDifferent animals live in different environments having different refractive indexes involving water, air and often both. The eyes are therefor required to bend light different amounts leading to different mechanisms of focus being used in different environments. The air/cornea interface involves a larger difference in refractive index than hydrated structures within the eye. As a result, animals living in air have most of the bending of light achieved at the air/cornea interface with the lens being involved in finer focus of the image.\\n3D reconstruction based on measurements taken from a 20 year old human male focusing from 26mm to infinity (4.85 dioptre change). Side & back views shown. Most image distortions near the center are due to model being limited to 512 faces to make up the lens. Peripheral distortions are also present in animal lenses though are typically covered by the iris (anatomy)\\nThe previous video of the eye lens changing shape with focus is placed into context as the lens in this video is placed into the context of a virtual eye.\\nGenerally mammals, birds and reptiles living in air vary their eyes' optical power by subtly and precisely changing the shape of the elastic lens using the ciliary body.\\nThe small difference in refractive index between water and the hydrated cornea means fish and amphibians need to bend the light more using the internal structures of the eye. Therefore, eyes evolved in water have a mechanism involving changing the distance between a rigid rounder more refractive lens and the retina using less uniform muscles rather than subtly changing the shape of the lens itself using circularly arranged muscles.\\nLand based animals and the shape changing lens[edit]\\nVarying forms of direct experimental proof outlined in this article show that most non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of their lenses.\\nWhat is less well understood is how the subtle, precise and very quick changes in lens shape are made. Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. When considering vertebrates, aspects of all models may play varying roles in lens focus. The models can be broadly divided into two camps. Those models that stress the importance of external forces acting on a more passively elastic lens and other models that include forces that may be generated by the lens internally.\\nExternal forces[edit]\\nThe model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Others such as Helmholtz and Huxley refined the model in the mid-1800s explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by Helmholtz in 1909. The model may be summarized like this. Normally the lens is held under tension by its suspending ligaments and capsule being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts, stretching the ciliary body and relieving some of the tension on the suspensory ligaments, allowing the lens to elastically round up a bit, increasing refractive power. Changing focus to an object at a greater distance requires a thinner less curved lens. This is achieved by relaxing some of the sphincter like ciliary muscles allowing the ciliarly body to spring back, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried none has come close enough to working using only the Helmholtz mechanisms.\\nSchachar model of lens focus\\nSchachar has proposed a model for land based vertebrates that was not well received. The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles, while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling.\\nThe \\\"catenary\\\" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change its curve when the poles are moved closer together. This model requires precise fluid movement of the lens front only rather than trying to change the shape of the lens as a whole. While this concept may be involved in the focusing it has been shown by Scheimpflug photography that the rear of the lens also changes shape in the living eye.\\nInternal forces[edit]\\nTracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant\\nWrinkled lens fibers in picture below compared to straight fibers above\\nWhen Thomas Young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. This type of model is termed intracapsular accommodation as it relies on activity within the lens. In a 1911 Nobel lecture Allvar Gullstrand spoke on \\\"How I found the intracapsular mechanism of accommodation\\\" and this aspect of lens focusing continues to be investigated. Young spent time searching for the nerves that could stimulate the lens to contract without success. Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence. Pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. Membrane proteins such as aquaporins which allow water to flow into and out of cells are the most abundant membrane protein in the lens. Connexins which allow electrical coupling of cells are also prevalent. Electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens. As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.\\nHuman eyes[edit]\\nDuane's classical curves showing the amplitude or width of accommodation as changing with age. Mean (B) and approximate lower (A) and upper (C) standard deviations are shown.\\nThe young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye. This dramatic change in focal power of the eye of approximately 15 dioptres (the reciprocal of focal length in metres) occurs as a consequence of a reduction in zonular tension induced by ciliary muscle contraction. This process can occur in as little as 224 ± 30 milliseconds in bright light. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude can decline so that the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are emmetropic (i.e., do not require optical correction for distance vision) will need an optical aid for near vision; those who are myopic (nearsighted and require an optical correction for distance or far vision), will find that they see better at near without their distance correction; and those who are hyperopic (farsighted) will find that they may need a correction for both distance and near vision. Note that these effects are most noticeable when the pupil is large; i.e. in dim light. The age-related decline in accommodation occurs almost universally to less than 2 dioptres by the time a person reaches 45 to 50 years, by which time most of the population will have noticed a decrease in their ability to focus on close objects and hence require glasses for reading or bifocal lenses. Accommodation decreases to about 1 dioptre at the age of 70 years. The dependency of accommodation amplitude on age is graphically summarized by Duane's classical curves.\\nTheories on how humans focus[edit]\\nHelmholtz—The most widely held theory of accommodation is that proposed by Hermann von Helmholtz in 1855. When viewing a far object, the circularly arranged ciliary muscle relaxes allowing the lens zonules and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera. When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.\\nSchachar—Ronald A. Schachar has proposed in 1992 what has been called a \\\"rather bizarre geometric theory\\\" which claims that focus by the human lens is associated with increased tension on the lens via the equatorial zonules; that when the ciliary muscle contracts, equatorial zonular tension is increased, causing the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing. The increased equatorial zonular tension keeps the lens stable and flattens the peripheral lens surface during accommodation. As a consequence, gravity does not affect the amplitude of accommodation and primary spherical aberration shifts in the negative direction during accommodation. The theory has not found much independent support.\\nCatenary—D. Jackson Coleman proposes that the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye. Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape. It is in this lens shape that the mechanically reproducible state of a steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross section, of a catenary occurs. The anterior capsule and the zonule form a trampoline shape or hammock shaped surface that is totally reproducible depending on the circular dimensions, i.e. the diameter of the ciliary body (Müeller's muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need to support an equatorial traction force to flatten the lens.\\nInduced effects of accommodation[edit]\\nWhen humans accommodate to a near object, they also converge their eyes and constrict their pupils. The combination of these three movements (accommodation, convergence and miosis) is under the control of the Edinger-Westphal nucleus and is referred to as the near triad, or accommodation reflex. While it is well understood that proper convergence is necessary to prevent diplopia, the functional role of the pupillary constriction remains less clear. Arguably, it may increase the depth of field by reducing the aperture of the eye, and thus reduce the amount of accommodation needed to bring the image in focus on the retina.\\nThere is a measurable ratio (Matthiessen's ratio) between how much convergence takes place because of accommodation (AC/A ratio, CA/C ratio). Abnormalities with this can lead to binocular vision problems.\\nAnomalies of accommodation described in humans[edit]\\nThere are many types of accommodation anomalies. It can be broadly classified into two, decreased accommodation and increased accommodation. Decreased accommodation may occur due to physiological (presbyopia), pharmacological (cycloplegia) or pathological. Excessive accommodation and spasm of accommodation are types of increased accommodation.\\nPresbyopia[edit]\\nPresbyopia, physiological insufficiency of accommodation due to age related changes in lens (decreased elasticity and increased hardness) and ciliary muscle power is the commonest form of accommodative dysfunction. It will cause gradual decrease in near vision.\\nAccommodative insufficiency[edit]\\nAccommodative insufficiency is the condition where amplitude of accommodation of a person is lesser compared to physiological limits for their age. Premature sclerosis of lens or ciliary muscle weaknesses due to systemic or local cases may cause accommodative insufficiency.\\nAccommodative insufficiency is further categorised into different categories.\\nIll-sustained accommodation[edit]\\nIll-sustained accommodation is a condition similar to accommodative insufficiency. In this, range of accommodation will be normal, but after excessive near work accommodative power will decrease.\\nParalysis of accommodation[edit]\\nIn paralysis of accommodation, amplitude of accommodation is either markedly reduced or completely absent (cycloplegia). It may occur due to ciliary muscle paralysis or occulomotor nerve paralysis. Parasympatholytic drugs like atropine will also cause paralysis of accommodation.\\nUnequal accommodation[edit]\\nIf there is amplitude of accommodation between the eyes differ 0.5 dioptre or more, it is considered as unequal. Organic diseases, head trauma or functional amblyopia may be responsible for unequal accommodation.\\nAccommodative infacility[edit]\\nAccommodative infacility is also known as accommodative inertia. In this condition there will be difficulty in changing accommodation from one point to other. There may be difficulty in adjusting focus from distance from near. It is a comparatively rare condition.\\nSpasm of accommodation[edit]\\nSpasm of accommodation also known as ciliary spasm is a condition of abnormally excessive accommodation which is out of voluntary control of the person. Vision may be blurred due to induced pseudomyopia.\\nAccommodative excess[edit]\\nAccommodative excess occurs when an individual uses more than normal accommodation for performing certain near work. Modern definitions simply regard it as an inability to relax accommodation readily.\\nAquatic animals[edit]\\nDiving bird (Cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.\\nBony fish eye. Note the harder more spherical lens than in land based animals and a none circular muscle to pull the lens backward\\nAquatic animals include some that also thrive in the air so focusing mechanisms vary more than in those that are only land based. Some whales and seals are able to focus above and below water having two areas of retina with high numbers of rods and cones rather than one as in humans. Having two high resolution area of retina presumably allows two axis of vision one for above and one for below water. In reptiles and birds, the ciliary body which supports the lens via suspensory ligaments also touches the lens with a number of pads on its inner surface. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in mammals. With vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus.\\nIn cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal. In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle.\\nIn the simplest vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards.\\nWhile not vertebrate, brief mention is made here of the convergent evolution of vertebrate and Molluscan eyes. The most complex Molluscan eye is the Cephalopod eye which is superficially similar structure and function to a vertebrate eye, including accommodation, while differing in basic ways such as having a two part lens and no cornea. The fundamental requirements of optics must be filled by all eyes with lenses using the tissues at their disposal so superficially eyes all tend to look similar. It is the way optical requirements are met using different cell types and structural mechanisms that varies among animals.\"\n },\n {\n \"doc_id\": 4244157888613556700,\n \"text\": \"holtz and Huxley refined the model in the mid 1800s explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by Helmholtz in 1909. The model may be summarized like this. Normally the lens is held under tension by its suspending ligaments being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts relieving some of the tension on the ligaments, allowing the lens to elastically round up a bit, increasing refractive power. Changing focus to an object at a greater distance requires a thinner\"\n },\n {\n \"doc_id\": 9053365922261031000,\n \"text\": \"Changing the curvature of the lens is carried out by the [ ciliary muscles\\n](/wiki/Ciliary_muscle \\\"Ciliary muscle\\\") surrounding the lens; this process is\\nknown as \\\"accommodation\\\". Accommodation narrows the inner diameter of the\\nciliary body, which actually relaxes the fibers of the suspensory ligament\\nattached to the periphery of the lens, and also allows the lens to relax into\\na more convex, or globular, shape. A more convex lens refracts light more\\nstrongly and focuses\"\n },\n {\n \"doc_id\": 797130488406740100,\n \"text\": \"Nearsightedness occurs when there is a mismatch between the focusing power of\\nthe eye and the length of the eye. Light rays are focused in front of the\\nretina, rather than directly on it. As a result, what you see is blurry. Most\\nof the eye's focusing power comes from the cornea.\"\n },\n {\n \"doc_id\": 758996407282252700,\n \"text\": \" to the retina rather than changing lens shape. Accommodation is analogous to the focusing of a photographic camera via changing its lenses. In land vertebrates the lens is flatter on its anterior side than on its posterior side, while in fish the lens is often close to spherical.\\nAccommodation in humans is well studied and allows artificial means of supplementing our focus such as glasses for correction of sight as we age. The refractive power of a younger human lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power of about 60 di\"\n },\n {\n \"doc_id\": 6052385935703241000,\n \"text\": \"Normal vision occurs when light is focused directly on the retina rather than\\nin front or behind it. A person with normal vision can see objects clearly\\nnear and faraway. Nearsightedness results in blurred vision when the visual\\nimage is focused in front of the retina, rather than directly on it. It occurs\\nwhen the physical length of the eye is greater than the optical length. For\\nthis reason, nearsightedness often develops in the rapidly growing school-aged\\nchild or teenager, and progresses during the growth years, requiring frequent\\nchanges in glasses or contact lenses. A\"\n },\n {\n \"doc_id\": 1509484961915717600,\n \"text\": \"Myopia occurs if the eyeball is too long or the cornea (the clear front cover\\nof the eye) is too curved. As a result, the light entering the eye isn't\\nfocused correctly, and distant objects look blurred. Myopia affects nearly 30%\\nof the U.S. population. While the exact cause of myopia is unknown, there is\\nsignificant evidence that many people inherit myopia, or at least the tendency\\nto develop myopia. If one or both parents are nearsighted, there is an\\nincreased chance their children will be nearsighted.\"\n },\n {\n \"doc_id\": 4249337666591342000,\n \"text\": \"Function[edit]\\nFocusing[edit]\\nAn image that is partially in focus, but mostly out of focus in varying degrees.\\nEye and detailed ray path including one intraocular lens layer\\nIn humans the widely quoted Helmholtz mechanism of focusing, also called accommodation, is often referred to as a \\\"model\\\". Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. When considering all vertebrates aspects of all models may play varying roles in lens focus.\\nThe shape changing lens of many land based vertebrates[edit]\\n3D reconstruction of lens in a living 20 year old human male focusing from 0 dioptres (infinity) to 4.85 dioptres (26mm) side & back views\\nExternal forces[edit]\\nTwo horse lenses suspended on water by cling wrap with 4 approximately parallel lasers directed through them. The 1 cm spaced grid indicates an accommodated, i.e. relaxed, near focus, focal length of around 6cm\\nThe model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Others such as Helmholtz and Huxley refined the model in the mid 1800s explaining how the ciliary muscle contracts rounding the lens to focus near and this model was popularized by Helmholtz in 1909. The model may be summarized like this. Normally the lens is held under tension by its suspending ligaments being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts relieving some of the tension on the ligaments, allowing the lens to elastically round up a bit, increasing refractive power. Changing focus to an object at a greater distance requires a thinner less curved lens. This is achieved by relaxing some of the sphincter like ciliary muscles. While not referenced this presumably allows the pressure in the eyeball to again expand it outwards, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried none has come close enough to working using only the Helmholtz mechanisms.\\nSchachar model of lens focus\\nSchachar has proposed a model for land based vertebrates that was not well received. The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling.\\nThe \\\"catenary\\\" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change it's curve when the poles are moved closer together. This model requires fluid movement of the lens front only rather than trying to change the shape of the lens as a whole.\\nInternal forces[edit]\\nTracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant\\nWrinkled lens fibers in picture below compared to straight fibers above\\nWhen Thomas Young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. This type of model is termed intracapsular accommodation as it relies on activity within the lens. In a 1911 Nobel lecture Allvar Gullstrand spoke on \\\"How I found the intracapsular mechanism of accommodation\\\" and this aspect of lens focusing continues to be investigated. Young spent time searching for the nerves that could stimulate the lens to contract without success. Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence. Pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. Membrane proteins such as aquaporins which allow water to flow into and out of cells are the most abundant membrane protein in the lens. Connexins which allow electrical coupling of cells are also prevalent. Electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens. As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.\\nLenses of birds, reptiles, amphibians, fish and others[edit]\\nDiving bird (Cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.\\nBony fish eye. Note the spherical lens and muscle to pull the lens backward\\nIn reptiles and birds, the ciliary body which supports the lens via suspensory ligaments also touches the lens with a number of pads on its inner surface. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the suspensory ligaments usually perform this function in mammals. With vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus.\\nIn cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not similar in either type of animal. In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle.\\nIn the simplest vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards.\\nWhile not vertebrate, brief mention is made here of the convergent evolution of vertebrate and Molluscan eyes. The most complex Molluscan eye is the Cephalopod eye which is superficially similar structure and function to a vertebrate eye, including accommodation, while differing in basic ways such as having a two part lens and no cornea. The fundamental requirements of optics must be filled by all eyes with lenses using the tissues at their disposal so superficially eyes all tend to look similar. It is the way optical requirements are met using different cell types and structural mechanisms that varies among animals.\\nCrystallins and transparency[edit]\\nGraph showing optical density (OD) of the human crystalline lens for newborn, 30-year-old, and 65-year-old from wavelengths 300-1400 nm.\\nCrystallins are water-soluble proteins that compose over 90% of the protein within the lens. The three main crystallin types found in the human eye are α-, β-, and γ-crystallins. Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in lens fibers, thus increasing the index of refraction of the lens while maintaining its transparency. β and γ crystallins are found primarily in the lens, while subunits of α -crystallin have been isolated from other parts of the eye and the body. α-crystallin proteins belong to a larger superfamily of molecular chaperone proteins, and so it is believed that the crystallin proteins were evolutionarily recruited from chaperone proteins for optical purposes. The chaperone functions of α-crystallin may also help maintain the lens proteins, which must last a human for their entire lifetime.\\nAnother important factor in maintaining the transparency of the lens is the absence of light-scattering organelles such as the nucleus, endoplasmic reticulum, and mitochondria within the mature lens fibers. Lens fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the lens fibers; disruptions/mutations in certain cytoskeletal elements can lead to the loss of transparency.\\nThe lens blocks most ultraviolet light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea. The pigment responsible for blocking the light is 3-hydroxykynurenine glucoside, a product of tryptophan catabolism in the lens epithelium. High intensity ultraviolet light can harm the retina, and artificial intraocular lenses are therefore manufactured to also block ultraviolet light. People lacking a lens (a condition known as aphakia) perceive ultraviolet light as whitish blue or whitish-violet.\\nNourishment[edit]\\nThe lens is metabolically active and requires nourishment in order to maintain its growth and transparency. Compared to other tissues in the eye, however, the lens has considerably lower energy demands.\\nBy nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery. Beginning in the fourth month of development, the hyaloid artery and its related vasculature begin to atrophy and completely disappear by birth. In the postnatal eye, Cloquet's canal marks the former location of the hyaloid artery.\\nChannels regulate lens transport.\\nAfter regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor. Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the lens and out of the equatorial regions, a dynamic that is maintained by the Na/K-ATPase pumps located in the equatorially positioned cells of the lens epithelium. The interaction of these pumps with water channels into cells called aquaporins, molecules less than 100 daltons in size among cells via gap junctions, and calcium using transporters/regulators (TRPV channels) results in a flow of nutrients throughout the lens.\\nGlucose is the primary energy source for the lens. As mature lens fibers do not have mitochondria, approximately 80% of the glucose is metabolized via anaerobic metabolism. The remaining fraction of glucose is shunted primarily down the pentose phosphate pathway. The lack of aerobic respiration means that the lens consumes very little oxygen.\"\n },\n {\n \"doc_id\": 6445900096251042000,\n \"text\": \"A transparent biconvex body just lies at the back of the iris, called the\\ncrystalline lens composed of transparent flexible material, so that the lens\\ncan adjust its shape. It is fixed with the support of suspensory ligaments\\nwhich are controlled by ciliary muscles. When these muscles contract or\\nexpand, the curvature of the lens increases or decreases accordingly. The\\nciliary muscles also act involuntary. We see different objects at different\\ndistances at different moments. Without our knowing, immediately, the\\ncurvature of the lens changes its shape\"\n },\n {\n \"doc_id\": 5192538267906782000,\n \"text\": \"It’s not the “…elongated eye lens…” that causes myopia, but the elongated globe, or eye ball. The lens has some ability to fine tune the focus of the eye from near to far vision (which diminishes with age), but not enough to permanently correct for a globe that is too long axially (myopia), or one that is too short (hyperopia).\"\n },\n {\n \"doc_id\": 6787073046764652000,\n \"text\": \" nearsighted person sees near objects\\nclearly, while objects in the distance are blurred. Farsightedness is the\\nresult of the visual image being focused behind the retina rather than\\ndirectly on it. It may be caused by the eyeball being too small or the\\nfocusing power being too weak. Farsightedness is often present from birth, but\\nchildren can often tolerate moderate amounts without difficulty and most\\noutgrow the condition. A farsighted person sees faraway objects clearly, while\\nobjects that are near are blurred.\"\n },\n {\n \"doc_id\": 1717641018414186000,\n \"text\": \" lens focuses light at the\\nback of the eye. About 9mm in diameter and 4 mm thick, the crystalline lens\\nprovides perhaps 20% of the refracting power of the eye. The index ranges from\\nabout 1.406 at the center to about 1.386 in outer layers making it a gradient\\nindex lens. It is pliable, and changes shape for close focusing. This is known\\nas accommodation.\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":5277346963269157000,"text":"If you have myopia, your eyes are focused up close"},{"doc_id":6483076863330530000,"text":" Focus that happens in front of the retina of your eye instead\nof at the retina results in myopia (nearsightedness)."},{"doc_id":8583842396495696000,"text":"while the question is correct- light entering the eye from optical infinity (ie distance) comes to focus in front of the retina in myopia, and forms an “apparent “ focus behind the retina in hyperopia, to understand correction we need to consider the same situation, but with a slight tweak. But before then we need to also understand the principle of reversibility in ray tracing, and lens orientation."},{"doc_id":7748910388324795000,"text":" required is for objects at infinite distance. More is required the closer object gets. The eye is set up so this minimum is when eye lens muscles are in relaxed state. The refraction power can only be increased by the muscles."},{"doc_id":2570414022684135400,"text":" divergent light rays from near objects onto the retina,\nallowing closer objects to be brought into better focus. [31] [32]"},{"doc_id":6751751125259898000,"text":"So think of binoculars. If you focus them near, then things in the distance are blurry. If you focus them at far objects, then nearer ones are blurry."},{"doc_id":5962416175974752000,"text":"Profile photo for Bill Otto\nBill Otto\nTop Writer Eyesight & Optometry at QuoraAuthor has 8.2K answers and 66.2M answer views\n·\nFeb 21\nRelated\nWhy and how are long-distance images blurry in myopia and opposite in hypermyopia?\nI think you meant to write “hyperopia.”"},{"doc_id":3116876100822912000,"text":" to adjust its focal length, so that\nthe image can be formed on the retina.\n\nThere is a large space between the lens and the retina, called posterior\nchamber. It is filled with the vitreous humor which is a transparent colorless\njelly-like substance."},{"doc_id":8838327111474969000,"text":"ations that might otherwise result from the changing shape while better fitting mathematical modeling.\nThe \"catenary\" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change it's curve when the poles are moved closer together."},{"doc_id":7125326944439106000,"text":" form in the eye of a nearsighted person?\nRelated questions\nHow do diverging lenses correct nearsightedness?\nHow does focused light in front of the retina make us near-sighted and not otherwise? Does it have anything to do with image formation?\nWhy does light focusing in front of the retina mostly cause problems with seeing objects that are far (nearsightedness), and light focusing past the retina causes problems with seeing objects that are close (farsightedness)?\nHow do one’s eyes go from farsighted to nearsighted?\nI"},{"doc_id":4002039531316687400,"text":"Related questions\nHow do diverging lenses correct nearsightedness?\nHow does focused light in front of the retina make us near-sighted and not otherwise? Does it have anything to do with image formation?\nWhy does light focusing in front of the retina mostly cause problems with seeing objects that are far (nearsightedness), and light focusing past the retina causes problems with seeing objects that are close (farsightedness)?\nHow do one’s eyes go from farsighted to nearsighted?\nI am a farsighted person but my lenses is"},{"doc_id":7037178924202905000,"text":"Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image.\nAccommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled.\nThe main ways animals may change focus are:"},{"doc_id":738652577321205800,"text":" less curved lens. This is achieved by relaxing some of the sphincter like ciliary muscles. While not referenced this presumably allows the pressure in the eyeball to again expand it outwards, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried none has come close enough to working using only the Helmholtz mechanisms.\nSchachar model of lens focus\nSchachar has proposed a model for land based vertebrates that was not well received. The theory allows"},{"doc_id":4070135853981967400,"text":"A normal young eye, focuses on distant objects and accommodation allows automatic focusing on near objects. Hyperopia is when your eyes are focused beyond far so nothing is in focus. Your accommodation can add enough focusing power to just see far objects clearly."},{"doc_id":3962391993003015700,"text":"Sponsored by Grammarly\nHit the mark with every message.\nPredict how your reader will react, see how you sound, and get the response you want—with Grammarly.\nRelated questions\nFor a nearsighted person, is the image focused in front of the retina? True or False?\nMyopia and hyperopia causes are explainable by the eye lense getting stretched or not, but how does by focal condition occur?\nCan mild hyperopia/farsightedness be reversed without the use of corrective lenses or surgery?\nWhy is my eye bleeding"},{"doc_id":3271388781564688400,"text":"Nearsightedness is when light entering the eye is focused incorrectly. This\nmakes distant objects appear blurred. Nearsightedness is a type of refractive\nerror of the eye."},{"doc_id":5281582486101445000,"text":"Profile photo for Valente Hernandez Perez\nValente Hernandez Perez\nReduced my myopia naturallyAuthor has 3.4K answers and 3.5M answer views\n·\n3y\nRelated\nHow do diverging lenses correct nearsightedness?\nNearsightedness occurs when the eye can only see near objects but not far objects. This is because the light coming from far objects focus in front of the retina, rather than on the retina. Diverging lenses, also called concave lenses, move the converging point of the light backwards towards the retina. Therefore, diverging"},{"doc_id":8538690308462247000,"text":"\nIn a nearsighted the image of a far object is focused in front of the retina while in a farsighted the image is focused behind the retina. How are you going to correct these eye defect s using diverging and converging lenses? Explain.\nSort\nProfile photo for Malcolm Maciver\nMalcolm Maciver\nOptometrist & AcademicAuthor has 429 answers and 779K answer views\n·\n3y\nWell - while not the most eloquently asked - I think this is a good point to begin an explanation."},{"doc_id":9055938428256027000,"text":"Sponsored by Online Shopping Tools\nAmazon Hates When Prime Members Do This, But They Can't Stop You.\nThis simple trick can save tons of money on Amazon, but most Prime members are ignoring it.\nRelated questions\nHow do diverging lenses correct nearsightedness?\nHow does focused light in front of the retina make us near-sighted and not otherwise? Does it have anything to do with image formation?\nWhy does light focusing in front of the retina mostly cause problems with seeing objects that are far (nearsightedness), and light focusing past the retina causes problems"},{"doc_id":597238679023231400,"text":"Lenses cannot refract light rays at their edges as well as closer to the\ncentre. The image produced by any lens is therefore somewhat blurry around the\nedges ( [ spherical aberration ](/wiki/Spherical_aberration \"Spherical\naberration\") ). It can be minimized by screening out peripheral light rays and\nlooking only at the better-focused centre. In the eye, the pupil serves this\npurpose by constricting while the eye is focused on nearby objects. Small\napertures also give an increase in [ depth of field ](/wiki/"},{"doc_id":3948332616404524500,"text":" and farsightedness (hyperopia), and how are these vision problems treated?\nWhat is the possible disease if one retina is larger than the retina in other eye?\nIf you have presbyopia, does far object focus in front of the retina and close object focus behind the retina?\nA nearsighted person has a near point of 12 cm and a far point of 17 cm. If the corrective lens is 2.0 cm from his eye, what lens power will enable this person to see distant objects clearly (answer should be in diopters)?\nWhere does the image"},{"doc_id":8923114265773640000,"text":"The adjustment to close-range vision involves three processes to focus an\nimage on the retina."},{"doc_id":2141862257330952000,"text":" with seeing objects that are close (farsightedness)?\nHow do one’s eyes go from farsighted to nearsighted?\nI am a farsighted person but my lenses is for nearsighted. Will this affect my eyes?\nProfile photo for Xaun Loc\nXaun Loc\nAdult With an IQ Above Room Temperature! (1948–present)Author has 6K answers and 6.5M answer views\n·\n2y\nOriginally Answered: In a nearsighted the image of a far object is focused in front of the retina while"},{"doc_id":1314637696850278000,"text":"[\n ](/wiki/File:Arizona_eye_model.png) Arizona Eye Model.\n\"A\" is accommodation in diopters."},{"doc_id":3708577464021101600,"text":"\nProfile photo for Assistant\nAssistant\nBot\n·\nNov 28\nTo correct nearsightedness, which is also known as myopia, a diverging lens is used. This type of lens spreads out the light rays before they enter the eye, which helps to move the focal point back to the retina. This allows the individual to see distant objects more clearly."},{"doc_id":7836815286624988000,"text":"Nearsightedness, or myopia, as it is medically termed, is a vision condition\nin which people can see close objects clearly, but objects farther away appear\nblurred."},{"doc_id":6534254037188262000,"text":"Myopia (nearsightedness) is a common condition that’s usually diagnosed before\nage 20. It affects your distance vision — you can see objects that are near,\nbut you have trouble viewing objects that are farther away like grocery store\naisle markers or road signs. Myopia treatments include glasses, contact lenses\nor surgery."},{"doc_id":933988202701675800,"text":" This model requires fluid movement of the lens front only rather than trying to change the shape of the lens as a whole.\nInternal forces[edit]\nTracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant\nWrinkled lens fibers in picture below compared to straight fibers above\nWhen Thomas Young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. This type of model is termed intr"},{"doc_id":4928957121947894000,"text":"## Near response [ [ edit\n](/w/index.php?title=Human_eye&action=edit§ion=15 \"Edit section: Near\nresponse\") ]"},{"doc_id":1081809044795926500,"text":" to focus on the retina in myopes."},{"doc_id":4531302417764727000,"text":"[edit]\n3D reconstruction of lens in a living 20 year old human male focusing from 0 dioptres (infinity) to 4.85 dioptres (26mm) side & back views\nExternal forces[edit]\nTwo horse lenses suspended on water by cling wrap with 4 approximately parallel lasers directed through them. The 1 cm spaced grid indicates an accommodated, i.e. relaxed, near focus, focal length of around 6cm\nThe model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Others such as Helm"},{"doc_id":7697060254554999000,"text":"[ ](mailto:?subject=Cleveland Clinic - Myopia\n\\(Nearsightedness\\)&body=https://my.clevelandclinic.org/health/diseases/8579-myopia-\nnearsightedness)"},{"doc_id":5665544169939711000,"text":"Function[edit]\nFocusing[edit]\nAn image that is partially in focus, but mostly out of focus in varying degrees.\nEye and detailed ray path including one intraocular lens layer\nIn humans the widely quoted Helmholtz mechanism of focusing, also called accommodation, is often referred to as a \"model\". Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. When considering all vertebrates aspects of all models may play varying roles in lens focus.\nThe shape changing lens of many land based vertebrates"},{"doc_id":6993670070181808000,"text":""},{"doc_id":4210204835744713000,"text":"Depth_of_field\n\"Depth of field\"), allowing a broader range of \"in focus\" vision. In this way\nthe pupil has a dual purpose for near vision: to reduce spherical aberration\nand increase depth of field. [31]"},{"doc_id":7224089652970956000,"text":"[ Healthy Eyes ](/healthy-eyes \"Healthy Eyes\") / [ Eye and Vision Conditions\n](/healthy-eyes/eye-and-vision-conditions \"Eye and Vision Conditions\") / [\nMyopia ](/healthy-eyes/eye-and-vision-conditions/myopia \"Myopia\")"},{"doc_id":7926337537666380000,"text":"Profile photo for Raphael\nRaphael\nAn African student optometrist\n·\n7y\nRelated\nCan you cure far-sightedness by looking at objects up-close?\nThe answer is simply no.Far-sightedness(hypermetropia) can be diagnosed only when one is looking at far(6m and beyond)...this why your optometrist asks you to look at a test chart(VA) placed at 6m from ur examination chair.During looking at far your lens is relaxed and there is no accommodation.When looking at near,your lens contracts temporarily to"},{"doc_id":416839722049606660,"text":"Profile photo for Sharon Breivogel-Leonard\nSharon Breivogel-Leonard\nOptician and Contact Lens Practitioner licensed in N.Y.Author has 3K answers and 14.5M answer views\n·\n4y\nRelated\nShouldn’t elongated eye lens converge the rays of light behind retina? If yes, then how is elongation of eye lens a cause of myopia?\n“Shouldn’t elongated eye lens converge the rays of light behind retina? If yes, then how is elongation of eye"},{"doc_id":6712287497928144000,"text":" for nearsighted. Will this affect my eyes?\nFor a nearsighted person, is the image focused in front of the retina? True or False?\nMyopia and hyperopia causes are explainable by the eye lense getting stretched or not, but how does by focal condition occur?\nCan mild hyperopia/farsightedness be reversed without the use of corrective lenses or surgery?\nWhy is my eye bleeding behind the retina?\nWhich one is worse: being nearsighted or farsighted?\nCan you explain the difference between nearsightedness (myopia)"},{"doc_id":9060855836708261000,"text":"Myopia is the medical name for nearsightedness, which means that you can see\nobjects that are near clearly but have difficulty seeing objects that are\nfarther away. For example, if you’re nearsighted, you may not be able to make\nout highway signs until they’re just a few feet away."},{"doc_id":4921471637836497000,"text":" in a farsighted the image is focused behind the retina. How are you going to correct these eye defect s using diverging and converging lenses? Explain.\nA2A: In a nearsighted person, the image of a far object is focused in front of the retina while in a farsighted person, the image is focused behind the retina. How would you correct these eye defects using diverging and converging lenses?"},{"doc_id":1316363963287033600,"text":"The focus needs to be precise. If it isn’t, what you’re looking at appears\nblurry. Your eye has muscles that can make subtle changes to the shape of your\neye, moving the focus point so it lands correctly on the retina.\n\nWhen light lands on the cells of your retinas, those cells send signals to\nyour brain. The signals are like coded messages describing everything they can\nabout the light. That includes the color, how intense it is and any other\nrelevant details. Your brain decodes and processes the signals and uses them\n"},{"doc_id":7433487709782678000,"text":"* * *\n\nImage credit The image formation on the retina"},{"doc_id":3391843269234439000,"text":" a screen or image sensor, is known as a real image. This is the principle of the camera, and also of the human eye, in which the retina serves as the image sensor.\nThe focusing adjustment of a camera adjusts S2, as using an image distance different from that required by this formula produces a defocused (fuzzy) image for an object at a distance of S1 from the camera. Put another way, modifying S2 causes objects at a different S1 to come into perfect focus.\nVirtual image formation using a positive lens as a magnifying glass.\nIn"},{"doc_id":9010197787198001000,"text":" lenses correct nearsightedness by focusing the light coming from far objects onto the retina."},{"doc_id":8588554961387381000,"text":". As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.\n"},{"doc_id":7035571819567415000,"text":"Image credit The diagram of the human eye\n\n* * *"},{"doc_id":7664261621066329000,"text":"��ிரமநாயகம்)\nM.Sc. Retired as Head of Department of Physics. Government of Tamilnadu, lndia.Author has 10.2K answers and 6.9M answer views\n·\n3y\nRelated\nA myopia patient could see objects blurred. So if the image is formed before retina (which works as a screen) then how that person could see the object even blur. if the image is not forming in retina"},{"doc_id":9089020966453622000,"text":" function in mammals. With vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus.\nIn cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process,"},{"doc_id":7836039674261284000,"text":" mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberr"},{"doc_id":5776985908033602000,"text":"# Myopia (nearsightedness)"},{"doc_id":3197771491313399000,"text":"When a creature with binocular vision looks at an object, the eyes must rotate\naround a vertical axis so that the projection of the image is in the centre of\nthe retina in both eyes. To look at a nearby object, the eyes rotate 'towards\neach other' ( [ convergence ](/wiki/Convergence_\\(eye\\) \"Convergence \\(eye\\)\")\n), while for an object farther away they rotate 'away from each other' ( [\ndivergence ](/wiki/Divergence_\\(eye\\) \"Diver"},{"doc_id":3258428016166494700,"text":"Myopia (Nearsightedness)"},{"doc_id":1274739173265799200,"text":"Profile photo for Francis Vincent\nFrancis Vincent\nAuthor has 7.2K answers and 6.3M answer views\n3y\nRelated\nMy right eye sees far objects better but cannot focus as well on nearer objects as my left eye does. The opposite is on my left eye (cannot see far objects, better on near). Is this normal?\nHaving one far sighted eye and one near sighted eye is called antimetropia and isn't very common. I had that when I was younger. My optometrist said I didn't need glasses as long as it"},{"doc_id":4503927224653083000,"text":" behind the retina?\nWhich one is worse: being nearsighted or farsighted?\nProfile photo for Don Vater\nDon Vater\nStudied Optometry at Indiana University (Graduated 1970)Author has 551 answers and 793K answer views\n·\n3y\nRelated\nHow do diverging lenses correct nearsightedness?\nWith nearsightedness (myopia) the eyeball length is too long and the light rays focus in front of the retina. Divergent (negative/concave) lenses make the light rays diverge to focus back further on"},{"doc_id":2139784360858340900,"text":"* [ ](/wiki/File:Blausen_0389_EyeAnatomy_02.png)"},{"doc_id":9212096145168112000,"text":"Myopia; Shortsightedness; Refractive error - nearsightedness"},{"doc_id":6370306629959697000,"text":"Light energy enters the eye through the cornea, through the pupil and then\nthrough the lens. The lens shape is changed for near focus (accommodation) and\nis controlled by the ciliary muscle. Between the two lenses, there are four [\noptical surfaces ](/wiki/Optical_surfaces \"Optical surfaces\") which each [\nrefract ](/wiki/Refraction \"Refraction\") light as it travels along the optical\npath. One basic model describing the geometry of the optical system is the\nArizona Eye Model. [2]"},{"doc_id":5963560923440762000,"text":"* Some people may experience blurred distance vision only at night. With \"night myopia,\" low light makes it difficult for the eyes to focus properly. Or the increased pupil size during dark conditions allows more peripheral, unfocused light rays to enter the eye.\n* People who do an excessive amount of near-vision work may experience a false or \"pseudo\" myopia. Their blurred distance vision is caused by overuse of the eyes' focusing mechanism. After long periods of near work, their eyes are unable to refocus to see clearly in the distance. Clear distance vision usually returns after resting the"},{"doc_id":7726529561464266000,"text":"A nearsighted person sees close-up objects clearly, but objects in the\ndistance are blurred. Squinting will tend to make far away objects seem\nclearer."},{"doc_id":4485262315221988000,"text":"The lens , which sits behind the iris, changes its shape to focus light onto\nthe retina. Small muscles (ciliary muscles) contract to cause the lens to\nbecome thicker, which allows the lens to focus on nearby objects. The ciliary\nmuscles relax to cause the lens to become thinner when it focuses on distant\nobjects. These lens changes are limited in dogs. The retina contains the\ncells that sense light (photoreceptors). The most sensitive area of the retina\nis called the area centralis in dogs; this area contains thousands of"},{"doc_id":4868953833671812000,"text":"Common eye defects like myopia (nearsightedness), hyperopia (farsightedness)\nand astigmatism are often called \"refractive errors\". They affect how the eyes\nbend or \"refract\" light and they can usually be corrected by relatively simple\ncompensating lenses or by surgery changing the shape of the cornea in order to\nimprove the refractive state of the eye. Reshaping of the cornea is frequently\nperformed by photoablation using laser.\n\nMyopia (Nearsightedness)"},{"doc_id":8189386028845468000,"text":" am a farsighted person but my lenses is for nearsighted. Will this affect my eyes?\nFor a nearsighted person, is the image focused in front of the retina? True or False?\nAboutCareersPrivacyTermsContactLanguagesYour Ad ChoicesPress© Quora, Inc. 2024"},{"doc_id":5803745890755941000,"text":"# Myopia (Nearsightedness)"},{"doc_id":8400401202443150000,"text":" which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. Such a non-homogeneous"},{"doc_id":6376945335873103000,"text":""},{"doc_id":1254291911034754300,"text":" angular size of the moon seen from earth of about 0.5°.\nIn the extreme case where an object is an infinite distance away, S1 = ∞, S2 = f and M = −f/∞ = 0, indicating that the object would be imaged to a single point in the focal plane. In fact, the diameter of the projected spot is not actually zero, since diffraction places a lower limit on the size of the point spread function. This is called the diffraction limit.\nImages of black letters in a thin convex lens of focal length f"},{"doc_id":6706918406178198000,"text":"* [ Refractive errors and refraction ](/eye-exam/refraction.htm)\n* [ Visual Acuity: 20/20 Vision ](/eye-exam/2020-vision.htm)\n* [ Hyperopia (farsightedness) ](/conditions/hyperopia.htm)"},{"doc_id":348459215555378240,"text":"Main article: [ Vergence ](/wiki/Vergence \"Vergence\")"},{"doc_id":6365536122890732000,"text":"On the other hand, to correct farsightedness, also known as hyperopia, a converging lens is used. This type of lens brings the light rays together before they enter the eye, which helps to move the focal point forward to the retina. This allows the individual to see nearby objects more clearly."},{"doc_id":8600789022803291000,"text":"* [ ](/wiki/File:Blausen_0388_EyeAnatomy_01.png)"},{"doc_id":8311068887189887000,"text":" the eye's [ optic axis\n](/wiki/Optic_axis \"Optic axis\"). In order, along the optic axis, the optical\ncomponents consist of a first lens (the [ cornea—the clear part of the eye\n](/wiki/Cornea \"Cornea\") ) that accounts for most of the optical power of the\neye and accomplishes most of the [ focusing of light ](/wiki/Focus_\\(optics\\)\n\"Focus \\(optics\\)\") from the outside world; then an [ aperture\n](/wiki"},{"doc_id":3617328645619534000,"text":"### What causes myopia?"},{"doc_id":4844955335891414000,"text":"Myopia affects a significant percentage of people. It’s an [ eye\n](https://my.clevelandclinic.org/health/body/21823-eyes) focus disorder that’s\nnormally corrected with eyeglasses, contact lenses or surgery."},{"doc_id":3855191632893827000,"text":"The lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. Along with the cornea, aqueous and vitreous humours it refracts light, focusing it onto the retina. In many land animals the shape of the lens can be altered, effectively changing the focal length of the eye, enabling them to focus on objects at various distances. This adjustment of the lens is known as accommodation (see also below). In many fully aquatic vertebrates such as fish other methods of accommodation are used such as changing the lens's position relative"},{"doc_id":451401416707211000,"text":"* [ Refractive Errors ](/refractive-errors/) __"},{"doc_id":3143293289961343000,"text":" between object and image is given by\nn\n\n2\n\n\nv\n\n\n+\n\n\n\nn\n\n1\n\n\nu\n\n\n=\n\n\n\n\nn\n\n2\n\n\n−\n\nn\n\n1\n\n\n\nR\n\n\n\n\n{\\displaystyle {\\frac {n_{2}}{v}}+{\\frac {n_{1}}{u}}={\\frac {n_{2}-n_{1}}{R}}}\nwhere R is the radius of the spherical surface, n2 is the refractive index of the surface, and n1 is"},{"doc_id":5498563415514282000,"text":"Farsightedness or hyperopia, is a vision condition in which distant objects\nare seen clearly, but close objects does not come into proper focus.\nFarsightedness occurs if the eyeball is too short or the cornea has too little\ncurvature. Light entering the eye is therefore not focused correctly. Common\nsigns of farsightedness include difficulty in concentrating and maintaining a\nclear focus on near objects. Also eye strain, fatigue and/or headaches after\nclose work. Aching or burning of the eyes and irritability or nervousness\n"},{"doc_id":631565061695999000,"text":"effective) focal length of the lens;\nn is the refractive index of the lens material;\nR1 is the (signed, see below) radius of curvature of the lens surface closer to the light source;\nR2 is the radius of curvature of the lens surface farther from the light source; and\nd is the thickness of the lens (the distance along the lens axis between the two surface vertices).\nThe focal length f is with respect to the principal planes of the lens, and the locations of the planes \n\n\n\n\nh\n\n1\n\n\n\n\n"},{"doc_id":4597998300836652000,"text":" then how it could be possible to see image? \nThe image is formed before the retina. If a screen is placed at this position, the image will be clear and sharp. Why?"},{"doc_id":2059059180795264800,"text":"The lens focuses light into the retina\n\n### Retina"},{"doc_id":7180154509523189000,"text":"The use of glasses or contact lenses does not affect the normal progression of\nmyopia -- they simply focus the light so the nearsighted person can see\ndistant objects clearly. However, it is important to not prescribe glasses or\ncontact lenses that are too strong. Hard contact lenses will sometimes hide\nthe progression of nearsightedness, but vision will still get worse \"under\"\nthe contact lens."},{"doc_id":6454700608850699000,"text":"2}}}\\,.}\nThe distance from the front focal point of the combined lenses to the first lens is called the front focal length (FFL):\nFFL\n\n=\n\n\n\n\nf\n\n1\n\n\n(\n\nf\n\n2\n\n\n−\nd\n)\n\n\n(\n\nf\n\n1\n\n\n+\n\nf\n\n2\n\n\n)\n−\nd\n\n\n\n,\n.\n\n\n{\\displaystyle {\\text{FFL}}={\\frac {f_{1}(f_{2}-d)}{("}],"string":"[\n {\n \"doc_id\": 5277346963269157000,\n \"text\": \"If you have myopia, your eyes are focused up close\"\n },\n {\n \"doc_id\": 6483076863330530000,\n \"text\": \" Focus that happens in front of the retina of your eye instead\\nof at the retina results in myopia (nearsightedness).\"\n },\n {\n \"doc_id\": 8583842396495696000,\n \"text\": \"while the question is correct- light entering the eye from optical infinity (ie distance) comes to focus in front of the retina in myopia, and forms an “apparent “ focus behind the retina in hyperopia, to understand correction we need to consider the same situation, but with a slight tweak. But before then we need to also understand the principle of reversibility in ray tracing, and lens orientation.\"\n },\n {\n \"doc_id\": 7748910388324795000,\n \"text\": \" required is for objects at infinite distance. More is required the closer object gets. The eye is set up so this minimum is when eye lens muscles are in relaxed state. The refraction power can only be increased by the muscles.\"\n },\n {\n \"doc_id\": 2570414022684135400,\n \"text\": \" divergent light rays from near objects onto the retina,\\nallowing closer objects to be brought into better focus. [31] [32]\"\n },\n {\n \"doc_id\": 6751751125259898000,\n \"text\": \"So think of binoculars. If you focus them near, then things in the distance are blurry. If you focus them at far objects, then nearer ones are blurry.\"\n },\n {\n \"doc_id\": 5962416175974752000,\n \"text\": \"Profile photo for Bill Otto\\nBill Otto\\nTop Writer Eyesight & Optometry at QuoraAuthor has 8.2K answers and 66.2M answer views\\n·\\nFeb 21\\nRelated\\nWhy and how are long-distance images blurry in myopia and opposite in hypermyopia?\\nI think you meant to write “hyperopia.”\"\n },\n {\n \"doc_id\": 3116876100822912000,\n \"text\": \" to adjust its focal length, so that\\nthe image can be formed on the retina.\\n\\nThere is a large space between the lens and the retina, called posterior\\nchamber. It is filled with the vitreous humor which is a transparent colorless\\njelly-like substance.\"\n },\n {\n \"doc_id\": 8838327111474969000,\n \"text\": \"ations that might otherwise result from the changing shape while better fitting mathematical modeling.\\nThe \\\"catenary\\\" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change it's curve when the poles are moved closer together.\"\n },\n {\n \"doc_id\": 7125326944439106000,\n \"text\": \" form in the eye of a nearsighted person?\\nRelated questions\\nHow do diverging lenses correct nearsightedness?\\nHow does focused light in front of the retina make us near-sighted and not otherwise? Does it have anything to do with image formation?\\nWhy does light focusing in front of the retina mostly cause problems with seeing objects that are far (nearsightedness), and light focusing past the retina causes problems with seeing objects that are close (farsightedness)?\\nHow do one’s eyes go from farsighted to nearsighted?\\nI\"\n },\n {\n \"doc_id\": 4002039531316687400,\n \"text\": \"Related questions\\nHow do diverging lenses correct nearsightedness?\\nHow does focused light in front of the retina make us near-sighted and not otherwise? Does it have anything to do with image formation?\\nWhy does light focusing in front of the retina mostly cause problems with seeing objects that are far (nearsightedness), and light focusing past the retina causes problems with seeing objects that are close (farsightedness)?\\nHow do one’s eyes go from farsighted to nearsighted?\\nI am a farsighted person but my lenses is\"\n },\n {\n \"doc_id\": 7037178924202905000,\n \"text\": \"Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image.\\nAccommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled.\\nThe main ways animals may change focus are:\"\n },\n {\n \"doc_id\": 738652577321205800,\n \"text\": \" less curved lens. This is achieved by relaxing some of the sphincter like ciliary muscles. While not referenced this presumably allows the pressure in the eyeball to again expand it outwards, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried none has come close enough to working using only the Helmholtz mechanisms.\\nSchachar model of lens focus\\nSchachar has proposed a model for land based vertebrates that was not well received. The theory allows\"\n },\n {\n \"doc_id\": 4070135853981967400,\n \"text\": \"A normal young eye, focuses on distant objects and accommodation allows automatic focusing on near objects. Hyperopia is when your eyes are focused beyond far so nothing is in focus. Your accommodation can add enough focusing power to just see far objects clearly.\"\n },\n {\n \"doc_id\": 3962391993003015700,\n \"text\": \"Sponsored by Grammarly\\nHit the mark with every message.\\nPredict how your reader will react, see how you sound, and get the response you want—with Grammarly.\\nRelated questions\\nFor a nearsighted person, is the image focused in front of the retina? True or False?\\nMyopia and hyperopia causes are explainable by the eye lense getting stretched or not, but how does by focal condition occur?\\nCan mild hyperopia/farsightedness be reversed without the use of corrective lenses or surgery?\\nWhy is my eye bleeding\"\n },\n {\n \"doc_id\": 3271388781564688400,\n \"text\": \"Nearsightedness is when light entering the eye is focused incorrectly. This\\nmakes distant objects appear blurred. Nearsightedness is a type of refractive\\nerror of the eye.\"\n },\n {\n \"doc_id\": 5281582486101445000,\n \"text\": \"Profile photo for Valente Hernandez Perez\\nValente Hernandez Perez\\nReduced my myopia naturallyAuthor has 3.4K answers and 3.5M answer views\\n·\\n3y\\nRelated\\nHow do diverging lenses correct nearsightedness?\\nNearsightedness occurs when the eye can only see near objects but not far objects. This is because the light coming from far objects focus in front of the retina, rather than on the retina. Diverging lenses, also called concave lenses, move the converging point of the light backwards towards the retina. Therefore, diverging\"\n },\n {\n \"doc_id\": 8538690308462247000,\n \"text\": \"\\nIn a nearsighted the image of a far object is focused in front of the retina while in a farsighted the image is focused behind the retina. How are you going to correct these eye defect s using diverging and converging lenses? Explain.\\nSort\\nProfile photo for Malcolm Maciver\\nMalcolm Maciver\\nOptometrist & AcademicAuthor has 429 answers and 779K answer views\\n·\\n3y\\nWell - while not the most eloquently asked - I think this is a good point to begin an explanation.\"\n },\n {\n \"doc_id\": 9055938428256027000,\n \"text\": \"Sponsored by Online Shopping Tools\\nAmazon Hates When Prime Members Do This, But They Can't Stop You.\\nThis simple trick can save tons of money on Amazon, but most Prime members are ignoring it.\\nRelated questions\\nHow do diverging lenses correct nearsightedness?\\nHow does focused light in front of the retina make us near-sighted and not otherwise? Does it have anything to do with image formation?\\nWhy does light focusing in front of the retina mostly cause problems with seeing objects that are far (nearsightedness), and light focusing past the retina causes problems\"\n },\n {\n \"doc_id\": 597238679023231400,\n \"text\": \"Lenses cannot refract light rays at their edges as well as closer to the\\ncentre. The image produced by any lens is therefore somewhat blurry around the\\nedges ( [ spherical aberration ](/wiki/Spherical_aberration \\\"Spherical\\naberration\\\") ). It can be minimized by screening out peripheral light rays and\\nlooking only at the better-focused centre. In the eye, the pupil serves this\\npurpose by constricting while the eye is focused on nearby objects. Small\\napertures also give an increase in [ depth of field ](/wiki/\"\n },\n {\n \"doc_id\": 3948332616404524500,\n \"text\": \" and farsightedness (hyperopia), and how are these vision problems treated?\\nWhat is the possible disease if one retina is larger than the retina in other eye?\\nIf you have presbyopia, does far object focus in front of the retina and close object focus behind the retina?\\nA nearsighted person has a near point of 12 cm and a far point of 17 cm. If the corrective lens is 2.0 cm from his eye, what lens power will enable this person to see distant objects clearly (answer should be in diopters)?\\nWhere does the image\"\n },\n {\n \"doc_id\": 8923114265773640000,\n \"text\": \"The adjustment to close-range vision involves three processes to focus an\\nimage on the retina.\"\n },\n {\n \"doc_id\": 2141862257330952000,\n \"text\": \" with seeing objects that are close (farsightedness)?\\nHow do one’s eyes go from farsighted to nearsighted?\\nI am a farsighted person but my lenses is for nearsighted. Will this affect my eyes?\\nProfile photo for Xaun Loc\\nXaun Loc\\nAdult With an IQ Above Room Temperature! (1948–present)Author has 6K answers and 6.5M answer views\\n·\\n2y\\nOriginally Answered: In a nearsighted the image of a far object is focused in front of the retina while\"\n },\n {\n \"doc_id\": 1314637696850278000,\n \"text\": \"[\\n ](/wiki/File:Arizona_eye_model.png) Arizona Eye Model.\\n\\\"A\\\" is accommodation in diopters.\"\n },\n {\n \"doc_id\": 3708577464021101600,\n \"text\": \"\\nProfile photo for Assistant\\nAssistant\\nBot\\n·\\nNov 28\\nTo correct nearsightedness, which is also known as myopia, a diverging lens is used. This type of lens spreads out the light rays before they enter the eye, which helps to move the focal point back to the retina. This allows the individual to see distant objects more clearly.\"\n },\n {\n \"doc_id\": 7836815286624988000,\n \"text\": \"Nearsightedness, or myopia, as it is medically termed, is a vision condition\\nin which people can see close objects clearly, but objects farther away appear\\nblurred.\"\n },\n {\n \"doc_id\": 6534254037188262000,\n \"text\": \"Myopia (nearsightedness) is a common condition that’s usually diagnosed before\\nage 20. It affects your distance vision — you can see objects that are near,\\nbut you have trouble viewing objects that are farther away like grocery store\\naisle markers or road signs. Myopia treatments include glasses, contact lenses\\nor surgery.\"\n },\n {\n \"doc_id\": 933988202701675800,\n \"text\": \" This model requires fluid movement of the lens front only rather than trying to change the shape of the lens as a whole.\\nInternal forces[edit]\\nTracing of Scheimpflug photographs of 20 year old human lens being thicker focusing near and thinner when focusing far. Internal layering of the lens is also significant\\nWrinkled lens fibers in picture below compared to straight fibers above\\nWhen Thomas Young proposed the changing of the human lens's shape as the mechanism for focal accommodation in 1801 he thought the lens may be a muscle capable of contraction. This type of model is termed intr\"\n },\n {\n \"doc_id\": 4928957121947894000,\n \"text\": \"## Near response [ [ edit\\n](/w/index.php?title=Human_eye&action=edit§ion=15 \\\"Edit section: Near\\nresponse\\\") ]\"\n },\n {\n \"doc_id\": 1081809044795926500,\n \"text\": \" to focus on the retina in myopes.\"\n },\n {\n \"doc_id\": 4531302417764727000,\n \"text\": \"[edit]\\n3D reconstruction of lens in a living 20 year old human male focusing from 0 dioptres (infinity) to 4.85 dioptres (26mm) side & back views\\nExternal forces[edit]\\nTwo horse lenses suspended on water by cling wrap with 4 approximately parallel lasers directed through them. The 1 cm spaced grid indicates an accommodated, i.e. relaxed, near focus, focal length of around 6cm\\nThe model of a shape changing lens of humans was proposed by Young in a lecture on the 27th Nov 1800. Others such as Helm\"\n },\n {\n \"doc_id\": 7697060254554999000,\n \"text\": \"[ ](mailto:?subject=Cleveland Clinic - Myopia\\n\\\\(Nearsightedness\\\\)&body=https://my.clevelandclinic.org/health/diseases/8579-myopia-\\nnearsightedness)\"\n },\n {\n \"doc_id\": 5665544169939711000,\n \"text\": \"Function[edit]\\nFocusing[edit]\\nAn image that is partially in focus, but mostly out of focus in varying degrees.\\nEye and detailed ray path including one intraocular lens layer\\nIn humans the widely quoted Helmholtz mechanism of focusing, also called accommodation, is often referred to as a \\\"model\\\". Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animals. When considering all vertebrates aspects of all models may play varying roles in lens focus.\\nThe shape changing lens of many land based vertebrates\"\n },\n {\n \"doc_id\": 6993670070181808000,\n \"text\": \"\"\n },\n {\n \"doc_id\": 4210204835744713000,\n \"text\": \"Depth_of_field\\n\\\"Depth of field\\\"), allowing a broader range of \\\"in focus\\\" vision. In this way\\nthe pupil has a dual purpose for near vision: to reduce spherical aberration\\nand increase depth of field. [31]\"\n },\n {\n \"doc_id\": 7224089652970956000,\n \"text\": \"[ Healthy Eyes ](/healthy-eyes \\\"Healthy Eyes\\\") / [ Eye and Vision Conditions\\n](/healthy-eyes/eye-and-vision-conditions \\\"Eye and Vision Conditions\\\") / [\\nMyopia ](/healthy-eyes/eye-and-vision-conditions/myopia \\\"Myopia\\\")\"\n },\n {\n \"doc_id\": 7926337537666380000,\n \"text\": \"Profile photo for Raphael\\nRaphael\\nAn African student optometrist\\n·\\n7y\\nRelated\\nCan you cure far-sightedness by looking at objects up-close?\\nThe answer is simply no.Far-sightedness(hypermetropia) can be diagnosed only when one is looking at far(6m and beyond)...this why your optometrist asks you to look at a test chart(VA) placed at 6m from ur examination chair.During looking at far your lens is relaxed and there is no accommodation.When looking at near,your lens contracts temporarily to\"\n },\n {\n \"doc_id\": 416839722049606660,\n \"text\": \"Profile photo for Sharon Breivogel-Leonard\\nSharon Breivogel-Leonard\\nOptician and Contact Lens Practitioner licensed in N.Y.Author has 3K answers and 14.5M answer views\\n·\\n4y\\nRelated\\nShouldn’t elongated eye lens converge the rays of light behind retina? If yes, then how is elongation of eye lens a cause of myopia?\\n“Shouldn’t elongated eye lens converge the rays of light behind retina? If yes, then how is elongation of eye\"\n },\n {\n \"doc_id\": 6712287497928144000,\n \"text\": \" for nearsighted. Will this affect my eyes?\\nFor a nearsighted person, is the image focused in front of the retina? True or False?\\nMyopia and hyperopia causes are explainable by the eye lense getting stretched or not, but how does by focal condition occur?\\nCan mild hyperopia/farsightedness be reversed without the use of corrective lenses or surgery?\\nWhy is my eye bleeding behind the retina?\\nWhich one is worse: being nearsighted or farsighted?\\nCan you explain the difference between nearsightedness (myopia)\"\n },\n {\n \"doc_id\": 9060855836708261000,\n \"text\": \"Myopia is the medical name for nearsightedness, which means that you can see\\nobjects that are near clearly but have difficulty seeing objects that are\\nfarther away. For example, if you’re nearsighted, you may not be able to make\\nout highway signs until they’re just a few feet away.\"\n },\n {\n \"doc_id\": 4921471637836497000,\n \"text\": \" in a farsighted the image is focused behind the retina. How are you going to correct these eye defect s using diverging and converging lenses? Explain.\\nA2A: In a nearsighted person, the image of a far object is focused in front of the retina while in a farsighted person, the image is focused behind the retina. How would you correct these eye defects using diverging and converging lenses?\"\n },\n {\n \"doc_id\": 1316363963287033600,\n \"text\": \"The focus needs to be precise. If it isn’t, what you’re looking at appears\\nblurry. Your eye has muscles that can make subtle changes to the shape of your\\neye, moving the focus point so it lands correctly on the retina.\\n\\nWhen light lands on the cells of your retinas, those cells send signals to\\nyour brain. The signals are like coded messages describing everything they can\\nabout the light. That includes the color, how intense it is and any other\\nrelevant details. Your brain decodes and processes the signals and uses them\\n\"\n },\n {\n \"doc_id\": 7433487709782678000,\n \"text\": \"* * *\\n\\nImage credit The image formation on the retina\"\n },\n {\n \"doc_id\": 3391843269234439000,\n \"text\": \" a screen or image sensor, is known as a real image. This is the principle of the camera, and also of the human eye, in which the retina serves as the image sensor.\\nThe focusing adjustment of a camera adjusts S2, as using an image distance different from that required by this formula produces a defocused (fuzzy) image for an object at a distance of S1 from the camera. Put another way, modifying S2 causes objects at a different S1 to come into perfect focus.\\nVirtual image formation using a positive lens as a magnifying glass.\\nIn\"\n },\n {\n \"doc_id\": 9010197787198001000,\n \"text\": \" lenses correct nearsightedness by focusing the light coming from far objects onto the retina.\"\n },\n {\n \"doc_id\": 8588554961387381000,\n \"text\": \". As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.\\n\"\n },\n {\n \"doc_id\": 7035571819567415000,\n \"text\": \"Image credit The diagram of the human eye\\n\\n* * *\"\n },\n {\n \"doc_id\": 7664261621066329000,\n \"text\": \"��ிரமநாயகம்)\\nM.Sc. Retired as Head of Department of Physics. Government of Tamilnadu, lndia.Author has 10.2K answers and 6.9M answer views\\n·\\n3y\\nRelated\\nA myopia patient could see objects blurred. So if the image is formed before retina (which works as a screen) then how that person could see the object even blur. if the image is not forming in retina\"\n },\n {\n \"doc_id\": 9089020966453622000,\n \"text\": \" function in mammals. With vision in fish and amphibians, the lens is fixed in shape, and focusing is instead achieved by moving the lens forwards or backwards within the eye using a muscle called the retractor lentus.\\nIn cartilaginous fish, the suspensory ligaments are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process,\"\n },\n {\n \"doc_id\": 7836039674261284000,\n \"text\": \" mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberr\"\n },\n {\n \"doc_id\": 5776985908033602000,\n \"text\": \"# Myopia (nearsightedness)\"\n },\n {\n \"doc_id\": 3197771491313399000,\n \"text\": \"When a creature with binocular vision looks at an object, the eyes must rotate\\naround a vertical axis so that the projection of the image is in the centre of\\nthe retina in both eyes. To look at a nearby object, the eyes rotate 'towards\\neach other' ( [ convergence ](/wiki/Convergence_\\\\(eye\\\\) \\\"Convergence \\\\(eye\\\\)\\\")\\n), while for an object farther away they rotate 'away from each other' ( [\\ndivergence ](/wiki/Divergence_\\\\(eye\\\\) \\\"Diver\"\n },\n {\n \"doc_id\": 3258428016166494700,\n \"text\": \"Myopia (Nearsightedness)\"\n },\n {\n \"doc_id\": 1274739173265799200,\n \"text\": \"Profile photo for Francis Vincent\\nFrancis Vincent\\nAuthor has 7.2K answers and 6.3M answer views\\n3y\\nRelated\\nMy right eye sees far objects better but cannot focus as well on nearer objects as my left eye does. The opposite is on my left eye (cannot see far objects, better on near). Is this normal?\\nHaving one far sighted eye and one near sighted eye is called antimetropia and isn't very common. I had that when I was younger. My optometrist said I didn't need glasses as long as it\"\n },\n {\n \"doc_id\": 4503927224653083000,\n \"text\": \" behind the retina?\\nWhich one is worse: being nearsighted or farsighted?\\nProfile photo for Don Vater\\nDon Vater\\nStudied Optometry at Indiana University (Graduated 1970)Author has 551 answers and 793K answer views\\n·\\n3y\\nRelated\\nHow do diverging lenses correct nearsightedness?\\nWith nearsightedness (myopia) the eyeball length is too long and the light rays focus in front of the retina. Divergent (negative/concave) lenses make the light rays diverge to focus back further on\"\n },\n {\n \"doc_id\": 2139784360858340900,\n \"text\": \"* [ ](/wiki/File:Blausen_0389_EyeAnatomy_02.png)\"\n },\n {\n \"doc_id\": 9212096145168112000,\n \"text\": \"Myopia; Shortsightedness; Refractive error - nearsightedness\"\n },\n {\n \"doc_id\": 6370306629959697000,\n \"text\": \"Light energy enters the eye through the cornea, through the pupil and then\\nthrough the lens. The lens shape is changed for near focus (accommodation) and\\nis controlled by the ciliary muscle. Between the two lenses, there are four [\\noptical surfaces ](/wiki/Optical_surfaces \\\"Optical surfaces\\\") which each [\\nrefract ](/wiki/Refraction \\\"Refraction\\\") light as it travels along the optical\\npath. One basic model describing the geometry of the optical system is the\\nArizona Eye Model. [2]\"\n },\n {\n \"doc_id\": 5963560923440762000,\n \"text\": \"* Some people may experience blurred distance vision only at night. With \\\"night myopia,\\\" low light makes it difficult for the eyes to focus properly. Or the increased pupil size during dark conditions allows more peripheral, unfocused light rays to enter the eye.\\n* People who do an excessive amount of near-vision work may experience a false or \\\"pseudo\\\" myopia. Their blurred distance vision is caused by overuse of the eyes' focusing mechanism. After long periods of near work, their eyes are unable to refocus to see clearly in the distance. Clear distance vision usually returns after resting the\"\n },\n {\n \"doc_id\": 7726529561464266000,\n \"text\": \"A nearsighted person sees close-up objects clearly, but objects in the\\ndistance are blurred. Squinting will tend to make far away objects seem\\nclearer.\"\n },\n {\n \"doc_id\": 4485262315221988000,\n \"text\": \"The lens , which sits behind the iris, changes its shape to focus light onto\\nthe retina. Small muscles (ciliary muscles) contract to cause the lens to\\nbecome thicker, which allows the lens to focus on nearby objects. The ciliary\\nmuscles relax to cause the lens to become thinner when it focuses on distant\\nobjects. These lens changes are limited in dogs. The retina contains the\\ncells that sense light (photoreceptors). The most sensitive area of the retina\\nis called the area centralis in dogs; this area contains thousands of\"\n },\n {\n \"doc_id\": 4868953833671812000,\n \"text\": \"Common eye defects like myopia (nearsightedness), hyperopia (farsightedness)\\nand astigmatism are often called \\\"refractive errors\\\". They affect how the eyes\\nbend or \\\"refract\\\" light and they can usually be corrected by relatively simple\\ncompensating lenses or by surgery changing the shape of the cornea in order to\\nimprove the refractive state of the eye. Reshaping of the cornea is frequently\\nperformed by photoablation using laser.\\n\\nMyopia (Nearsightedness)\"\n },\n {\n \"doc_id\": 8189386028845468000,\n \"text\": \" am a farsighted person but my lenses is for nearsighted. Will this affect my eyes?\\nFor a nearsighted person, is the image focused in front of the retina? True or False?\\nAboutCareersPrivacyTermsContactLanguagesYour Ad ChoicesPress© Quora, Inc. 2024\"\n },\n {\n \"doc_id\": 5803745890755941000,\n \"text\": \"# Myopia (Nearsightedness)\"\n },\n {\n \"doc_id\": 8400401202443150000,\n \"text\": \" which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable. The most basic form, seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. Such a non-homogeneous\"\n },\n {\n \"doc_id\": 6376945335873103000,\n \"text\": \"\"\n },\n {\n \"doc_id\": 1254291911034754300,\n \"text\": \" angular size of the moon seen from earth of about 0.5°.\\nIn the extreme case where an object is an infinite distance away, S1 = ∞, S2 = f and M = −f/∞ = 0, indicating that the object would be imaged to a single point in the focal plane. In fact, the diameter of the projected spot is not actually zero, since diffraction places a lower limit on the size of the point spread function. This is called the diffraction limit.\\nImages of black letters in a thin convex lens of focal length f\"\n },\n {\n \"doc_id\": 6706918406178198000,\n \"text\": \"* [ Refractive errors and refraction ](/eye-exam/refraction.htm)\\n* [ Visual Acuity: 20/20 Vision ](/eye-exam/2020-vision.htm)\\n* [ Hyperopia (farsightedness) ](/conditions/hyperopia.htm)\"\n },\n {\n \"doc_id\": 348459215555378240,\n \"text\": \"Main article: [ Vergence ](/wiki/Vergence \\\"Vergence\\\")\"\n },\n {\n \"doc_id\": 6365536122890732000,\n \"text\": \"On the other hand, to correct farsightedness, also known as hyperopia, a converging lens is used. This type of lens brings the light rays together before they enter the eye, which helps to move the focal point forward to the retina. This allows the individual to see nearby objects more clearly.\"\n },\n {\n \"doc_id\": 8600789022803291000,\n \"text\": \"* [ ](/wiki/File:Blausen_0388_EyeAnatomy_01.png)\"\n },\n {\n \"doc_id\": 8311068887189887000,\n \"text\": \" the eye's [ optic axis\\n](/wiki/Optic_axis \\\"Optic axis\\\"). In order, along the optic axis, the optical\\ncomponents consist of a first lens (the [ cornea—the clear part of the eye\\n](/wiki/Cornea \\\"Cornea\\\") ) that accounts for most of the optical power of the\\neye and accomplishes most of the [ focusing of light ](/wiki/Focus_\\\\(optics\\\\)\\n\\\"Focus \\\\(optics\\\\)\\\") from the outside world; then an [ aperture\\n](/wiki\"\n },\n {\n \"doc_id\": 3617328645619534000,\n \"text\": \"### What causes myopia?\"\n },\n {\n \"doc_id\": 4844955335891414000,\n \"text\": \"Myopia affects a significant percentage of people. It’s an [ eye\\n](https://my.clevelandclinic.org/health/body/21823-eyes) focus disorder that’s\\nnormally corrected with eyeglasses, contact lenses or surgery.\"\n },\n {\n \"doc_id\": 3855191632893827000,\n \"text\": \"The lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. Along with the cornea, aqueous and vitreous humours it refracts light, focusing it onto the retina. In many land animals the shape of the lens can be altered, effectively changing the focal length of the eye, enabling them to focus on objects at various distances. This adjustment of the lens is known as accommodation (see also below). In many fully aquatic vertebrates such as fish other methods of accommodation are used such as changing the lens's position relative\"\n },\n {\n \"doc_id\": 451401416707211000,\n \"text\": \"* [ Refractive Errors ](/refractive-errors/) __\"\n },\n {\n \"doc_id\": 3143293289961343000,\n \"text\": \" between object and image is given by\\nn\\n\\n2\\n\\n\\nv\\n\\n\\n+\\n\\n\\n\\nn\\n\\n1\\n\\n\\nu\\n\\n\\n=\\n\\n\\n\\n\\nn\\n\\n2\\n\\n\\n−\\n\\nn\\n\\n1\\n\\n\\n\\nR\\n\\n\\n\\n\\n{\\\\displaystyle {\\\\frac {n_{2}}{v}}+{\\\\frac {n_{1}}{u}}={\\\\frac {n_{2}-n_{1}}{R}}}\\nwhere R is the radius of the spherical surface, n2 is the refractive index of the surface, and n1 is\"\n },\n {\n \"doc_id\": 5498563415514282000,\n \"text\": \"Farsightedness or hyperopia, is a vision condition in which distant objects\\nare seen clearly, but close objects does not come into proper focus.\\nFarsightedness occurs if the eyeball is too short or the cornea has too little\\ncurvature. Light entering the eye is therefore not focused correctly. Common\\nsigns of farsightedness include difficulty in concentrating and maintaining a\\nclear focus on near objects. Also eye strain, fatigue and/or headaches after\\nclose work. Aching or burning of the eyes and irritability or nervousness\\n\"\n },\n {\n \"doc_id\": 631565061695999000,\n \"text\": \"effective) focal length of the lens;\\nn is the refractive index of the lens material;\\nR1 is the (signed, see below) radius of curvature of the lens surface closer to the light source;\\nR2 is the radius of curvature of the lens surface farther from the light source; and\\nd is the thickness of the lens (the distance along the lens axis between the two surface vertices).\\nThe focal length f is with respect to the principal planes of the lens, and the locations of the planes \\n\\n\\n\\n\\nh\\n\\n1\\n\\n\\n\\n\\n\"\n },\n {\n \"doc_id\": 4597998300836652000,\n \"text\": \" then how it could be possible to see image? \\nThe image is formed before the retina. If a screen is placed at this position, the image will be clear and sharp. Why?\"\n },\n {\n \"doc_id\": 2059059180795264800,\n \"text\": \"The lens focuses light into the retina\\n\\n### Retina\"\n },\n {\n \"doc_id\": 7180154509523189000,\n \"text\": \"The use of glasses or contact lenses does not affect the normal progression of\\nmyopia -- they simply focus the light so the nearsighted person can see\\ndistant objects clearly. However, it is important to not prescribe glasses or\\ncontact lenses that are too strong. Hard contact lenses will sometimes hide\\nthe progression of nearsightedness, but vision will still get worse \\\"under\\\"\\nthe contact lens.\"\n },\n {\n \"doc_id\": 6454700608850699000,\n \"text\": \"2}}}\\\\,.}\\nThe distance from the front focal point of the combined lenses to the first lens is called the front focal length (FFL):\\nFFL\\n\\n=\\n\\n\\n\\n\\nf\\n\\n1\\n\\n\\n(\\n\\nf\\n\\n2\\n\\n\\n−\\nd\\n)\\n\\n\\n(\\n\\nf\\n\\n1\\n\\n\\n+\\n\\nf\\n\\n2\\n\\n\\n)\\n−\\nd\\n\\n\\n\\n,\\n.\\n\\n\\n{\\\\displaystyle {\\\\text{FFL}}={\\\\frac {f_{1}(f_{2}-d)}{(\"\n }\n]","isConversational":false}}},{"rowIdx":95,"cells":{"query":{"kind":"string","value":"I've been reading about hybridization in nature and came across an interesting case involving the gelada and the hamadryas baboon. Despite these two species belonging to different genera and having diverged millions of years ago, interbreeding still occurs in the wild. What are the potential evolutionary implications of such intergeneric hybridization? How common is this phenomenon, and what conditions might facilitate or hinder the formation of hybrid zones between distinct genera? Are there any notable examples of intergeneric hybrids that have been documented to survive and possibly thrive in natural environments?"},"positive_passages":{"kind":"list like","value":[{"doc_id":8889518085160947000,"text":"Post-zygotic isolation[edit]\nA number of mechanisms which act after fertilization preventing successful inter-population crossing are discussed below.\nZygote mortality and non-viability of hybrids[edit]\nA type of incompatibility that is found as often in plants as in animals occurs when the egg or ovule is fertilized but the zygote does not develop, or it develops and the resulting individual has a reduced viability. This is the case for crosses between species of the frog order, where widely differing results are observed depending upon the species involved. In some crosses there is no segmentation of the zygote (or it may be that the hybrid is extremely non-viable and changes occur from the first mitosis). In others, normal segmentation occurs in the blastula but gastrulation fails. Finally, in other crosses, the initial stages are normal but errors occur in the final phases of embryo development. This indicates differentiation of the embryo development genes (or gene complexes) in these species and these differences determine the non-viability of the hybrids.\nSimilar results are observed in mosquitoes of the genus Culex, but the differences are seen between reciprocal crosses, from which it is concluded that the same effect occurs in the interaction between the genes of the cell nucleus (inherited from both parents) as occurs in the genes of the cytoplasmic organelles which are inherited solely from the female progenitor through the cytoplasm of the ovule.\nIn Angiosperms, the successful development of the embryo depends on the normal functioning of its endosperm.\nThe failure of endosperm development and its subsequent abortion has been observed in many interploidal crosses (that is, those between populations with a particular degree of intra or interspecific ploidy), and in certain crosses in species with the same level of ploidy. The collapse of the endosperm, and the subsequent abortion of the hybrid embryo is one of the most common post-fertilization reproductive isolation mechanism found in angiosperms.\nHybrid sterility[edit]\nMules are hybrids with interspecific sterility.\nA hybrid may have normal viability but is typically deficient in terms of reproduction or is sterile. This is demonstrated by the mule and in many other well known hybrids. In all of these cases sterility is due to the interaction between the genes of the two species involved; to chromosomal imbalances due to the different number of chromosomes in the parent species; or to nucleus-cytoplasmic interactions such as in the case of Culex described above.\nHinnies and mules are hybrids resulting from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus Equus, but Equus caballus has 64 chromosomes, while Equus asinus only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not cross. In order to obtain mules or hinnies it is necessary to train the progenitors to accept copulation between the species or create them through artificial insemination.\nThe sterility of many interspecific hybrids in angiosperms has been widely recognised and studied.\nInterspecific sterility of hybrids in plants has multiple possible causes. These may be genetic, related to the genomes, or the interaction between nuclear and cytoplasmic factors, as will be discussed in the corresponding section. Nevertheless, in plants, hybridization is a stimulus for the creation of new species – the contrary to the situation in animals.\nAlthough the hybrid may be sterile, it can continue to multiply in the wild by asexual reproduction, whether vegetative propagation or apomixis or the production of seeds.\nIndeed, interspecific hybridization can be associated with polyploidia and, in this way, the origin of new species that are called allopolyploids. Rosa canina, for example, is the result of multiple hybridizations. The common wheat (Triticum aestivum) is an allohexaploid (allopolyploid with six chromosome sets) that contains the genomes of three different species."}],"string":"[\n {\n \"doc_id\": 8889518085160947000,\n \"text\": \"Post-zygotic isolation[edit]\\nA number of mechanisms which act after fertilization preventing successful inter-population crossing are discussed below.\\nZygote mortality and non-viability of hybrids[edit]\\nA type of incompatibility that is found as often in plants as in animals occurs when the egg or ovule is fertilized but the zygote does not develop, or it develops and the resulting individual has a reduced viability. This is the case for crosses between species of the frog order, where widely differing results are observed depending upon the species involved. In some crosses there is no segmentation of the zygote (or it may be that the hybrid is extremely non-viable and changes occur from the first mitosis). In others, normal segmentation occurs in the blastula but gastrulation fails. Finally, in other crosses, the initial stages are normal but errors occur in the final phases of embryo development. This indicates differentiation of the embryo development genes (or gene complexes) in these species and these differences determine the non-viability of the hybrids.\\nSimilar results are observed in mosquitoes of the genus Culex, but the differences are seen between reciprocal crosses, from which it is concluded that the same effect occurs in the interaction between the genes of the cell nucleus (inherited from both parents) as occurs in the genes of the cytoplasmic organelles which are inherited solely from the female progenitor through the cytoplasm of the ovule.\\nIn Angiosperms, the successful development of the embryo depends on the normal functioning of its endosperm.\\nThe failure of endosperm development and its subsequent abortion has been observed in many interploidal crosses (that is, those between populations with a particular degree of intra or interspecific ploidy), and in certain crosses in species with the same level of ploidy. The collapse of the endosperm, and the subsequent abortion of the hybrid embryo is one of the most common post-fertilization reproductive isolation mechanism found in angiosperms.\\nHybrid sterility[edit]\\nMules are hybrids with interspecific sterility.\\nA hybrid may have normal viability but is typically deficient in terms of reproduction or is sterile. This is demonstrated by the mule and in many other well known hybrids. In all of these cases sterility is due to the interaction between the genes of the two species involved; to chromosomal imbalances due to the different number of chromosomes in the parent species; or to nucleus-cytoplasmic interactions such as in the case of Culex described above.\\nHinnies and mules are hybrids resulting from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus Equus, but Equus caballus has 64 chromosomes, while Equus asinus only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not cross. In order to obtain mules or hinnies it is necessary to train the progenitors to accept copulation between the species or create them through artificial insemination.\\nThe sterility of many interspecific hybrids in angiosperms has been widely recognised and studied.\\nInterspecific sterility of hybrids in plants has multiple possible causes. These may be genetic, related to the genomes, or the interaction between nuclear and cytoplasmic factors, as will be discussed in the corresponding section. Nevertheless, in plants, hybridization is a stimulus for the creation of new species – the contrary to the situation in animals.\\nAlthough the hybrid may be sterile, it can continue to multiply in the wild by asexual reproduction, whether vegetative propagation or apomixis or the production of seeds.\\nIndeed, interspecific hybridization can be associated with polyploidia and, in this way, the origin of new species that are called allopolyploids. Rosa canina, for example, is the result of multiple hybridizations. The common wheat (Triticum aestivum) is an allohexaploid (allopolyploid with six chromosome sets) that contains the genomes of three different species.\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":2028903019636597200,"text":"Genetics[edit]\nPre-copulatory mechanisms in animals[edit]\nThe genetics of ethological isolation barriers will be discussed first. Pre-copulatory isolation occurs when the genes necessary for the sexual reproduction of one species differ from the equivalent genes of another species, such that if a male of species A and a female of species B are placed together they are unable to copulate. Study of the genetics involved in this reproductive barrier tries to identify the genes that govern distinct sexual behaviors in the two species. The males of Drosophila melanogaster and those of D. simulans conduct an elaborate courtship with their respective females, which are different for each species, but the differences between the species are more quantitative than qualitative. In fact the simulans males are able to hybridize with the melanogaster females. Although there are lines of the latter species that can easily cross there are others that are hardly able to. Using this difference, it is possible to assess the minimum number of genes involved in pre-copulatory isolation between the melanogaster and simulans species and their chromosomal location.\nIn experiments, flies of the D. melanogaster line, which hybridizes readily with simulans, were crossed with another line that it does not hybridize with, or rarely. The females of the segregated populations obtained by this cross were placed next to simulans males and the percentage of hybridization was recorded, which is a measure of the degree of reproductive isolation. It was concluded from this experiment that 3 of the 8 chromosomes of the haploid complement of D. melanogaster carry at least one gene that affects isolation, such that substituting one chromosome from a line of low isolation with another of high isolation reduces the hybridization frequency. In addition, interactions between chromosomes are detected so that certain combinations of the chromosomes have a multiplying effect.\nCross incompatibility or incongruence in plants is also determined by major genes that are not associated at the self-incompatibility S locus.\nPost-copulation or fertilization mechanisms in animals[edit]\nReproductive isolation between species appears, in certain cases, a long time after fertilization and the formation of the zygote, as happens – for example – in the twin species Drosophila pavani and D. gaucha. The hybrids between both species are not sterile, in the sense that they produce viable gametes, ovules and spermatozoa. However, they cannot produce offspring as the sperm of the hybrid male do not survive in the semen receptors of the females, be they hybrids or from the parent lines. In the same way, the sperm of the males of the two parent species do not survive in the reproductive tract of the hybrid female. This type of post-copulatory isolation appears as the most efficient system for maintaining reproductive isolation in many species.\nThe development of a zygote into an adult is a complex and delicate process of interactions between genes and the environment that must be carried out precisely, and if there is any alteration in the usual process, caused by the absence of a necessary gene or the presence of a different one, it can arrest the normal development causing the non-viability of the hybrid or its sterility. It should be borne in mind that half of the chromosomes and genes of a hybrid are from one species and the other half come from the other. If the two species are genetically different, there is little possibility that the genes from both will act harmoniously in the hybrid. From this perspective, only a few genes would be required in order to bring about post copulatory isolation, as opposed to the situation described previously for pre-copulatory isolation.\nIn many species where pre-copulatory reproductive isolation does not exist, hybrids are produced but they are of only one sex. This is the case for the hybridization between females of Drosophila simulans and Drosophila melanogaster males: the hybridized females die early in their development so that only males are seen among the offspring. However, populations of D. simulans have been recorded with genes that permit the development of adult hybrid females, that is, the viability of the females is \"rescued\". It is assumed that the normal activity of these speciation genes is to \"inhibit\" the expression of the genes that allow the growth of the hybrid. There will also be regulator genes.\nA number of these genes have been found in the melanogaster species group. The first to be discovered was \"Lhr\" (Lethal hybrid rescue) located in Chromosome II of D. simulans. This dominant allele allows the development of hybrid females from the cross between simulans females and melanogaster males. A different gene, also located on Chromosome II of D. simulans is \"Shfr\" that also allows the development of female hybrids, its activity being dependent on the temperature at which development occurs. Other similar genes have been located in distinct populations of species of this group. In short, only a few genes are needed for an effective post copulatory isolation barrier mediated through the non-viability of the hybrids.\nAs important as identifying an isolation gene is knowing its function. The Hmr gene, linked to the X chromosome and implicated in the viability of male hybrids between D. melanogaster and D. simulans, is a gene from the proto-oncogene family myb, that codes for a transcriptional regulator. Two variants of this gene function perfectly well in each separate species, but in the hybrid they do not function correctly, possibly due to the different genetic background of each species. Examination of the allele sequence of the two species shows that change of direction substitutions are more abundant than synonymous substitutions, suggesting that this gene has been subject to intense natural selection.\nThe Dobzhansky–Muller model proposes that reproductive incompatibilities between species are caused by the interaction of the genes of the respective species. It has been demonstrated recently that Lhr has functionally diverged in D. simulans and will interact with Hmr which, in turn, has functionally diverged in D. melanogaster to cause the lethality of the male hybrids. Lhr is located in a heterochromatic region of the genome and its sequence has diverged between these two species in a manner consistent with the mechanisms of positive selection. An important unanswered question is whether the genes detected correspond to old genes that initiated the speciation favoring hybrid non-viability, or are modern genes that have appeared post-speciation by mutation, that are not shared by the different populations and that suppress the effect of the primitive non-viability genes. The OdsH (abbreviation of Odysseus) gene causes partial sterility in the hybrid between Drosophila simulans and a related species, D. mauritiana, which is only encountered on Mauritius, and is of recent origin. This gene shows monophyly in both species and also has been subject to natural selection. It is thought that it is a gene that intervenes in the initial stages of speciation, while other genes that differentiate the two species show polyphyly. Odsh originated by duplication in the genome of Drosophila and has evolved at very high rates in D. mauritania, while its paralogue, unc-4, is nearly identical between the species of the group melanogaster. Seemingly, all these cases illustrate the manner in which speciation mechanisms originated in nature, therefore they are collectively known as \"speciation genes\", or possibly, gene sequences with a normal function within the populations of a species that diverge rapidly in response to positive selection thereby forming reproductive isolation barriers with other species. In general, all these genes have functions in the transcriptional regulation of other genes.\nThe Nup96 gene is another example of the evolution of the genes implicated in post-copulatory isolation. It regulates the production of one of the approximately 30 proteins required to form a nuclear pore. In each of the simulans groups of Drosophila the protein from this gene interacts with the protein from another, as yet undiscovered, gene on the X chromosome in order to form a functioning pore. However, in a hybrid the pore that is formed is defective and causes sterility. The differences in the sequences of Nup96 have been subject to adaptive selection, similar to the other examples of speciation genes described above.\nPost-copulatory isolation can also arise between chromosomally differentiated populations due to chromosomal translocations and inversions. If, for example, a reciprocal translocation is fixed in a population, the hybrid produced between this population and one that does not carry the translocation will not have a complete meiosis. This will result in the production of unequal gametes containing unequal numbers of chromosomes with a reduced fertility. In certain cases, complete translocations exist that involve more than two chromosomes, so that the meiosis of the hybrids is irregular and their fertility is zero or nearly zero. Inversions can also give rise to abnormal gametes in heterozygous individuals but this effect has little importance compared to translocations. An example of chromosomal changes causing sterility in hybrids comes from the study of Drosophila nasuta and D. albomicans which are twin species from the Indo-Pacific region. There is no sexual isolation between them and the F1 hybrid is fertile. However, the F2 hybrids are relatively infertile and leave few descendants which have a skewed ratio of the sexes. The reason is that the X chromosome of albomicans is translocated and linked to an autosome which causes abnormal meiosis in hybrids. Robertsonian translocations are variations in the numbers of chromosomes that arise from either: the fusion of two acrocentric chromosomes into a single chromosome with two arms, causing a reduction in the haploid number, or conversely; or the fission of one chromosome into two acrocentric chromosomes, in this case increasing the haploid number. The hybrids of two populations with differing numbers of chromosomes can experience a certain loss of fertility, and therefore a poor adaptation, because of irregular meiosis.\nIn plants[edit]\n\nFurther information: Reproductive coevolution in Ficus\nA large variety of mechanisms have been demonstrated to reinforce reproductive isolation between closely related plant species that either historically lived or currently live in sympatry. This phenomenon is driven by strong selection against hybrids, typically resulting from instances in which hybrids suffer reduced fitness. Such negative fitness consequences have been proposed to be the result of negative epistasis in hybrid genomes and can also result from the effects of hybrid sterility. In such cases, selection gives rise to population-specific isolating mechanisms to prevent either fertilization by interspecific gametes or the development of hybrid embryos.\nBecause many sexually reproducing species of plants are exposed to a variety of interspecific gametes, natural selection has given rise to a variety of mechanisms to prevent the production of hybrids. These mechanisms can act at different stages in the developmental process and are typically divided into two categories, pre-fertilization and post-fertilization, indicating at which point the barrier acts to prevent either zygote formation or development. In the case of angiosperms and other pollinated species, pre-fertilization mechanisms can be further subdivided into two more categories, pre-pollination and post-pollination, the difference between the two being whether or not a pollen tube is formed. (Typically when pollen encounters a receptive stigma, a series of changes occur which ultimately lead to the growth of a pollen tube down the style, allowing for the formation of the zygote.) Empirical investigation has demonstrated that these barriers act at many different developmental stages and species can have none, one, or many barriers to hybridization with interspecifics.\nExamples of pre-fertilization mechanisms[edit]\nA well-documented example of a pre-fertilization isolating mechanism comes from study of Louisiana iris species. These iris species were fertilized with interspecific and conspecific pollen loads and it was demonstrated by measure of hybrid progeny success that differences in pollen-tube growth between interspecific and conspecific pollen led to a lower fertilization rate by interspecific pollen. This demonstrates how a specific point in the reproductive process is manipulated by a particular isolating mechanism to prevent hybrids.\nAnother well-documented example of a pre-fertilization isolating mechanism in plants comes from study of the 2 wind-pollinated birch species. Study of these species led to the discovery that mixed conspecific and interspecific pollen loads still result in 98% conspecific fertilization rates, highlighting the effectiveness of such barriers. In this example, pollen tube incompatibility and slower generative mitosis have been implicated in the post-pollination isolation mechanism.\nExamples of post-fertilization mechanisms[edit]\nCrosses between diploid and tetraploid species of Paspalum provide evidence of a post-fertilization mechanism preventing hybrid formation when pollen from tetraploid species was used to fertilize a female of a diploid species. There were signs of fertilization and even endosperm formation but subsequently this endosperm collapsed. This demonstrates evidence of an early post-fertilization isolating mechanism, in which the hybrid early embryo is detected and selectively aborted. This process can also occur later during development in which developed, hybrid seeds are selectively aborted.\nEffects of hybrid necrosis[edit]\nPlant hybrids often suffer from an autoimmune syndrome known as hybrid necrosis. In the hybrids, specific gene products contributed by one of the parents may be inappropriately recognized as foreign and pathogenic, and thus trigger pervasive cell death throughout the plant. In at least one case, a pathogen receptor, encoded by the most variable gene family in plants, was identified as being responsible for hybrid necrosis.\nChromosomal rearrangements in yeast[edit]\nIn brewers' yeast Saccharomyces cerevisiae, chromosomal rearrangements are a major mechanism to reproductively isolate different strains. Hou et al. showed that reproductive isolation acts postzygotically and could be attributed to chromosomal rearrangements. These authors crossed 60 natural isolates sampled from diverse niches with the reference strain S288c and identified 16 cases of reproductive isolation with reduced offspring viabilities, and identified reciprocal chromosomal translocations in a large fraction of isolates.\nIncompatibility caused by microorganisms[edit]\nIn addition to the genetic causes of reproductive isolation between species there is another factor that can cause post zygotic isolation: the presence of microorganisms in the cytoplasm of certain species. The presence of these organisms in a species and their absence in another causes the non-viability of the corresponding hybrid. For example, in the semi-species of the group D. paulistorum the hybrid females are fertile but the males are sterile, this is due to the presence of a Wolbachia in the cytoplasm which alters spermatogenesis leading to sterility. It is interesting that incompatibility or isolation can also arise at an intraspecific level. Populations of D. simulans have been studied that show hybrid sterility according to the direction of the cross. The factor determining sterility has been found to be the presence or absence of a microorganism Wolbachia and the populations tolerance or susceptibility to these organisms. This inter population incompatibility can be eliminated in the laboratory through the administration of a specific antibiotic to kill the microorganism. Similar situations are known in a number of insects, as around 15% of species show infections caused by this symbiont. It has been suggested that, in some cases, the speciation process has taken place because of the incompatibility caused by this bacteria. Two wasp species Nasonia giraulti and N. longicornis carry two different strains of Wolbachia. Crosses between an infected population and one free from infection produces a nearly total reproductive isolation between the semi-species. However, if both species are free from the bacteria or both are treated with antibiotics there is no reproductive barrier. Wolbachia also induces incompatibility due to the weakness of the hybrids in populations of spider mites (Tetranychus urticae), between Drosophila recens and D. subquinaria and between species of Diabrotica (beetle) and Gryllus (cricket)."},{"doc_id":5139828354633502000,"text":"\")\n* [ intralocus ](/wiki/Intralocus_sexual_conflict \"Intralocus sexual conflict\")\n* [ Interspecies breeding ](/wiki/Hybrid_\\(biology\\) \"Hybrid \\(biology\\)\")"},{"doc_id":4765997756196682000,"text":" some generations the two sub populations were mixed again. Subsequent matings occurred between individuals belonging to the same adapted group.\nOn the other hand, interspecific hybridization barriers can also arise as a result of the adaptive divergence that accompanies allopatric speciation. This mechanism has been experimentally proved by an experiment carried out by Diane Dodd on D. pseudoobscura. A single population of flies was divided into two, with one of the populations fed with starch-based food and the other with maltose-based food. This meant that each sub population was adapted to each food"},{"doc_id":8354333819587692000,"text":" closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example.\nSpeciation has been observed multiple times under both controlled laboratory conditions and in nature. In sexually reproducing organisms, speciation results from reproductive isolation followed by"},{"doc_id":4694161439350746000,"text":" how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many"},{"doc_id":7406906637871403000,"text":"Pre-zygotic isolation[edit]\nPre-zygotic isolation mechanisms are the most economic in terms of the natural selection of a population, as resources are not wasted on the production of a descendant that is weak, non-viable or sterile. These mechanisms include physiological or systemic barriers to fertilization.\nTemporal or habitat isolation[edit]\nSee also: Allochronic speciation\nThe Central Valley in California prevents the two salamander populations from interacting with each other which is an example of habitat isolation. After many generations the two salamander gene pools will become mutated caused by natural selection. The mutation will change the DNA sequence of the two populations enough that the salamander populations can no longer successfully breed between each other making the populations of salamander become classified as different species.\nAny of the factors that prevent potentially fertile individuals from meeting will reproductively isolate the members of distinct species. The types of barriers that can cause this isolation include: different habitats, physical barriers, and a difference in the time of sexual maturity or flowering.\nAn example of the ecological or habitat differences that impede the meeting of potential pairs occurs in two fish species of the family Gasterosteidae (sticklebacks). One species lives all year round in fresh water, mainly in small streams. The other species lives in the sea during winter, but in spring and summer individuals migrate to river estuaries to reproduce. The members of the two populations are reproductively isolated due to their adaptations to distinct salt concentrations.\nAn example of reproductive isolation due to differences in the mating season are found in the toad species Bufo americanus and Bufo fowleri. The members of these species can be successfully crossed in the laboratory producing healthy, fertile hybrids. However, mating does not occur in the wild even though the geographical distribution of the two species overlaps. The reason for the absence of inter-species mating is that B. americanus mates in early summer and B. fowleri in late summer.\nCertain plant species, such as Tradescantia canaliculata and T. subaspera, are sympatric throughout their geographic distribution, yet they are reproductively isolated as they flower at different times of the year. In addition, one species grows in sunny areas and the other in deeply shaded areas.\nBehavioral isolation[edit]\nThe different mating rituals of animal species creates extremely powerful reproductive barriers, termed sexual or behavior isolation, that isolate apparently similar species in the majority of the groups of the animal kingdom. In dioecious species, males and females have to search for a partner, be in proximity to each other, carry out the complex mating rituals and finally copulate or release their gametes into the environment in order to breed.\nThe songs of birds, insects and many other animals are part of a ritual to attract potential partners of their own species. The song presents specific patterns recognizable only by members of the same species, and therefore represents a mechanism of reproductive isolation. This recording is the song of a species of cicada, recorded in New Zealand.\nMating dances, the songs of males to attract females or the mutual grooming of pairs, are all examples of typical courtship behavior that allows both recognition and reproductive isolation. This is because each of the stages of courtship depend on the behavior of the partner. The male will only move onto the second stage of the exhibition if the female shows certain responses in her behavior. He will only pass onto the third stage when she displays a second key behavior. The behaviors of both interlink, are synchronized in time and lead finally to copulation or the liberation of gametes into the environment. No animal that is not physiologically suitable for fertilization can complete this demanding chain of behavior. In fact, the smallest difference in the courting patterns of two species is enough to prevent mating (for example, a specific song pattern acts as an isolation mechanism in distinct species of grasshopper of the genus Chorthippus).\nEven where there are minimal morphological differences between species, differences in behavior can be enough to prevent mating. For example, Drosophila melanogaster and D. simulans which are considered twin species due to their morphological similarity, do not mate even if they are kept together in a laboratory. Drosophila ananassae and D. pallidosa are twin species from Melanesia. In the wild they rarely produce hybrids, although in the laboratory it is possible to produce fertile offspring. Studies of their sexual behavior show that the males court the females of both species but the females show a marked preference for mating with males of their own species. A different regulator region has been found on Chromosome II of both species that affects the selection behavior of the females.\nPheromones play an important role in the sexual isolation of insect species. These compounds serve to identify individuals of the same species and of the same or different sex. Evaporated molecules of volatile pheromones can serve as a wide-reaching chemical signal. In other cases, pheromones may be detected only at a short distance or by contact.\nIn species of the melanogaster group of Drosophila, the pheromones of the females are mixtures of different compounds, there is a clear dimorphism in the type and/or quantity of compounds present for each sex. In addition, there are differences in the quantity and quality of constituent compounds between related species, it is assumed that the pheromones serve to distinguish between individuals of each species. An example of the role of pheromones in sexual isolation is found in 'corn borers' in the genus Ostrinia. There are two twin species in Europe that occasionally cross. The females of both species produce pheromones that contain a volatile compound which has two isomers, E and Z; 99% of the compound produced by the females of one species is in the E isomer form, while the females of the other produce 99% isomer Z. The production of the compound is controlled by just one locus and the interspecific hybrid produces an equal mix of the two isomers. The males, for their part, almost exclusively detect the isomer emitted by the females of their species, such that the hybridization although possible is scarce. The perception of the males is controlled by one gene, distinct from the one for the production of isomers, the heterozygous males show a moderate response to the odour of either type. In this case, just 2 'loci' produce the effect of ethological isolation between species that are genetically very similar.\nSexual isolation between two species can be asymmetrical. This can happen when the mating that produces descendants only allows one of the two species to function as the female progenitor and the other as the male, while the reciprocal cross does not occur. For instance, half of the wolves tested in the Great Lakes area of America show mitochondrial DNA sequences of coyotes, while mitochondrial DNA from wolves is never found in coyote populations. This probably reflects an asymmetry in inter-species mating due to the difference in size of the two species as male wolves take advantage of their greater size in order to mate with female coyotes, while female wolves and male coyotes do not mate.\nMechanical isolation[edit]\nThe flowers of many species of Angiosperm have evolved to attract and reward a single or a few pollinator species (insects, birds, mammals). Their wide diversity of form, colour, fragrance and presence of nectar is, in many cases, the result of coevolution with the pollinator species. This dependency on its pollinator species also acts as a reproductive isolation barrier.\nMating pairs may not be able to couple successfully if their genitals are not compatible. The relationship between the reproductive isolation of species and the form of their genital organs was signaled for the first time in 1844 by the French entomologist Léon Dufour. Insects' rigid carapaces act in a manner analogous to a lock and key, as they will only allow mating between individuals with complementary structures, that is, males and females of the same species (termed co-specifics).\nEvolution has led to the development of genital organs with increasingly complex and divergent characteristics, which will cause mechanical isolation between species. Certain characteristics of the genital organs will often have converted them into mechanisms of isolation. However, numerous studies show that organs that are anatomically very different can be functionally compatible, indicating that other factors also determine the form of these complicated structures.\nMechanical isolation also occurs in plants and this is related to the adaptation and coevolution of each species in the attraction of a certain type of pollinator (where pollination is zoophilic) through a collection of morphophysiological characteristics of the flowers (called pollination syndrome), in such a way that the transport of pollen to other species does not occur.\nGametic isolation[edit]\nThe synchronous spawning of many species of coral in marine reefs means that inter-species hybridization can take place as the gametes of hundreds of individuals of tens of species are liberated into the same water at the same time. Approximately a third of all the possible crosses between species are compatible, in the sense that the gametes will fuse and lead to individual hybrids. This hybridization apparently plays a fundamental role in the evolution of coral species. However, the other two-thirds of possible crosses are incompatible. It has been observed that in sea urchins of the genus Strongylocentrotus the concentration of spermatocytes that allow 100% fertilization of the ovules of the same species is only able to fertilize 1.5% of the ovules of other species. This inability to produce hybrid offspring, despite the fact that the gametes are found at the same time and in the same place, is due to a phenomenon known as gamete incompatibility, which is often found between marine invertebrates, and whose physiological causes are not fully understood.\nIn some Drosophila crosses, the swelling of the female's vagina has been noted following insemination. This has the effect of consequently preventing the fertilization of the ovule by sperm of a different species.\nIn plants the pollen grains of a species can germinate in the stigma and grow in the style of other species. However, the growth of the pollen tubes may be detained at some point between the stigma and the ovules, in such a way that fertilization does not take place. This mechanism of reproductive isolation is common in the angiosperms and is called cross-incompatibility or incongruence. A relationship exists between self-incompatibility and the phenomenon of cross-incompatibility. In general crosses between individuals of a self-compatible species (SC) with individuals of a self-incompatible (SI) species give hybrid offspring. On the other hand, a reciprocal cross (SI x SC) will not produce offspring, because the pollen tubes will not reach the ovules. This is known as unilateral incompatibility, which also occurs when two SC or two SI species are crossed.\nIn coral reefs, gamete incompatibility prevents the formation of numerous inter-species hybrids."},{"doc_id":7259701613231482000,"text":"Multiple mechanisms[edit]\nIn general, the barriers that separate species do not consist of just one mechanism. The twin species of Drosophila, D. pseudoobscura and D. persimilis, are isolated from each other by habitat (persimilis generally lives in colder regions at higher altitudes), by the timing of the mating season (persimilis is generally more active in the morning and pseudoobscura at night) and by behavior during mating (the females of both species prefer the males of their respective species). In this way, although the distribution of these species overlaps in wide areas of the west of the United States of America, these isolation mechanisms are sufficient to keep the species separated. Such that, only a few fertile females have been found amongst the other species among the thousands that have been analyzed. However, when hybrids are produced between both species, the gene flow between the two will continue to be impeded as the hybrid males are sterile. Also, and in contrast with the great vigor shown by the sterile males, the descendants of the backcrosses of the hybrid females with the parent species are weak and notoriously non-viable. This last mechanism restricts even more the genetic interchange between the two species of fly in the wild."},{"doc_id":8234954082625799000,"text":" that solely mated with members of their own species produced more surviving descendants than the flies that mated solely with individuals of the other species. In the adjacent table it can be seen that for each generation the number of hybrids continuously decreased up to the tenth generation when hardly any interspecific hybrids were produced. It is evident that selection against the hybrids was very effective in increasing reproductive isolation between these species. From the third generation, the proportions of the hybrids were less than 5%. This confirmed that selection acts to reinforce the reproductive isolation of two genetically divergent populations if the hybrids formed by these species are less well"},{"doc_id":2795545685075461000,"text":" to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already. An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa cross"},{"doc_id":3648476275161450500,"text":" already. An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa crossbred to give the new species Arabidopsis suecica. This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.\nSpeciation events are important in the theory of punctuated equilibrium"},{"doc_id":6240594478791638000,"text":" monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.\nBarriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile. In this case,"},{"doc_id":2196319744834792700,"text":" and nonrandom mating, to allow reproductive isolation to evolve.\nOne type of sympatric speciation involves crossbreeding of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair"},{"doc_id":6771066120557716000,"text":"). First one is the primary hybridisation generating hybrid, second one is most widespread type of hybridogenesis.\nOther examples where hybridogenesis is at least one of modes of reproduction include i.e.\nIberian minnow Tropidophoxinellus alburnoides (Squalius pyrenaicus × hypothetical ancestor related with Anaecypris hispanica)\nspined loaches Cobitis hankugensis × C. longicorpus\nBacillus stick insects B. rossius × Bacillus grandii benazzii"},{"doc_id":6455540793167434000,"text":"Goldstein, P. Z., _et al_. Conservation genetics at the species boundary.\n_Conservation_ _Biology_ **14**, 120–131 (2000)\ndoi:10.1046/j.1523-1739.2000.98122.x"},{"doc_id":6428639301698572000,"text":" 2010Natur.468.1053R. doi : 10.1038/nature09710. hdl : 10230/25596. PMC 4306417. PMID 21179161. ^ Hammer MF (May 2013). \"Human Hybrids\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E"},{"doc_id":4759425386332560000,"text":"Selection[edit]\n\nFurther information: Laboratory experiments of speciation\nSelection for reproductive isolation between two Drosophila species.\n\n\nGeneration\n\nPercentage of hybrids\n\n\n1\n\n49\n\n\n2\n\n17.6\n\n\n3\n\n3.3\n\n\n4\n\n1.0\n\n\n5\n\n1.4\n\n\n10\n\n0.6\nIn 1950 K. F. Koopman reported results from experiments designed to examine the hypothesis that selection can increase reproductive isolation between populations. He used D. pseudoobsc"},{"doc_id":449019984534268200,"text":" a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character"},{"doc_id":5482007351581509000,"text":" genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms. As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.\nThe second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from"},{"doc_id":8702957417387419000,"text":" types of animals, with the gray tree frog being a particularly well-studied example.\nSpeciation has been observed multiple times under both controlled laboratory conditions and in nature. In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms. As selection and drift act independently on populations isolated from"},{"doc_id":3710299959466571000,"text":". 468 (7327): 1053–1060. Bibcode : 2010Natur.468.1053R. doi : 10.1038/nature09710. hdl : 10230/25596. PMC 4306417. PMID 21179161. ^ Hammer MF (May 2013). \"Human Hybrids\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222."},{"doc_id":1139347458278782600,"text":" a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.\nFinally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences"},{"doc_id":2469823221813689300,"text":" that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced"},{"doc_id":1465423531852362000,"text":"20221114193201/https://pubs.geoscienceworld.org/gsa/geology/article/46/2/135/524864/Precise-age-of-Bangiomorpha-pubescens-dates-the) from the original on 2022-11-14 . Retrieved 2021-10-28 .\n19. ** ^ ** Gray, J. C.; Goddard, M. R. (2012). Bonsall, Michael (ed.). \"Gene-flow between niches facilitates local adaptation in sexual populations"},{"doc_id":6925360346047986000,"text":" displacement, which is when two species become more distinct in appearance.\nGeographical isolation of finches on the Galápagos Islands produced over a dozen new species.\nFinally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences and nonrandom mating, to allow reproductive isolation to evolve.\nOne type of sympatric speciation involves crossbreeding of two related species"},{"doc_id":5817479360179624000,"text":" type over a number of generations. After the populations had diverged over many generations, the groups were again mixed; it was observed that the flies would mate only with others from their adapted population. This indicates that the mechanisms of reproductive isolation can arise even though the interspecific hybrids are not selected against."},{"doc_id":6430109045870276000,"text":" adapted than their parents.\nThese discoveries allowed certain assumptions to be made regarding the origin of reproductive isolation mechanisms in nature. Namely, if selection reinforces the degree of reproductive isolation that exists between two species due to the poor adaptive value of the hybrids, it is expected that the populations of two species located in the same area will show a greater reproductive isolation than populations that are geographically separated (see reinforcement). This mechanism for \"reinforcing\" hybridization barriers in sympatric populations is also known as the \"Wallace effect\", as it was first proposed by Alfred Russel Wallace at the end of"},{"doc_id":1230949997641124000,"text":" of reproduction of hybrids. Hybridogenetic hybrids (for example AB genome), usually females, during gametogenesis exclude one of parental genomes (A) and produce gametes with unrecombined genome of second parental species (B), instead of containing mixed recombined parental genomes. First genome (A) is restored by fertilization of these gametes with gametes from the first species (AA, sexual host, usually male).\nSo hybridogenesis is not completely asexual, but instead hemiclonal: half of genome is passed to the next generation clonally, unrecomb"},{"doc_id":4658965309438925000,"text":" related species for the needed stimulus. Some salamanders of the genus Ambystoma are gynogenetic and appear to have been so for over a million years. It is believed that the success of those salamanders may be due to rare fertilization of eggs by males, introducing new material to the gene pool, which may result from perhaps only one mating out of a million. In addition, the amazon molly is known to reproduce by gynogenesis.\nHybridogenesis[edit]\n\nSee also: Hybridogenesis in water frogs\nHybridogenesis is a mode"},{"doc_id":8740337636363372000,"text":" linear or branched progression but involved interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans."},{"doc_id":8863286223059836000,"text":"code : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \"Mosaic humans, the hybrid species\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold"},{"doc_id":1278503641006759200,"text":" : 10.1038/nature09710. hdl : 10230/25596. PMC 4306417. PMID 21179161. ^ Hammer MF (May 2013). \"Human Hybrids\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \"Mosaic humans, the"},{"doc_id":6891327462150478000,"text":"iens sapiens.\nA large part of the controversy stems from the vagueness of the term \"species\", as it is generally used to distinguish two genetically isolated populations, but admixture between modern humans and Neanderthals is known to have occurred. However, the absence of Neanderthal-derived patrilineal Y-chromosome and matrilineal mitochondrial DNA (mtDNA) in modern humans, along with the underrepresentation of Neanderthal X chromosome DNA, could imply reduced fertility or frequent sterility of some hybrid crosses, representing a partial biological reproductive barrier between the groups,"},{"doc_id":317753063721401660,"text":"combination \"Homologous\nrecombination\") that occurs during [ meiosis ](/wiki/Meiosis \"Meiosis\").\n[48]"},{"doc_id":571901081079250000,"text":"(identifier\\) \"S2CID \\(identifier\\)\") [ 7714974 ](https://api.semanticscholar.org/CorpusID:7714974).\n24. ** ^ ** Colegrave, N. (2002). \"Sex releases the speed limit on evolution\". _Nature_. **420** (6916): 664–6. [ Bibcode ](/wiki/Bibcode_\\(identifier\\) \"Bibcode \\(identifier\\)\") : [ 2002Natur.420..664C ]("},{"doc_id":2111678933010816500,"text":"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H"},{"doc_id":6981789283127673000,"text":" the rest of their species, separation may eventually produce organisms that cannot interbreed.\nThe second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.\nThe third mode is parapatric speciation. This is similar to peripatric speciation in"},{"doc_id":7456392705571888000,"text":" coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\nGene flow\n\nFurther information: Gene flow\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the"},{"doc_id":1624798045956767200,"text":" twinning propensity, resulting in not only vespertilionidae's increased ability to produce twins but the increased likelihood of the genus's reproductive survival. This means that despite the genus's high exposure to factors that would seemingly increase mortality rates, vespertilionidae counteracts their environmental conditions through the evolutionary adaptation of dizygotic twins.\nThe prevalence of dizygotic twinning in monkeys is thought to be an \"insurance adaptation\" for mothers reproducing at the end of their fertile years. While dizygotic twinning has been observed in species such as gorillas and"},{"doc_id":4641475335747405000,"text":"00-756-2 \"Special:BookSources/978-1-62100-756-2\")\n23. ** ^ ** Gorelick, Root (2010). [ \"Sex reduces genetic variation: a multidisciplinary review\" ](https://doi.org/10.1111%2Fj.1558-5646.2010.01173.x). _Evolution_. **65** (4): 1088–1098. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)"},{"doc_id":5764818063332034000,"text":"6417. PMID 21179161. ^ Hammer MF (May 2013). \"Human Hybrids\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \"Mosaic humans, the hybrid species\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc"},{"doc_id":2462182602478930400,"text":"bred to give the new species Arabidopsis suecica. This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.\nSpeciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short \"bursts\" of evolution interspersed with relatively long periods of stasis"},{"doc_id":5560836972122283000,"text":" mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.\nSeveral studies report that the mutations implicated in adaptation reflect common mutation biases though others dispute this interpretation.\nGenetic hitchhiking\n\nFurther information: Genetic hitchhiking, Hill–Robertson effect, and Selective sweep\nRecombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome"},{"doc_id":238258965686082980,"text":"\nMain article: Speciation\n\nFurther information: Assortative mating and Panmixia\nThe four geographic modes of speciation\nSpeciation is the process where a species diverges into two or more descendant species.\nThere are multiple ways to define the concept of \"species.\" The choice of definition is dependent on the particularities of the species concerned. For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: inter"},{"doc_id":7326570392395027000,"text":" sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might"},{"doc_id":4253207277175081500,"text":" Hybrids\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018."},{"doc_id":4276364739988023000,"text":" the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.\nA common"},{"doc_id":204678165237244900,"text":"term use, the BSC like other species concepts is not without controversy, for example, because genetic recombination among prokaryotes is not an intrinsic aspect of reproduction; this is called the species problem. Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.\nBarriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on"},{"doc_id":1322102312395196000,"text":" the 19th century, and it has been experimentally demonstrated in both plants and animals.\nThe sexual isolation between Drosophila miranda and D. pseudoobscura, for example, is more or less pronounced according to the geographic origin of the flies being studied. Flies from regions where the distribution of the species is superimposed show a greater sexual isolation than exists between populations originating in distant regions.\nReproductive isolation can be caused by allopatric speciation. A population of Drosophila was divided into sub populations selected to adapt to different food types. After"},{"doc_id":6759693808606828000,"text":"The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species.\nThe mechanisms of reproductive isolation have been classified in a number of ways. Zoologist Ernst Mayr classified the mechanisms of reproductive isolation in two broad categories: pre-zygotic for those that act before fertilization (or before mating in the case of animals) and post-zygotic for those that act after it. The mechanisms are genetically controlled and can appear in species whose geographic distributions overlap (sympatric speciation) or are separate (allopatric speciation)."},{"doc_id":3939273447862958000,"text":" is thought to be an \"insurance adaptation\" for mothers reproducing at the end of their fertile years. While dizygotic twinning has been observed in species such as gorillas and chimpanzees, monkeys in the cebidae genus are found to be more likely to produce twins because of their small size and insect-based diet (Varella, 2018). This is because their small size indicates shorter gestation periods and the rapid maturation of offspring, resulting in a shorter lifespan where organisms are rapidly replaced by newer generations. The smaller size of the cebidae genus also makes these species more susceptible"},{"doc_id":3920312823126678000,"text":"Hybrid sex: Haldane's rule[edit]\nHaldane's rule states that when one of the two sexes is absent in interspecific hybrids between two specific species, then the sex that is not produced, is rare or is sterile is the heterozygous (or heterogametic) sex. In mammals, at least, there is growing evidence to suggest that this is due to high rates of mutation of the genes determining masculinity in the Y chromosome.\nIt has been suggested that Haldane's rule simply reflects the fact that the male sex is more sensitive than the female when the sex-determining genes are included in a hybrid genome. But there are also organisms in which the heterozygous sex is the female: birds and butterflies and the law is followed in these organisms. Therefore, it is not a problem related to sexual development, nor with the sex chromosomes. Haldane proposed that the stability of hybrid individual development requires the full gene complement of each parent species, so that the hybrid of the heterozygous sex is unbalanced (i.e. missing at least one chromosome from each of the parental species). For example, the hybrid male obtained by crossing D. melanogaster females with D. simulans males, which is non-viable, lacks the X chromosome of D. simulans."},{"doc_id":7398087953810060000,"text":" how few Denisovan bones are known, the discovery of a first-generation hybrid indicates interbreeding was very common between these species, and Neanderthal migration across Eurasia likely occurred sometime after 120,000 years ago."},{"doc_id":8811423195234019000,"text":"ura and D. persimilis in these experiments. When the flies of these species are kept at 16 °C approximately a third of the matings are interspecific. In the experiment equal numbers of males and females of both species were placed in containers suitable for their survival and reproduction. The progeny of each generation were examined in order to determine if there were any interspecific hybrids. These hybrids were then eliminated. An equal number of males and females of the resulting progeny were then chosen to act as progenitors of the next generation. As the hybrids were destroyed in each generation the flies"},{"doc_id":7315091402935209000,"text":" and natural selection. Through their studies on Vespertilionidae and Cebidae species, scientists Guilherme Siniciato Terra Garbino (2021) and Marco Varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propensities. In his study on the evolution of litter size in bats, Garbino discovered that the vespertilionidae genus has higher twinning propensities as a result of their high"},{"doc_id":1660289872362549800,"text":"Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging\nconditions induces cellular aggregation accompanied by high frequency [\ngenetic marker ](/wiki/Genetic_marker \"Genetic marker\") exchange [52] [53]\nAjon et al. [53] hypothesized that this cellular aggregation enhances\nspecies-specific DNA repair by homologous recombination. DNA transfer in\n_Sulfolobus_ may be an early form of sexual interaction similar to the more\nwell-studied bacterial transformation systems"},{"doc_id":3863667276857597400,"text":"See also[edit]\nChimera (genetics)\nMixed twins\nPolyandry in nature\nPolyspermy\nTwins"},{"doc_id":5950274991834669000,"text":"Yong E (July 2011). \"Mosaic humans, the hybrid species\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3.\n\nAckermann RR, Mackay A, Arnold ML (October 2015). \"The Hybrid Origin of \"Modern\" Humans\". Evolutionary Biology. 43 (1): 1–11. doi : 10.1007/s11692-015-9348-1. S"},{"doc_id":8650248059159904000,"text":" the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.\nNatural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection, may allow the evolution of cooperation.\nGenetic drift"},{"doc_id":2624959496645327000,"text":" interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans. Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees"},{"doc_id":6015808912048891000,"text":"(/wiki/Self-incompatibility \"Self-incompatibility\")\n* [ Sex ](/wiki/Sex \"Sex\")\n* [ Sexual intercourse ](/wiki/Sexual_intercourse \"Sexual intercourse\")\n* [ Transformation (genetics) ](/wiki/Transformation_\\(genetics\\) \"Transformation \\(genetics\\)\")"},{"doc_id":8158047257641537000,"text":" currently difficult to determine whether abrupt boundaries exist, or whether properties may change gradually with intermediate phenotypes."},{"doc_id":2381713283919416300,"text":" allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.\nThe third mode is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been"},{"doc_id":937806436257521400,"text":" the greater the evolutionary distance between\nspecies, the lower percentage of REs mapped (Fig. [ 2a, b\n](/articles/s41467-021-22100-8#Fig2) ). Of particular note, intergenic\nenhancers had a lower mapping rate compared to promoters and genic enhancers\nat all evolutionary distances (Student’s _t_ -test, _p_ -value < 0.05), while\nthe mapping rates between promoters and genic enhancers were not significantly\ndifferent at any evolutionary distance. We then checked if the mapped"},{"doc_id":6739160733741488000,"text":", and remain circulating in a large population. Genetic drift is also a major driving force in some species. It is possible that the combined effects of some or all of these factors cause the genetic diversity.\nMHC diversity has also been suggested as a possible indicator for conservation, because large, stable populations tend to display greater MHC diversity, than smaller, isolated populations. Small, fragmented populations that have experienced a population bottleneck typically have lower MHC diversity. For example, relatively low MHC diversity has been observed in the cheetah (Acinonyx jubatus), Eurasian beaver"},{"doc_id":1091313024442821200,"text":" of modern humans as H. sapiens sapiens.\nA large part of the controversy stems from the vagueness of the term \"species\", as it is generally used to distinguish two genetically isolated populations, but admixture between modern humans and Neanderthals is known to have occurred. However, the absence of Neanderthal-derived patrilineal Y-chromosome and matrilineal mitochondrial DNA (mtDNA) in modern humans, along with the underrepresentation of Neanderthal X chromosome DNA, could imply reduced fertility or frequent sterility of some hybrid crosses, representing a partial"},{"doc_id":5871278720061880000,"text":" hybrid species\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold ML (October 2015). \"The Hybrid Origin of \"Modern\" Humans\". Evolutionary Biology. 43 (1): 1–11. doi : 10.1007/s11692-015-9348-1. S2CID 14329491. ^ Noonan JP (May 2010). \""},{"doc_id":506009288803000260,"text":", which is where one organism acts to help raise a relative's offspring. This activity is selected for because if the helping individual contains alleles which promote the helping activity, it is likely that its kin will also contain these alleles and thus those alleles will be passed on. Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.\nSpeciation is the process where a species diverges into two or more descendant species.\nThere are multiple ways to define the concept of \"species.\" The choice of definition is dependent on the particularities of the species"},{"doc_id":1289797645621291300,"text":"ulty/Michod/Downloads/IGE%20review%20sex.pdf) (PDF) from the original on 2016-12-30 . Retrieved 2013-04-22 .\n21. ^ _**a** _ _**b** _ Bernstein, Harris; Bernstein, Carol (2010). \"Evolutionary Origin of Recombination during Meiosis\". _BioScience_. **60** (7): 498–505. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\")"},{"doc_id":7220667487780981000,"text":"a017764.full) from the original on 2022-09-13 . Retrieved 2022-09-13 – via Cold Spring Harbor.\n3. ** ^ ** Can Song, ShaoJun Liu (2012). [ \"Polyploid Organisms\" ](https://doi.org/10.1007%2Fs11427-012-4310-2). _Science China Life Sciences_. **55** (4): 301–311. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier"},{"doc_id":5162834233464248000,"text":" concerned. For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic. The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Defined by evolutionary biologist Ernst Mayr in 1942, the BSC states that \"species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.\" Despite its wide and long-"},{"doc_id":6515319333001444000,"text":" (July 2011). \"Mosaic humans, the hybrid species\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold ML (October 2015). \"The Hybrid Origin of \"Modern\" Humans\". Evolutionary Biology. 43 (1): 1–11. doi : 10.1007/s11692-015-9348-1. S2CID 14329"},{"doc_id":4849045386553743000,"text":" be caused by the movement of mice between inland and coastal populations, or the movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.\nGene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to e"},{"doc_id":9174348665583201000,"text":" likelihood of reversing their naturally-selected traits that have been passed on for generations. When considering this phenomenon in relation to twinning, larger mammals not commonly associated with high twinning propensities can perhaps produce twins as an adaptive response to their human-controlled environment. Additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment (Rickard, 2022, p.2). Thus if a species lives in a controlled environment with a low infant mortality rate, the frequency of the \"twinning trait\" could increase, leading to"},{"doc_id":1914184315045595100,"text":" reproduce, as more resources become available, more organisms can take advantage of these resources. Thus, monkeys that are smaller and have more access to food, such as the cebidae genus, have the ability to produce more offspring at a quicker pace. In terms of dizygotic twinning, it has been observed that older mothers within the cebidae genus have a higher chance of producing twins than those at the beginning stages of their fertility. Despite their access to resources, the cebidae genus has a high mortality rate attributed to their size, meaning that in order to \"keep up\""},{"doc_id":4763986652893559000,"text":" incidence of twinning will be comparatively less.\"Ashraf's insight not only illuminates the rarity of twinning among large mammals in the natural world, but directs our attention to the increased twinning propensity of animals under human care. This increased twinning propensity is thought to be either caused by random mutation facilitated by genetic drift, or the positive selection of the \"twinning\" trait in human-controlled conditions. Due to the removal of natural predators and unpredictable environmental conditions with the increase of human-provided food and medical care, species residing in nature reserves, zoos, etc., carry an increased"},{"doc_id":1891700658853292500,"text":"breeding, ecological and phylogenetic. The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Defined by evolutionary biologist Ernst Mayr in 1942, the BSC states that \"species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.\" Despite its wide and long-term use, the BSC like other species concepts is not without controversy, for example, because genetic recombination among prokaryotes is not an intrinsic aspect of reproduction; this is called the species problem. Some researchers have attempted a unifying"},{"doc_id":5786062632813836000,"text":"Rickard, 2022, p.2). Thus if a species lives in a controlled environment with a low infant mortality rate, the frequency of the \"twinning trait\" could increase, leading to a higher likelihood of producing twin offspring. In the case of the monozygotic twin calves in India, their existence could be connected to a new, positively selected adaptation of twinning attributed to species living under human care (Ward, 2014, p.7-11).\nSpecies with small physicalities and quick reproductive cycles carry high twinning propensities as a result of increased predation and high"},{"doc_id":823817488587350900,"text":"osity is not completely preserved when crossing over occurs in central fusion. In the case of pre-meiotic doubling, recombination, if it happens, occurs between identical sister chromatids.\nIf terminal fusion (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half the mother's genetic material is present in the offspring and the offspring are mostly homozygous. This is because at anaphase II the sister chromatids are separated and whatever heterozygosity is present is due to crossing over. In the case of endomitosis"},{"doc_id":2901697379073162000,"text":"* [ Amphimixis (psychology) ](/wiki/Amphimixis_\\(psychology\\) \"Amphimixis \\(psychology\\)\")\n* [ Anisogamy ](/wiki/Anisogamy \"Anisogamy\")\n* [ Biological reproduction ](/wiki/Biological_reproduction \"Biological reproduction\")\n* [ Hermaphroditism ](/wiki/Hermaphroditism \"Hermaphroditism\")\n* [ Isogamy ](/wiki/Isogamy \"Is"},{"doc_id":3909554072535573000,"text":" conditions. Due to the removal of natural predators and unpredictable environmental conditions with the increase of human-provided food and medical care, species residing in nature reserves, zoos, etc., carry an increased likelihood of reversing their naturally-selected traits that have been passed on for generations. When considering this phenomenon in relation to twinning, larger mammals not commonly associated with high twinning propensities can perhaps produce twins as an adaptive response to their human-controlled environment. Additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment ("},{"doc_id":496138738223089400,"text":"Unusual twinnings[edit]\nBi-paternal twins[edit]\nThis phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%.\nMixed twins[edit]\n\nMain article: Mixed twins\nDizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in London in 1993 to a white mother and Caribbean father.\nMonozygotic twins of different sexes[edit]\nAmong monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). When monozygotic twins are born with different sexes it is because of chromosomal defects. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic.\nAnother abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with Turner syndrome (45,X). In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.\nSemi-identical (sesquizygotic) twins[edit]\nMonozygotic twins can develop differently, due to their genes being differently activated. More unusual are \"semi-identical twins\", also known as \"sesquizygotic\". As of 2019, only two cases have been reported. These \"half-identical twins\" are hypothesized to occur when an ovum is fertilized by two sperm. The cell assorts the chromosomes by heterogonesis and the cell divides into two, with each daughter cell now containing the correct number of chromosomes. The cells continue to develop into a morula. If the morula then undergoes a twinning event, two embryos will be formed, with different paternal genes but identical maternal genes.\nTwin calves of the Hereford breed in Miles City, Montana\nIn 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father's genetic makeup. The twins were both found to be chimeras. One was an intersex XX, and one a XY male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.\nThe likely genetic basis of semi-identical twins was reported in 2019 by Michael Gabbett and Nicholas Fisk. In their seminal publication, Gabbett, Fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon. The reported twins shared 100% of their maternal chromosomes and 78% of their paternal genomic information. The authors presented evidence that two sperm from the same man fertilized an ovum simultaneously. The chromosomes assorted themselves through heterogonesis to form three cell lines. The purely paternal cell line died out due to genomic imprinting lethality, while the other two cell lines, each consisting of the same maternal DNA but only 50% identical paternal DNA, formed a morula which subsequently split into twins.\n\nMirror image twins[edit]\nMirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus). If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image. The term \"mirror image\" is used because the twins, when facing each other, appear as matching reflections."},{"doc_id":8435128282461875000,"text":" movement of mice between inland and coastal populations, or the movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.\nGene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to eukaryotes such"},{"doc_id":6837946335336115000,"text":", Madagascar, and New Zealand in the years 300 to 1280 CE. Human evolution was not a simple linear or branched progression but involved interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans. Human evolution is characterized by a"},{"doc_id":7645290932902685000,"text":"the-evolution-of-sex-824/) ) − [ Nature journal ](/wiki/Nature_\\(journal\\) \"Nature \\(journal\\)\") (2008)"},{"doc_id":502307981460160960,"text":"ylogenetic) fusions appear to be non-fusion processes such as, for example, the de novo formation of intercalary growth in a ring zone at or below the base of primordia. Therefore, \"it is now increasingly acknowledged that the term 'fusion,' as applied to phylogeny (as in 'congenital fusion') is ill-advised.\""},{"doc_id":1441427302521822700,"text":" When a multimer is formed from polypeptides produced by two different mutant alleles of a particular gene, the mixed multimer may exhibit greater functional activity than the unmixed multimers formed by each of the mutants alone. In such a case, the phenomenon is referred to as intragenic complementation (also called inter-allelic complementation). Intragenic complementation has been demonstrated in many different genes in a variety of organisms including the fungi Neurospora crassa, Saccharomyces cerevisiae and Schizosaccharomyces pom"},{"doc_id":2313772491907400000,"text":". Due to their short life cycles, the cebidae genus is more inclined to produce dizygotic twins in their older reproductive years, thus signaling that the trait of high twinning propensity is one that is passed down in service of this genus's survival."},{"doc_id":6850787044714879000,"text":" PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \"Mosaic humans, the hybrid species\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold ML (October 2015). \"The Hybrid Origin of \"Modern\" Humans\". Evolutionary Biology. 43 (1): 1–11. doi"},{"doc_id":4450846448131686000,"text":" mortality rates. As scientists continue to study the origin of dizygotic twinning in the animal kingdom, many have turned to species that demonstrated an increased output of twins during periods of evolutionary distress and natural selection. Through their studies on Vespertilionidae and Cebidae species, scientists Guilherme Siniciato Terra Garbino (2021) and Marco Varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propens"},{"doc_id":5332197148514614000,"text":"1. [ Stabilizing Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-1)\n2. [ Directional Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-2)\n3. [ Diversifying Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-3"},{"doc_id":8349740326968695000,"text":"* [ Non-reproductive behavior ](/wiki/Non-reproductive_sexual_behavior_in_animals \"Non-reproductive sexual behavior in animals\")\n* [ Fisher's principle ](/wiki/Fisher%27s_principle \"Fisher's principle\")"},{"doc_id":64028681676661330,"text":" to predators, thus triggering the heightened pace of birth, maturation, reproduction, and death. Meanwhile, cebidae's insectivorous existence can be correlated with this genus's heightened ability to reproduce, as more resources become available, more organisms can take advantage of these resources. Thus, monkeys that are smaller and have more access to food, such as the cebidae genus, have the ability to produce more offspring at a quicker pace. In terms of dizygotic twinning, it has been observed that older mothers within the cebidae genus have a higher chance of producing twins than"},{"doc_id":4556164713948176400,"text":" those at the beginning stages of their fertility. Despite their access to resources, the cebidae genus has a high mortality rate attributed to their size, meaning that in order to \"keep up\" their quickened lifecycle, they must produce an excess of offspring in ensuring generational survival. The positively-selected adaptation of twinning counteracts the genus's high mortality rate by giving older mothers the chance to produce more than one offspring. This not only increases the likelihood that one or more of these offspring will reach reproductive maturity, but gives the mother a chance to birth at least one viable offspring despite their age"},{"doc_id":2266893002215886300,"text":"ifier\\) \"S2CID \\(identifier\\)\") [ 1179013 ](https://api.semanticscholar.org/CorpusID:1179013).\n26. ** ^ ** Dimijian, G. G. (2005). Evolution of sexuality: biology and behavior. Proceedings (Baylor University. Medical Center), 18, 244–258.\n27. ^ _**a** _ _**b** _ _**c** _ _**d** _ Ruppert, Fox & Barnes (2004) , pp. 537"},{"doc_id":3972252304274929000,"text":" inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.\nA special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness"},{"doc_id":3440409639012102000,"text":" a higher likelihood of producing twin offspring. In the case of the monozygotic twin calves in India, their existence could be connected to a new, positively selected adaptation of twinning attributed to species living under human care (Ward, 2014, p.7-11).\nSpecies with small physicalities and quick reproductive cycles carry high twinning propensities as a result of increased predation and high mortality rates. As scientists continue to study the origin of dizygotic twinning in the animal kingdom, many have turned to species that demonstrated an increased output of twins during periods of evolutionary distress"},{"doc_id":1869446651260227300,"text":" in particular shifts in allele frequency and adaptation. Macroevolution the outcome of long periods of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved. However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In"},{"doc_id":5173656170064837000,"text":" to other parts of the system. These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.\nNatural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as"}],"string":"[\n {\n \"doc_id\": 2028903019636597200,\n \"text\": \"Genetics[edit]\\nPre-copulatory mechanisms in animals[edit]\\nThe genetics of ethological isolation barriers will be discussed first. Pre-copulatory isolation occurs when the genes necessary for the sexual reproduction of one species differ from the equivalent genes of another species, such that if a male of species A and a female of species B are placed together they are unable to copulate. Study of the genetics involved in this reproductive barrier tries to identify the genes that govern distinct sexual behaviors in the two species. The males of Drosophila melanogaster and those of D. simulans conduct an elaborate courtship with their respective females, which are different for each species, but the differences between the species are more quantitative than qualitative. In fact the simulans males are able to hybridize with the melanogaster females. Although there are lines of the latter species that can easily cross there are others that are hardly able to. Using this difference, it is possible to assess the minimum number of genes involved in pre-copulatory isolation between the melanogaster and simulans species and their chromosomal location.\\nIn experiments, flies of the D. melanogaster line, which hybridizes readily with simulans, were crossed with another line that it does not hybridize with, or rarely. The females of the segregated populations obtained by this cross were placed next to simulans males and the percentage of hybridization was recorded, which is a measure of the degree of reproductive isolation. It was concluded from this experiment that 3 of the 8 chromosomes of the haploid complement of D. melanogaster carry at least one gene that affects isolation, such that substituting one chromosome from a line of low isolation with another of high isolation reduces the hybridization frequency. In addition, interactions between chromosomes are detected so that certain combinations of the chromosomes have a multiplying effect.\\nCross incompatibility or incongruence in plants is also determined by major genes that are not associated at the self-incompatibility S locus.\\nPost-copulation or fertilization mechanisms in animals[edit]\\nReproductive isolation between species appears, in certain cases, a long time after fertilization and the formation of the zygote, as happens – for example – in the twin species Drosophila pavani and D. gaucha. The hybrids between both species are not sterile, in the sense that they produce viable gametes, ovules and spermatozoa. However, they cannot produce offspring as the sperm of the hybrid male do not survive in the semen receptors of the females, be they hybrids or from the parent lines. In the same way, the sperm of the males of the two parent species do not survive in the reproductive tract of the hybrid female. This type of post-copulatory isolation appears as the most efficient system for maintaining reproductive isolation in many species.\\nThe development of a zygote into an adult is a complex and delicate process of interactions between genes and the environment that must be carried out precisely, and if there is any alteration in the usual process, caused by the absence of a necessary gene or the presence of a different one, it can arrest the normal development causing the non-viability of the hybrid or its sterility. It should be borne in mind that half of the chromosomes and genes of a hybrid are from one species and the other half come from the other. If the two species are genetically different, there is little possibility that the genes from both will act harmoniously in the hybrid. From this perspective, only a few genes would be required in order to bring about post copulatory isolation, as opposed to the situation described previously for pre-copulatory isolation.\\nIn many species where pre-copulatory reproductive isolation does not exist, hybrids are produced but they are of only one sex. This is the case for the hybridization between females of Drosophila simulans and Drosophila melanogaster males: the hybridized females die early in their development so that only males are seen among the offspring. However, populations of D. simulans have been recorded with genes that permit the development of adult hybrid females, that is, the viability of the females is \\\"rescued\\\". It is assumed that the normal activity of these speciation genes is to \\\"inhibit\\\" the expression of the genes that allow the growth of the hybrid. There will also be regulator genes.\\nA number of these genes have been found in the melanogaster species group. The first to be discovered was \\\"Lhr\\\" (Lethal hybrid rescue) located in Chromosome II of D. simulans. This dominant allele allows the development of hybrid females from the cross between simulans females and melanogaster males. A different gene, also located on Chromosome II of D. simulans is \\\"Shfr\\\" that also allows the development of female hybrids, its activity being dependent on the temperature at which development occurs. Other similar genes have been located in distinct populations of species of this group. In short, only a few genes are needed for an effective post copulatory isolation barrier mediated through the non-viability of the hybrids.\\nAs important as identifying an isolation gene is knowing its function. The Hmr gene, linked to the X chromosome and implicated in the viability of male hybrids between D. melanogaster and D. simulans, is a gene from the proto-oncogene family myb, that codes for a transcriptional regulator. Two variants of this gene function perfectly well in each separate species, but in the hybrid they do not function correctly, possibly due to the different genetic background of each species. Examination of the allele sequence of the two species shows that change of direction substitutions are more abundant than synonymous substitutions, suggesting that this gene has been subject to intense natural selection.\\nThe Dobzhansky–Muller model proposes that reproductive incompatibilities between species are caused by the interaction of the genes of the respective species. It has been demonstrated recently that Lhr has functionally diverged in D. simulans and will interact with Hmr which, in turn, has functionally diverged in D. melanogaster to cause the lethality of the male hybrids. Lhr is located in a heterochromatic region of the genome and its sequence has diverged between these two species in a manner consistent with the mechanisms of positive selection. An important unanswered question is whether the genes detected correspond to old genes that initiated the speciation favoring hybrid non-viability, or are modern genes that have appeared post-speciation by mutation, that are not shared by the different populations and that suppress the effect of the primitive non-viability genes. The OdsH (abbreviation of Odysseus) gene causes partial sterility in the hybrid between Drosophila simulans and a related species, D. mauritiana, which is only encountered on Mauritius, and is of recent origin. This gene shows monophyly in both species and also has been subject to natural selection. It is thought that it is a gene that intervenes in the initial stages of speciation, while other genes that differentiate the two species show polyphyly. Odsh originated by duplication in the genome of Drosophila and has evolved at very high rates in D. mauritania, while its paralogue, unc-4, is nearly identical between the species of the group melanogaster. Seemingly, all these cases illustrate the manner in which speciation mechanisms originated in nature, therefore they are collectively known as \\\"speciation genes\\\", or possibly, gene sequences with a normal function within the populations of a species that diverge rapidly in response to positive selection thereby forming reproductive isolation barriers with other species. In general, all these genes have functions in the transcriptional regulation of other genes.\\nThe Nup96 gene is another example of the evolution of the genes implicated in post-copulatory isolation. It regulates the production of one of the approximately 30 proteins required to form a nuclear pore. In each of the simulans groups of Drosophila the protein from this gene interacts with the protein from another, as yet undiscovered, gene on the X chromosome in order to form a functioning pore. However, in a hybrid the pore that is formed is defective and causes sterility. The differences in the sequences of Nup96 have been subject to adaptive selection, similar to the other examples of speciation genes described above.\\nPost-copulatory isolation can also arise between chromosomally differentiated populations due to chromosomal translocations and inversions. If, for example, a reciprocal translocation is fixed in a population, the hybrid produced between this population and one that does not carry the translocation will not have a complete meiosis. This will result in the production of unequal gametes containing unequal numbers of chromosomes with a reduced fertility. In certain cases, complete translocations exist that involve more than two chromosomes, so that the meiosis of the hybrids is irregular and their fertility is zero or nearly zero. Inversions can also give rise to abnormal gametes in heterozygous individuals but this effect has little importance compared to translocations. An example of chromosomal changes causing sterility in hybrids comes from the study of Drosophila nasuta and D. albomicans which are twin species from the Indo-Pacific region. There is no sexual isolation between them and the F1 hybrid is fertile. However, the F2 hybrids are relatively infertile and leave few descendants which have a skewed ratio of the sexes. The reason is that the X chromosome of albomicans is translocated and linked to an autosome which causes abnormal meiosis in hybrids. Robertsonian translocations are variations in the numbers of chromosomes that arise from either: the fusion of two acrocentric chromosomes into a single chromosome with two arms, causing a reduction in the haploid number, or conversely; or the fission of one chromosome into two acrocentric chromosomes, in this case increasing the haploid number. The hybrids of two populations with differing numbers of chromosomes can experience a certain loss of fertility, and therefore a poor adaptation, because of irregular meiosis.\\nIn plants[edit]\\n\\nFurther information: Reproductive coevolution in Ficus\\nA large variety of mechanisms have been demonstrated to reinforce reproductive isolation between closely related plant species that either historically lived or currently live in sympatry. This phenomenon is driven by strong selection against hybrids, typically resulting from instances in which hybrids suffer reduced fitness. Such negative fitness consequences have been proposed to be the result of negative epistasis in hybrid genomes and can also result from the effects of hybrid sterility. In such cases, selection gives rise to population-specific isolating mechanisms to prevent either fertilization by interspecific gametes or the development of hybrid embryos.\\nBecause many sexually reproducing species of plants are exposed to a variety of interspecific gametes, natural selection has given rise to a variety of mechanisms to prevent the production of hybrids. These mechanisms can act at different stages in the developmental process and are typically divided into two categories, pre-fertilization and post-fertilization, indicating at which point the barrier acts to prevent either zygote formation or development. In the case of angiosperms and other pollinated species, pre-fertilization mechanisms can be further subdivided into two more categories, pre-pollination and post-pollination, the difference between the two being whether or not a pollen tube is formed. (Typically when pollen encounters a receptive stigma, a series of changes occur which ultimately lead to the growth of a pollen tube down the style, allowing for the formation of the zygote.) Empirical investigation has demonstrated that these barriers act at many different developmental stages and species can have none, one, or many barriers to hybridization with interspecifics.\\nExamples of pre-fertilization mechanisms[edit]\\nA well-documented example of a pre-fertilization isolating mechanism comes from study of Louisiana iris species. These iris species were fertilized with interspecific and conspecific pollen loads and it was demonstrated by measure of hybrid progeny success that differences in pollen-tube growth between interspecific and conspecific pollen led to a lower fertilization rate by interspecific pollen. This demonstrates how a specific point in the reproductive process is manipulated by a particular isolating mechanism to prevent hybrids.\\nAnother well-documented example of a pre-fertilization isolating mechanism in plants comes from study of the 2 wind-pollinated birch species. Study of these species led to the discovery that mixed conspecific and interspecific pollen loads still result in 98% conspecific fertilization rates, highlighting the effectiveness of such barriers. In this example, pollen tube incompatibility and slower generative mitosis have been implicated in the post-pollination isolation mechanism.\\nExamples of post-fertilization mechanisms[edit]\\nCrosses between diploid and tetraploid species of Paspalum provide evidence of a post-fertilization mechanism preventing hybrid formation when pollen from tetraploid species was used to fertilize a female of a diploid species. There were signs of fertilization and even endosperm formation but subsequently this endosperm collapsed. This demonstrates evidence of an early post-fertilization isolating mechanism, in which the hybrid early embryo is detected and selectively aborted. This process can also occur later during development in which developed, hybrid seeds are selectively aborted.\\nEffects of hybrid necrosis[edit]\\nPlant hybrids often suffer from an autoimmune syndrome known as hybrid necrosis. In the hybrids, specific gene products contributed by one of the parents may be inappropriately recognized as foreign and pathogenic, and thus trigger pervasive cell death throughout the plant. In at least one case, a pathogen receptor, encoded by the most variable gene family in plants, was identified as being responsible for hybrid necrosis.\\nChromosomal rearrangements in yeast[edit]\\nIn brewers' yeast Saccharomyces cerevisiae, chromosomal rearrangements are a major mechanism to reproductively isolate different strains. Hou et al. showed that reproductive isolation acts postzygotically and could be attributed to chromosomal rearrangements. These authors crossed 60 natural isolates sampled from diverse niches with the reference strain S288c and identified 16 cases of reproductive isolation with reduced offspring viabilities, and identified reciprocal chromosomal translocations in a large fraction of isolates.\\nIncompatibility caused by microorganisms[edit]\\nIn addition to the genetic causes of reproductive isolation between species there is another factor that can cause post zygotic isolation: the presence of microorganisms in the cytoplasm of certain species. The presence of these organisms in a species and their absence in another causes the non-viability of the corresponding hybrid. For example, in the semi-species of the group D. paulistorum the hybrid females are fertile but the males are sterile, this is due to the presence of a Wolbachia in the cytoplasm which alters spermatogenesis leading to sterility. It is interesting that incompatibility or isolation can also arise at an intraspecific level. Populations of D. simulans have been studied that show hybrid sterility according to the direction of the cross. The factor determining sterility has been found to be the presence or absence of a microorganism Wolbachia and the populations tolerance or susceptibility to these organisms. This inter population incompatibility can be eliminated in the laboratory through the administration of a specific antibiotic to kill the microorganism. Similar situations are known in a number of insects, as around 15% of species show infections caused by this symbiont. It has been suggested that, in some cases, the speciation process has taken place because of the incompatibility caused by this bacteria. Two wasp species Nasonia giraulti and N. longicornis carry two different strains of Wolbachia. Crosses between an infected population and one free from infection produces a nearly total reproductive isolation between the semi-species. However, if both species are free from the bacteria or both are treated with antibiotics there is no reproductive barrier. Wolbachia also induces incompatibility due to the weakness of the hybrids in populations of spider mites (Tetranychus urticae), between Drosophila recens and D. subquinaria and between species of Diabrotica (beetle) and Gryllus (cricket).\"\n },\n {\n \"doc_id\": 5139828354633502000,\n \"text\": \"\\\")\\n* [ intralocus ](/wiki/Intralocus_sexual_conflict \\\"Intralocus sexual conflict\\\")\\n* [ Interspecies breeding ](/wiki/Hybrid_\\\\(biology\\\\) \\\"Hybrid \\\\(biology\\\\)\\\")\"\n },\n {\n \"doc_id\": 4765997756196682000,\n \"text\": \" some generations the two sub populations were mixed again. Subsequent matings occurred between individuals belonging to the same adapted group.\\nOn the other hand, interspecific hybridization barriers can also arise as a result of the adaptive divergence that accompanies allopatric speciation. This mechanism has been experimentally proved by an experiment carried out by Diane Dodd on D. pseudoobscura. A single population of flies was divided into two, with one of the populations fed with starch-based food and the other with maltose-based food. This meant that each sub population was adapted to each food\"\n },\n {\n \"doc_id\": 8354333819587692000,\n \"text\": \" closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example.\\nSpeciation has been observed multiple times under both controlled laboratory conditions and in nature. In sexually reproducing organisms, speciation results from reproductive isolation followed by\"\n },\n {\n \"doc_id\": 4694161439350746000,\n \"text\": \" how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridisation in producing new species of animals is unclear, although cases have been seen in many\"\n },\n {\n \"doc_id\": 7406906637871403000,\n \"text\": \"Pre-zygotic isolation[edit]\\nPre-zygotic isolation mechanisms are the most economic in terms of the natural selection of a population, as resources are not wasted on the production of a descendant that is weak, non-viable or sterile. These mechanisms include physiological or systemic barriers to fertilization.\\nTemporal or habitat isolation[edit]\\nSee also: Allochronic speciation\\nThe Central Valley in California prevents the two salamander populations from interacting with each other which is an example of habitat isolation. After many generations the two salamander gene pools will become mutated caused by natural selection. The mutation will change the DNA sequence of the two populations enough that the salamander populations can no longer successfully breed between each other making the populations of salamander become classified as different species.\\nAny of the factors that prevent potentially fertile individuals from meeting will reproductively isolate the members of distinct species. The types of barriers that can cause this isolation include: different habitats, physical barriers, and a difference in the time of sexual maturity or flowering.\\nAn example of the ecological or habitat differences that impede the meeting of potential pairs occurs in two fish species of the family Gasterosteidae (sticklebacks). One species lives all year round in fresh water, mainly in small streams. The other species lives in the sea during winter, but in spring and summer individuals migrate to river estuaries to reproduce. The members of the two populations are reproductively isolated due to their adaptations to distinct salt concentrations.\\nAn example of reproductive isolation due to differences in the mating season are found in the toad species Bufo americanus and Bufo fowleri. The members of these species can be successfully crossed in the laboratory producing healthy, fertile hybrids. However, mating does not occur in the wild even though the geographical distribution of the two species overlaps. The reason for the absence of inter-species mating is that B. americanus mates in early summer and B. fowleri in late summer.\\nCertain plant species, such as Tradescantia canaliculata and T. subaspera, are sympatric throughout their geographic distribution, yet they are reproductively isolated as they flower at different times of the year. In addition, one species grows in sunny areas and the other in deeply shaded areas.\\nBehavioral isolation[edit]\\nThe different mating rituals of animal species creates extremely powerful reproductive barriers, termed sexual or behavior isolation, that isolate apparently similar species in the majority of the groups of the animal kingdom. In dioecious species, males and females have to search for a partner, be in proximity to each other, carry out the complex mating rituals and finally copulate or release their gametes into the environment in order to breed.\\nThe songs of birds, insects and many other animals are part of a ritual to attract potential partners of their own species. The song presents specific patterns recognizable only by members of the same species, and therefore represents a mechanism of reproductive isolation. This recording is the song of a species of cicada, recorded in New Zealand.\\nMating dances, the songs of males to attract females or the mutual grooming of pairs, are all examples of typical courtship behavior that allows both recognition and reproductive isolation. This is because each of the stages of courtship depend on the behavior of the partner. The male will only move onto the second stage of the exhibition if the female shows certain responses in her behavior. He will only pass onto the third stage when she displays a second key behavior. The behaviors of both interlink, are synchronized in time and lead finally to copulation or the liberation of gametes into the environment. No animal that is not physiologically suitable for fertilization can complete this demanding chain of behavior. In fact, the smallest difference in the courting patterns of two species is enough to prevent mating (for example, a specific song pattern acts as an isolation mechanism in distinct species of grasshopper of the genus Chorthippus).\\nEven where there are minimal morphological differences between species, differences in behavior can be enough to prevent mating. For example, Drosophila melanogaster and D. simulans which are considered twin species due to their morphological similarity, do not mate even if they are kept together in a laboratory. Drosophila ananassae and D. pallidosa are twin species from Melanesia. In the wild they rarely produce hybrids, although in the laboratory it is possible to produce fertile offspring. Studies of their sexual behavior show that the males court the females of both species but the females show a marked preference for mating with males of their own species. A different regulator region has been found on Chromosome II of both species that affects the selection behavior of the females.\\nPheromones play an important role in the sexual isolation of insect species. These compounds serve to identify individuals of the same species and of the same or different sex. Evaporated molecules of volatile pheromones can serve as a wide-reaching chemical signal. In other cases, pheromones may be detected only at a short distance or by contact.\\nIn species of the melanogaster group of Drosophila, the pheromones of the females are mixtures of different compounds, there is a clear dimorphism in the type and/or quantity of compounds present for each sex. In addition, there are differences in the quantity and quality of constituent compounds between related species, it is assumed that the pheromones serve to distinguish between individuals of each species. An example of the role of pheromones in sexual isolation is found in 'corn borers' in the genus Ostrinia. There are two twin species in Europe that occasionally cross. The females of both species produce pheromones that contain a volatile compound which has two isomers, E and Z; 99% of the compound produced by the females of one species is in the E isomer form, while the females of the other produce 99% isomer Z. The production of the compound is controlled by just one locus and the interspecific hybrid produces an equal mix of the two isomers. The males, for their part, almost exclusively detect the isomer emitted by the females of their species, such that the hybridization although possible is scarce. The perception of the males is controlled by one gene, distinct from the one for the production of isomers, the heterozygous males show a moderate response to the odour of either type. In this case, just 2 'loci' produce the effect of ethological isolation between species that are genetically very similar.\\nSexual isolation between two species can be asymmetrical. This can happen when the mating that produces descendants only allows one of the two species to function as the female progenitor and the other as the male, while the reciprocal cross does not occur. For instance, half of the wolves tested in the Great Lakes area of America show mitochondrial DNA sequences of coyotes, while mitochondrial DNA from wolves is never found in coyote populations. This probably reflects an asymmetry in inter-species mating due to the difference in size of the two species as male wolves take advantage of their greater size in order to mate with female coyotes, while female wolves and male coyotes do not mate.\\nMechanical isolation[edit]\\nThe flowers of many species of Angiosperm have evolved to attract and reward a single or a few pollinator species (insects, birds, mammals). Their wide diversity of form, colour, fragrance and presence of nectar is, in many cases, the result of coevolution with the pollinator species. This dependency on its pollinator species also acts as a reproductive isolation barrier.\\nMating pairs may not be able to couple successfully if their genitals are not compatible. The relationship between the reproductive isolation of species and the form of their genital organs was signaled for the first time in 1844 by the French entomologist Léon Dufour. Insects' rigid carapaces act in a manner analogous to a lock and key, as they will only allow mating between individuals with complementary structures, that is, males and females of the same species (termed co-specifics).\\nEvolution has led to the development of genital organs with increasingly complex and divergent characteristics, which will cause mechanical isolation between species. Certain characteristics of the genital organs will often have converted them into mechanisms of isolation. However, numerous studies show that organs that are anatomically very different can be functionally compatible, indicating that other factors also determine the form of these complicated structures.\\nMechanical isolation also occurs in plants and this is related to the adaptation and coevolution of each species in the attraction of a certain type of pollinator (where pollination is zoophilic) through a collection of morphophysiological characteristics of the flowers (called pollination syndrome), in such a way that the transport of pollen to other species does not occur.\\nGametic isolation[edit]\\nThe synchronous spawning of many species of coral in marine reefs means that inter-species hybridization can take place as the gametes of hundreds of individuals of tens of species are liberated into the same water at the same time. Approximately a third of all the possible crosses between species are compatible, in the sense that the gametes will fuse and lead to individual hybrids. This hybridization apparently plays a fundamental role in the evolution of coral species. However, the other two-thirds of possible crosses are incompatible. It has been observed that in sea urchins of the genus Strongylocentrotus the concentration of spermatocytes that allow 100% fertilization of the ovules of the same species is only able to fertilize 1.5% of the ovules of other species. This inability to produce hybrid offspring, despite the fact that the gametes are found at the same time and in the same place, is due to a phenomenon known as gamete incompatibility, which is often found between marine invertebrates, and whose physiological causes are not fully understood.\\nIn some Drosophila crosses, the swelling of the female's vagina has been noted following insemination. This has the effect of consequently preventing the fertilization of the ovule by sperm of a different species.\\nIn plants the pollen grains of a species can germinate in the stigma and grow in the style of other species. However, the growth of the pollen tubes may be detained at some point between the stigma and the ovules, in such a way that fertilization does not take place. This mechanism of reproductive isolation is common in the angiosperms and is called cross-incompatibility or incongruence. A relationship exists between self-incompatibility and the phenomenon of cross-incompatibility. In general crosses between individuals of a self-compatible species (SC) with individuals of a self-incompatible (SI) species give hybrid offspring. On the other hand, a reciprocal cross (SI x SC) will not produce offspring, because the pollen tubes will not reach the ovules. This is known as unilateral incompatibility, which also occurs when two SC or two SI species are crossed.\\nIn coral reefs, gamete incompatibility prevents the formation of numerous inter-species hybrids.\"\n },\n {\n \"doc_id\": 7259701613231482000,\n \"text\": \"Multiple mechanisms[edit]\\nIn general, the barriers that separate species do not consist of just one mechanism. The twin species of Drosophila, D. pseudoobscura and D. persimilis, are isolated from each other by habitat (persimilis generally lives in colder regions at higher altitudes), by the timing of the mating season (persimilis is generally more active in the morning and pseudoobscura at night) and by behavior during mating (the females of both species prefer the males of their respective species). In this way, although the distribution of these species overlaps in wide areas of the west of the United States of America, these isolation mechanisms are sufficient to keep the species separated. Such that, only a few fertile females have been found amongst the other species among the thousands that have been analyzed. However, when hybrids are produced between both species, the gene flow between the two will continue to be impeded as the hybrid males are sterile. Also, and in contrast with the great vigor shown by the sterile males, the descendants of the backcrosses of the hybrid females with the parent species are weak and notoriously non-viable. This last mechanism restricts even more the genetic interchange between the two species of fly in the wild.\"\n },\n {\n \"doc_id\": 8234954082625799000,\n \"text\": \" that solely mated with members of their own species produced more surviving descendants than the flies that mated solely with individuals of the other species. In the adjacent table it can be seen that for each generation the number of hybrids continuously decreased up to the tenth generation when hardly any interspecific hybrids were produced. It is evident that selection against the hybrids was very effective in increasing reproductive isolation between these species. From the third generation, the proportions of the hybrids were less than 5%. This confirmed that selection acts to reinforce the reproductive isolation of two genetically divergent populations if the hybrids formed by these species are less well\"\n },\n {\n \"doc_id\": 2795545685075461000,\n \"text\": \" to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair already. An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa cross\"\n },\n {\n \"doc_id\": 3648476275161450500,\n \"text\": \" already. An example of such a speciation event is when the plant species Arabidopsis thaliana and Arabidopsis arenosa crossbred to give the new species Arabidopsis suecica. This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.\\nSpeciation events are important in the theory of punctuated equilibrium\"\n },\n {\n \"doc_id\": 6240594478791638000,\n \"text\": \" monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.\\nBarriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile. In this case,\"\n },\n {\n \"doc_id\": 2196319744834792700,\n \"text\": \" and nonrandom mating, to allow reproductive isolation to evolve.\\nOne type of sympatric speciation involves crossbreeding of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. This allows the chromosomes from each parental species to form matching pairs during meiosis, since each parent's chromosomes are represented by a pair\"\n },\n {\n \"doc_id\": 6771066120557716000,\n \"text\": \"). First one is the primary hybridisation generating hybrid, second one is most widespread type of hybridogenesis.\\nOther examples where hybridogenesis is at least one of modes of reproduction include i.e.\\nIberian minnow Tropidophoxinellus alburnoides (Squalius pyrenaicus × hypothetical ancestor related with Anaecypris hispanica)\\nspined loaches Cobitis hankugensis × C. longicorpus\\nBacillus stick insects B. rossius × Bacillus grandii benazzii\"\n },\n {\n \"doc_id\": 6455540793167434000,\n \"text\": \"Goldstein, P. Z., _et al_. Conservation genetics at the species boundary.\\n_Conservation_ _Biology_ **14**, 120–131 (2000)\\ndoi:10.1046/j.1523-1739.2000.98122.x\"\n },\n {\n \"doc_id\": 6428639301698572000,\n \"text\": \" 2010Natur.468.1053R. doi : 10.1038/nature09710. hdl : 10230/25596. PMC 4306417. PMID 21179161. ^ Hammer MF (May 2013). \\\"Human Hybrids\\\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E\"\n },\n {\n \"doc_id\": 4759425386332560000,\n \"text\": \"Selection[edit]\\n\\nFurther information: Laboratory experiments of speciation\\nSelection for reproductive isolation between two Drosophila species.\\n\\n\\nGeneration\\n\\nPercentage of hybrids\\n\\n\\n1\\n\\n49\\n\\n\\n2\\n\\n17.6\\n\\n\\n3\\n\\n3.3\\n\\n\\n4\\n\\n1.0\\n\\n\\n5\\n\\n1.4\\n\\n\\n10\\n\\n0.6\\nIn 1950 K. F. Koopman reported results from experiments designed to examine the hypothesis that selection can increase reproductive isolation between populations. He used D. pseudoobsc\"\n },\n {\n \"doc_id\": 449019984534268200,\n \"text\": \" a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character\"\n },\n {\n \"doc_id\": 5482007351581509000,\n \"text\": \" genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms. As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.\\nThe second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from\"\n },\n {\n \"doc_id\": 8702957417387419000,\n \"text\": \" types of animals, with the gray tree frog being a particularly well-studied example.\\nSpeciation has been observed multiple times under both controlled laboratory conditions and in nature. In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms. As selection and drift act independently on populations isolated from\"\n },\n {\n \"doc_id\": 3710299959466571000,\n \"text\": \". 468 (7327): 1053–1060. Bibcode : 2010Natur.468.1053R. doi : 10.1038/nature09710. hdl : 10230/25596. PMC 4306417. PMID 21179161. ^ Hammer MF (May 2013). \\\"Human Hybrids\\\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222.\"\n },\n {\n \"doc_id\": 1139347458278782600,\n \"text\": \" a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.\\nFinally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences\"\n },\n {\n \"doc_id\": 2469823221813689300,\n \"text\": \" that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced\"\n },\n {\n \"doc_id\": 1465423531852362000,\n \"text\": \"20221114193201/https://pubs.geoscienceworld.org/gsa/geology/article/46/2/135/524864/Precise-age-of-Bangiomorpha-pubescens-dates-the) from the original on 2022-11-14 . Retrieved 2021-10-28 .\\n19. ** ^ ** Gray, J. C.; Goddard, M. R. (2012). Bonsall, Michael (ed.). \\\"Gene-flow between niches facilitates local adaptation in sexual populations\"\n },\n {\n \"doc_id\": 6925360346047986000,\n \"text\": \" displacement, which is when two species become more distinct in appearance.\\nGeographical isolation of finches on the Galápagos Islands produced over a dozen new species.\\nFinally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences and nonrandom mating, to allow reproductive isolation to evolve.\\nOne type of sympatric speciation involves crossbreeding of two related species\"\n },\n {\n \"doc_id\": 5817479360179624000,\n \"text\": \" type over a number of generations. After the populations had diverged over many generations, the groups were again mixed; it was observed that the flies would mate only with others from their adapted population. This indicates that the mechanisms of reproductive isolation can arise even though the interspecific hybrids are not selected against.\"\n },\n {\n \"doc_id\": 6430109045870276000,\n \"text\": \" adapted than their parents.\\nThese discoveries allowed certain assumptions to be made regarding the origin of reproductive isolation mechanisms in nature. Namely, if selection reinforces the degree of reproductive isolation that exists between two species due to the poor adaptive value of the hybrids, it is expected that the populations of two species located in the same area will show a greater reproductive isolation than populations that are geographically separated (see reinforcement). This mechanism for \\\"reinforcing\\\" hybridization barriers in sympatric populations is also known as the \\\"Wallace effect\\\", as it was first proposed by Alfred Russel Wallace at the end of\"\n },\n {\n \"doc_id\": 1230949997641124000,\n \"text\": \" of reproduction of hybrids. Hybridogenetic hybrids (for example AB genome), usually females, during gametogenesis exclude one of parental genomes (A) and produce gametes with unrecombined genome of second parental species (B), instead of containing mixed recombined parental genomes. First genome (A) is restored by fertilization of these gametes with gametes from the first species (AA, sexual host, usually male).\\nSo hybridogenesis is not completely asexual, but instead hemiclonal: half of genome is passed to the next generation clonally, unrecomb\"\n },\n {\n \"doc_id\": 4658965309438925000,\n \"text\": \" related species for the needed stimulus. Some salamanders of the genus Ambystoma are gynogenetic and appear to have been so for over a million years. It is believed that the success of those salamanders may be due to rare fertilization of eggs by males, introducing new material to the gene pool, which may result from perhaps only one mating out of a million. In addition, the amazon molly is known to reproduce by gynogenesis.\\nHybridogenesis[edit]\\n\\nSee also: Hybridogenesis in water frogs\\nHybridogenesis is a mode\"\n },\n {\n \"doc_id\": 8740337636363372000,\n \"text\": \" linear or branched progression but involved interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans.\"\n },\n {\n \"doc_id\": 8863286223059836000,\n \"text\": \"code : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \\\"Mosaic humans, the hybrid species\\\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold\"\n },\n {\n \"doc_id\": 1278503641006759200,\n \"text\": \" : 10.1038/nature09710. hdl : 10230/25596. PMC 4306417. PMID 21179161. ^ Hammer MF (May 2013). \\\"Human Hybrids\\\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \\\"Mosaic humans, the\"\n },\n {\n \"doc_id\": 6891327462150478000,\n \"text\": \"iens sapiens.\\nA large part of the controversy stems from the vagueness of the term \\\"species\\\", as it is generally used to distinguish two genetically isolated populations, but admixture between modern humans and Neanderthals is known to have occurred. However, the absence of Neanderthal-derived patrilineal Y-chromosome and matrilineal mitochondrial DNA (mtDNA) in modern humans, along with the underrepresentation of Neanderthal X chromosome DNA, could imply reduced fertility or frequent sterility of some hybrid crosses, representing a partial biological reproductive barrier between the groups,\"\n },\n {\n \"doc_id\": 317753063721401660,\n \"text\": \"combination \\\"Homologous\\nrecombination\\\") that occurs during [ meiosis ](/wiki/Meiosis \\\"Meiosis\\\").\\n[48]\"\n },\n {\n \"doc_id\": 571901081079250000,\n \"text\": \"(identifier\\\\) \\\"S2CID \\\\(identifier\\\\)\\\") [ 7714974 ](https://api.semanticscholar.org/CorpusID:7714974).\\n24. ** ^ ** Colegrave, N. (2002). \\\"Sex releases the speed limit on evolution\\\". _Nature_. **420** (6916): 664–6. [ Bibcode ](/wiki/Bibcode_\\\\(identifier\\\\) \\\"Bibcode \\\\(identifier\\\\)\\\") : [ 2002Natur.420..664C ](\"\n },\n {\n \"doc_id\": 2111678933010816500,\n \"text\": \"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H\"\n },\n {\n \"doc_id\": 6981789283127673000,\n \"text\": \" the rest of their species, separation may eventually produce organisms that cannot interbreed.\\nThe second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.\\nThe third mode is parapatric speciation. This is similar to peripatric speciation in\"\n },\n {\n \"doc_id\": 7456392705571888000,\n \"text\": \" coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\\nGene flow\\n\\nFurther information: Gene flow\\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the\"\n },\n {\n \"doc_id\": 1624798045956767200,\n \"text\": \" twinning propensity, resulting in not only vespertilionidae's increased ability to produce twins but the increased likelihood of the genus's reproductive survival. This means that despite the genus's high exposure to factors that would seemingly increase mortality rates, vespertilionidae counteracts their environmental conditions through the evolutionary adaptation of dizygotic twins.\\nThe prevalence of dizygotic twinning in monkeys is thought to be an \\\"insurance adaptation\\\" for mothers reproducing at the end of their fertile years. While dizygotic twinning has been observed in species such as gorillas and\"\n },\n {\n \"doc_id\": 4641475335747405000,\n \"text\": \"00-756-2 \\\"Special:BookSources/978-1-62100-756-2\\\")\\n23. ** ^ ** Gorelick, Root (2010). [ \\\"Sex reduces genetic variation: a multidisciplinary review\\\" ](https://doi.org/10.1111%2Fj.1558-5646.2010.01173.x). _Evolution_. **65** (4): 1088–1098. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\"\n },\n {\n \"doc_id\": 5764818063332034000,\n \"text\": \"6417. PMID 21179161. ^ Hammer MF (May 2013). \\\"Human Hybrids\\\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \\\"Mosaic humans, the hybrid species\\\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc\"\n },\n {\n \"doc_id\": 2462182602478930400,\n \"text\": \"bred to give the new species Arabidopsis suecica. This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.\\nSpeciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short \\\"bursts\\\" of evolution interspersed with relatively long periods of stasis\"\n },\n {\n \"doc_id\": 5560836972122283000,\n \"text\": \" mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.\\nSeveral studies report that the mutations implicated in adaptation reflect common mutation biases though others dispute this interpretation.\\nGenetic hitchhiking\\n\\nFurther information: Genetic hitchhiking, Hill–Robertson effect, and Selective sweep\\nRecombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome\"\n },\n {\n \"doc_id\": 238258965686082980,\n \"text\": \"\\nMain article: Speciation\\n\\nFurther information: Assortative mating and Panmixia\\nThe four geographic modes of speciation\\nSpeciation is the process where a species diverges into two or more descendant species.\\nThere are multiple ways to define the concept of \\\"species.\\\" The choice of definition is dependent on the particularities of the species concerned. For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: inter\"\n },\n {\n \"doc_id\": 7326570392395027000,\n \"text\": \" sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might\"\n },\n {\n \"doc_id\": 4253207277175081500,\n \"text\": \" Hybrids\\\" (PDF). Scientific American. 308 (5): 66–71. Bibcode : 2013SciAm.308e..66H. doi : 10.1038/scientificamerican0513-66. PMID 23627222. Archived from the original (PDF) on 24 August 2018.\"\n },\n {\n \"doc_id\": 4276364739988023000,\n \"text\": \" the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.\\nA common\"\n },\n {\n \"doc_id\": 204678165237244900,\n \"text\": \"term use, the BSC like other species concepts is not without controversy, for example, because genetic recombination among prokaryotes is not an intrinsic aspect of reproduction; this is called the species problem. Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be different ways to logically interpret the definition of a species.\\nBarriers to reproduction between two diverging sexual populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on\"\n },\n {\n \"doc_id\": 1322102312395196000,\n \"text\": \" the 19th century, and it has been experimentally demonstrated in both plants and animals.\\nThe sexual isolation between Drosophila miranda and D. pseudoobscura, for example, is more or less pronounced according to the geographic origin of the flies being studied. Flies from regions where the distribution of the species is superimposed show a greater sexual isolation than exists between populations originating in distant regions.\\nReproductive isolation can be caused by allopatric speciation. A population of Drosophila was divided into sub populations selected to adapt to different food types. After\"\n },\n {\n \"doc_id\": 6759693808606828000,\n \"text\": \"The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offspring are sterile. These barriers maintain the integrity of a species by reducing gene flow between related species.\\nThe mechanisms of reproductive isolation have been classified in a number of ways. Zoologist Ernst Mayr classified the mechanisms of reproductive isolation in two broad categories: pre-zygotic for those that act before fertilization (or before mating in the case of animals) and post-zygotic for those that act after it. The mechanisms are genetically controlled and can appear in species whose geographic distributions overlap (sympatric speciation) or are separate (allopatric speciation).\"\n },\n {\n \"doc_id\": 3939273447862958000,\n \"text\": \" is thought to be an \\\"insurance adaptation\\\" for mothers reproducing at the end of their fertile years. While dizygotic twinning has been observed in species such as gorillas and chimpanzees, monkeys in the cebidae genus are found to be more likely to produce twins because of their small size and insect-based diet (Varella, 2018). This is because their small size indicates shorter gestation periods and the rapid maturation of offspring, resulting in a shorter lifespan where organisms are rapidly replaced by newer generations. The smaller size of the cebidae genus also makes these species more susceptible\"\n },\n {\n \"doc_id\": 3920312823126678000,\n \"text\": \"Hybrid sex: Haldane's rule[edit]\\nHaldane's rule states that when one of the two sexes is absent in interspecific hybrids between two specific species, then the sex that is not produced, is rare or is sterile is the heterozygous (or heterogametic) sex. In mammals, at least, there is growing evidence to suggest that this is due to high rates of mutation of the genes determining masculinity in the Y chromosome.\\nIt has been suggested that Haldane's rule simply reflects the fact that the male sex is more sensitive than the female when the sex-determining genes are included in a hybrid genome. But there are also organisms in which the heterozygous sex is the female: birds and butterflies and the law is followed in these organisms. Therefore, it is not a problem related to sexual development, nor with the sex chromosomes. Haldane proposed that the stability of hybrid individual development requires the full gene complement of each parent species, so that the hybrid of the heterozygous sex is unbalanced (i.e. missing at least one chromosome from each of the parental species). For example, the hybrid male obtained by crossing D. melanogaster females with D. simulans males, which is non-viable, lacks the X chromosome of D. simulans.\"\n },\n {\n \"doc_id\": 7398087953810060000,\n \"text\": \" how few Denisovan bones are known, the discovery of a first-generation hybrid indicates interbreeding was very common between these species, and Neanderthal migration across Eurasia likely occurred sometime after 120,000 years ago.\"\n },\n {\n \"doc_id\": 8811423195234019000,\n \"text\": \"ura and D. persimilis in these experiments. When the flies of these species are kept at 16 °C approximately a third of the matings are interspecific. In the experiment equal numbers of males and females of both species were placed in containers suitable for their survival and reproduction. The progeny of each generation were examined in order to determine if there were any interspecific hybrids. These hybrids were then eliminated. An equal number of males and females of the resulting progeny were then chosen to act as progenitors of the next generation. As the hybrids were destroyed in each generation the flies\"\n },\n {\n \"doc_id\": 7315091402935209000,\n \"text\": \" and natural selection. Through their studies on Vespertilionidae and Cebidae species, scientists Guilherme Siniciato Terra Garbino (2021) and Marco Varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propensities. In his study on the evolution of litter size in bats, Garbino discovered that the vespertilionidae genus has higher twinning propensities as a result of their high\"\n },\n {\n \"doc_id\": 1660289872362549800,\n \"text\": \"Exposure of hyperthermophilic archaeal Sulfolobus species to DNA damaging\\nconditions induces cellular aggregation accompanied by high frequency [\\ngenetic marker ](/wiki/Genetic_marker \\\"Genetic marker\\\") exchange [52] [53]\\nAjon et al. [53] hypothesized that this cellular aggregation enhances\\nspecies-specific DNA repair by homologous recombination. DNA transfer in\\n_Sulfolobus_ may be an early form of sexual interaction similar to the more\\nwell-studied bacterial transformation systems\"\n },\n {\n \"doc_id\": 3863667276857597400,\n \"text\": \"See also[edit]\\nChimera (genetics)\\nMixed twins\\nPolyandry in nature\\nPolyspermy\\nTwins\"\n },\n {\n \"doc_id\": 5950274991834669000,\n \"text\": \"Yong E (July 2011). \\\"Mosaic humans, the hybrid species\\\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3.\\n\\nAckermann RR, Mackay A, Arnold ML (October 2015). \\\"The Hybrid Origin of \\\"Modern\\\" Humans\\\". Evolutionary Biology. 43 (1): 1–11. doi : 10.1007/s11692-015-9348-1. S\"\n },\n {\n \"doc_id\": 8650248059159904000,\n \"text\": \" the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.\\nNatural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection, may allow the evolution of cooperation.\\nGenetic drift\"\n },\n {\n \"doc_id\": 2624959496645327000,\n \"text\": \" interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans. Human evolution is characterized by a number of morphological, developmental, physiological, and behavioral changes that have taken place since the split between the last common ancestor of humans and chimpanzees\"\n },\n {\n \"doc_id\": 6015808912048891000,\n \"text\": \"(/wiki/Self-incompatibility \\\"Self-incompatibility\\\")\\n* [ Sex ](/wiki/Sex \\\"Sex\\\")\\n* [ Sexual intercourse ](/wiki/Sexual_intercourse \\\"Sexual intercourse\\\")\\n* [ Transformation (genetics) ](/wiki/Transformation_\\\\(genetics\\\\) \\\"Transformation \\\\(genetics\\\\)\\\")\"\n },\n {\n \"doc_id\": 8158047257641537000,\n \"text\": \" currently difficult to determine whether abrupt boundaries exist, or whether properties may change gradually with intermediate phenotypes.\"\n },\n {\n \"doc_id\": 2381713283919416300,\n \"text\": \" allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation after an increase in inbreeding increases selection on homozygotes, leading to rapid genetic change.\\nThe third mode is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been\"\n },\n {\n \"doc_id\": 937806436257521400,\n \"text\": \" the greater the evolutionary distance between\\nspecies, the lower percentage of REs mapped (Fig. [ 2a, b\\n](/articles/s41467-021-22100-8#Fig2) ). Of particular note, intergenic\\nenhancers had a lower mapping rate compared to promoters and genic enhancers\\nat all evolutionary distances (Student’s _t_ -test, _p_ -value < 0.05), while\\nthe mapping rates between promoters and genic enhancers were not significantly\\ndifferent at any evolutionary distance. We then checked if the mapped\"\n },\n {\n \"doc_id\": 6739160733741488000,\n \"text\": \", and remain circulating in a large population. Genetic drift is also a major driving force in some species. It is possible that the combined effects of some or all of these factors cause the genetic diversity.\\nMHC diversity has also been suggested as a possible indicator for conservation, because large, stable populations tend to display greater MHC diversity, than smaller, isolated populations. Small, fragmented populations that have experienced a population bottleneck typically have lower MHC diversity. For example, relatively low MHC diversity has been observed in the cheetah (Acinonyx jubatus), Eurasian beaver\"\n },\n {\n \"doc_id\": 1091313024442821200,\n \"text\": \" of modern humans as H. sapiens sapiens.\\nA large part of the controversy stems from the vagueness of the term \\\"species\\\", as it is generally used to distinguish two genetically isolated populations, but admixture between modern humans and Neanderthals is known to have occurred. However, the absence of Neanderthal-derived patrilineal Y-chromosome and matrilineal mitochondrial DNA (mtDNA) in modern humans, along with the underrepresentation of Neanderthal X chromosome DNA, could imply reduced fertility or frequent sterility of some hybrid crosses, representing a partial\"\n },\n {\n \"doc_id\": 5871278720061880000,\n \"text\": \" hybrid species\\\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold ML (October 2015). \\\"The Hybrid Origin of \\\"Modern\\\" Humans\\\". Evolutionary Biology. 43 (1): 1–11. doi : 10.1007/s11692-015-9348-1. S2CID 14329491. ^ Noonan JP (May 2010). \\\"\"\n },\n {\n \"doc_id\": 506009288803000260,\n \"text\": \", which is where one organism acts to help raise a relative's offspring. This activity is selected for because if the helping individual contains alleles which promote the helping activity, it is likely that its kin will also contain these alleles and thus those alleles will be passed on. Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.\\nSpeciation is the process where a species diverges into two or more descendant species.\\nThere are multiple ways to define the concept of \\\"species.\\\" The choice of definition is dependent on the particularities of the species\"\n },\n {\n \"doc_id\": 1289797645621291300,\n \"text\": \"ulty/Michod/Downloads/IGE%20review%20sex.pdf) (PDF) from the original on 2016-12-30 . Retrieved 2013-04-22 .\\n21. ^ _**a** _ _**b** _ Bernstein, Harris; Bernstein, Carol (2010). \\\"Evolutionary Origin of Recombination during Meiosis\\\". _BioScience_. **60** (7): 498–505. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\")\"\n },\n {\n \"doc_id\": 7220667487780981000,\n \"text\": \"a017764.full) from the original on 2022-09-13 . Retrieved 2022-09-13 – via Cold Spring Harbor.\\n3. ** ^ ** Can Song, ShaoJun Liu (2012). [ \\\"Polyploid Organisms\\\" ](https://doi.org/10.1007%2Fs11427-012-4310-2). _Science China Life Sciences_. **55** (4): 301–311. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\"\n },\n {\n \"doc_id\": 5162834233464248000,\n \"text\": \" concerned. For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms. Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological and phylogenetic. The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Defined by evolutionary biologist Ernst Mayr in 1942, the BSC states that \\\"species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.\\\" Despite its wide and long-\"\n },\n {\n \"doc_id\": 6515319333001444000,\n \"text\": \" (July 2011). \\\"Mosaic humans, the hybrid species\\\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold ML (October 2015). \\\"The Hybrid Origin of \\\"Modern\\\" Humans\\\". Evolutionary Biology. 43 (1): 1–11. doi : 10.1007/s11692-015-9348-1. S2CID 14329\"\n },\n {\n \"doc_id\": 4849045386553743000,\n \"text\": \" be caused by the movement of mice between inland and coastal populations, or the movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.\\nGene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to e\"\n },\n {\n \"doc_id\": 9174348665583201000,\n \"text\": \" likelihood of reversing their naturally-selected traits that have been passed on for generations. When considering this phenomenon in relation to twinning, larger mammals not commonly associated with high twinning propensities can perhaps produce twins as an adaptive response to their human-controlled environment. Additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment (Rickard, 2022, p.2). Thus if a species lives in a controlled environment with a low infant mortality rate, the frequency of the \\\"twinning trait\\\" could increase, leading to\"\n },\n {\n \"doc_id\": 1914184315045595100,\n \"text\": \" reproduce, as more resources become available, more organisms can take advantage of these resources. Thus, monkeys that are smaller and have more access to food, such as the cebidae genus, have the ability to produce more offspring at a quicker pace. In terms of dizygotic twinning, it has been observed that older mothers within the cebidae genus have a higher chance of producing twins than those at the beginning stages of their fertility. Despite their access to resources, the cebidae genus has a high mortality rate attributed to their size, meaning that in order to \\\"keep up\\\"\"\n },\n {\n \"doc_id\": 4763986652893559000,\n \"text\": \" incidence of twinning will be comparatively less.\\\"Ashraf's insight not only illuminates the rarity of twinning among large mammals in the natural world, but directs our attention to the increased twinning propensity of animals under human care. This increased twinning propensity is thought to be either caused by random mutation facilitated by genetic drift, or the positive selection of the \\\"twinning\\\" trait in human-controlled conditions. Due to the removal of natural predators and unpredictable environmental conditions with the increase of human-provided food and medical care, species residing in nature reserves, zoos, etc., carry an increased\"\n },\n {\n \"doc_id\": 1891700658853292500,\n \"text\": \"breeding, ecological and phylogenetic. The Biological Species Concept (BSC) is a classic example of the interbreeding approach. Defined by evolutionary biologist Ernst Mayr in 1942, the BSC states that \\\"species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.\\\" Despite its wide and long-term use, the BSC like other species concepts is not without controversy, for example, because genetic recombination among prokaryotes is not an intrinsic aspect of reproduction; this is called the species problem. Some researchers have attempted a unifying\"\n },\n {\n \"doc_id\": 5786062632813836000,\n \"text\": \"Rickard, 2022, p.2). Thus if a species lives in a controlled environment with a low infant mortality rate, the frequency of the \\\"twinning trait\\\" could increase, leading to a higher likelihood of producing twin offspring. In the case of the monozygotic twin calves in India, their existence could be connected to a new, positively selected adaptation of twinning attributed to species living under human care (Ward, 2014, p.7-11).\\nSpecies with small physicalities and quick reproductive cycles carry high twinning propensities as a result of increased predation and high\"\n },\n {\n \"doc_id\": 823817488587350900,\n \"text\": \"osity is not completely preserved when crossing over occurs in central fusion. In the case of pre-meiotic doubling, recombination, if it happens, occurs between identical sister chromatids.\\nIf terminal fusion (restitutional meiosis of anaphase II or the fusion of its products) occurs, a little over half the mother's genetic material is present in the offspring and the offspring are mostly homozygous. This is because at anaphase II the sister chromatids are separated and whatever heterozygosity is present is due to crossing over. In the case of endomitosis\"\n },\n {\n \"doc_id\": 2901697379073162000,\n \"text\": \"* [ Amphimixis (psychology) ](/wiki/Amphimixis_\\\\(psychology\\\\) \\\"Amphimixis \\\\(psychology\\\\)\\\")\\n* [ Anisogamy ](/wiki/Anisogamy \\\"Anisogamy\\\")\\n* [ Biological reproduction ](/wiki/Biological_reproduction \\\"Biological reproduction\\\")\\n* [ Hermaphroditism ](/wiki/Hermaphroditism \\\"Hermaphroditism\\\")\\n* [ Isogamy ](/wiki/Isogamy \\\"Is\"\n },\n {\n \"doc_id\": 3909554072535573000,\n \"text\": \" conditions. Due to the removal of natural predators and unpredictable environmental conditions with the increase of human-provided food and medical care, species residing in nature reserves, zoos, etc., carry an increased likelihood of reversing their naturally-selected traits that have been passed on for generations. When considering this phenomenon in relation to twinning, larger mammals not commonly associated with high twinning propensities can perhaps produce twins as an adaptive response to their human-controlled environment. Additionally, the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment (\"\n },\n {\n \"doc_id\": 496138738223089400,\n \"text\": \"Unusual twinnings[edit]\\nBi-paternal twins[edit]\\nThis phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%.\\nMixed twins[edit]\\n\\nMain article: Mixed twins\\nDizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in London in 1993 to a white mother and Caribbean father.\\nMonozygotic twins of different sexes[edit]\\nAmong monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). When monozygotic twins are born with different sexes it is because of chromosomal defects. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic.\\nAnother abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with Turner syndrome (45,X). In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.\\nSemi-identical (sesquizygotic) twins[edit]\\nMonozygotic twins can develop differently, due to their genes being differently activated. More unusual are \\\"semi-identical twins\\\", also known as \\\"sesquizygotic\\\". As of 2019, only two cases have been reported. These \\\"half-identical twins\\\" are hypothesized to occur when an ovum is fertilized by two sperm. The cell assorts the chromosomes by heterogonesis and the cell divides into two, with each daughter cell now containing the correct number of chromosomes. The cells continue to develop into a morula. If the morula then undergoes a twinning event, two embryos will be formed, with different paternal genes but identical maternal genes.\\nTwin calves of the Hereford breed in Miles City, Montana\\nIn 2007, a study reported a case of a pair of living twins, which shared an identical set of maternal chromosomes, while each having a distinct set of paternal chromosomes, albeit from the same man, and thus they most likely share half of their father's genetic makeup. The twins were both found to be chimeras. One was an intersex XX, and one a XY male. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.\\nThe likely genetic basis of semi-identical twins was reported in 2019 by Michael Gabbett and Nicholas Fisk. In their seminal publication, Gabbett, Fisk and colleagues documented a second case of sesquizygosis and presented molecular evidence of the phenomenon. The reported twins shared 100% of their maternal chromosomes and 78% of their paternal genomic information. The authors presented evidence that two sperm from the same man fertilized an ovum simultaneously. The chromosomes assorted themselves through heterogonesis to form three cell lines. The purely paternal cell line died out due to genomic imprinting lethality, while the other two cell lines, each consisting of the same maternal DNA but only 50% identical paternal DNA, formed a morula which subsequently split into twins.\\n\\nMirror image twins[edit]\\nMirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus). If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image. The term \\\"mirror image\\\" is used because the twins, when facing each other, appear as matching reflections.\"\n },\n {\n \"doc_id\": 8435128282461875000,\n \"text\": \" movement of mice between inland and coastal populations, or the movement of pollen between heavy-metal-tolerant and heavy-metal-sensitive populations of grasses.\\nGene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to eukaryotes such\"\n },\n {\n \"doc_id\": 6837946335336115000,\n \"text\": \", Madagascar, and New Zealand in the years 300 to 1280 CE. Human evolution was not a simple linear or branched progression but involved interbreeding between related species. Genomic research has shown that hybridization between substantially diverged lineages was common in human evolution. DNA evidence suggests that several genes of Neanderthal origin are present among all non sub-Saharan-African populations, and Neanderthals and other hominins, such as Denisovans, may have contributed up to 6% of their genome to present-day non sub-Saharan-African humans. Human evolution is characterized by a\"\n },\n {\n \"doc_id\": 7645290932902685000,\n \"text\": \"the-evolution-of-sex-824/) ) − [ Nature journal ](/wiki/Nature_\\\\(journal\\\\) \\\"Nature \\\\(journal\\\\)\\\") (2008)\"\n },\n {\n \"doc_id\": 502307981460160960,\n \"text\": \"ylogenetic) fusions appear to be non-fusion processes such as, for example, the de novo formation of intercalary growth in a ring zone at or below the base of primordia. Therefore, \\\"it is now increasingly acknowledged that the term 'fusion,' as applied to phylogeny (as in 'congenital fusion') is ill-advised.\\\"\"\n },\n {\n \"doc_id\": 1441427302521822700,\n \"text\": \" When a multimer is formed from polypeptides produced by two different mutant alleles of a particular gene, the mixed multimer may exhibit greater functional activity than the unmixed multimers formed by each of the mutants alone. In such a case, the phenomenon is referred to as intragenic complementation (also called inter-allelic complementation). Intragenic complementation has been demonstrated in many different genes in a variety of organisms including the fungi Neurospora crassa, Saccharomyces cerevisiae and Schizosaccharomyces pom\"\n },\n {\n \"doc_id\": 2313772491907400000,\n \"text\": \". Due to their short life cycles, the cebidae genus is more inclined to produce dizygotic twins in their older reproductive years, thus signaling that the trait of high twinning propensity is one that is passed down in service of this genus's survival.\"\n },\n {\n \"doc_id\": 6850787044714879000,\n \"text\": \" PMID 23627222. Archived from the original (PDF) on 24 August 2018. ^ Yong E (July 2011). \\\"Mosaic humans, the hybrid species\\\". New Scientist. 211 (2823): 34–38. Bibcode : 2011NewSc.211...34Y. doi : 10.1016/S0262-4079(11)61839-3. ^ Ackermann RR, Mackay A, Arnold ML (October 2015). \\\"The Hybrid Origin of \\\"Modern\\\" Humans\\\". Evolutionary Biology. 43 (1): 1–11. doi\"\n },\n {\n \"doc_id\": 4450846448131686000,\n \"text\": \" mortality rates. As scientists continue to study the origin of dizygotic twinning in the animal kingdom, many have turned to species that demonstrated an increased output of twins during periods of evolutionary distress and natural selection. Through their studies on Vespertilionidae and Cebidae species, scientists Guilherme Siniciato Terra Garbino (2021) and Marco Varella (2018) have proven that smaller species experiencing infertility in old age and/or unstable habits as a result of increased predation or human interference can experience have undergone natural selection in gaining even higher twinning propens\"\n },\n {\n \"doc_id\": 5332197148514614000,\n \"text\": \"1. [ Stabilizing Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-1)\\n2. [ Directional Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-2)\\n3. [ Diversifying Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-3\"\n },\n {\n \"doc_id\": 8349740326968695000,\n \"text\": \"* [ Non-reproductive behavior ](/wiki/Non-reproductive_sexual_behavior_in_animals \\\"Non-reproductive sexual behavior in animals\\\")\\n* [ Fisher's principle ](/wiki/Fisher%27s_principle \\\"Fisher's principle\\\")\"\n },\n {\n \"doc_id\": 64028681676661330,\n \"text\": \" to predators, thus triggering the heightened pace of birth, maturation, reproduction, and death. Meanwhile, cebidae's insectivorous existence can be correlated with this genus's heightened ability to reproduce, as more resources become available, more organisms can take advantage of these resources. Thus, monkeys that are smaller and have more access to food, such as the cebidae genus, have the ability to produce more offspring at a quicker pace. In terms of dizygotic twinning, it has been observed that older mothers within the cebidae genus have a higher chance of producing twins than\"\n },\n {\n \"doc_id\": 4556164713948176400,\n \"text\": \" those at the beginning stages of their fertility. Despite their access to resources, the cebidae genus has a high mortality rate attributed to their size, meaning that in order to \\\"keep up\\\" their quickened lifecycle, they must produce an excess of offspring in ensuring generational survival. The positively-selected adaptation of twinning counteracts the genus's high mortality rate by giving older mothers the chance to produce more than one offspring. This not only increases the likelihood that one or more of these offspring will reach reproductive maturity, but gives the mother a chance to birth at least one viable offspring despite their age\"\n },\n {\n \"doc_id\": 2266893002215886300,\n \"text\": \"ifier\\\\) \\\"S2CID \\\\(identifier\\\\)\\\") [ 1179013 ](https://api.semanticscholar.org/CorpusID:1179013).\\n26. ** ^ ** Dimijian, G. G. (2005). Evolution of sexuality: biology and behavior. Proceedings (Baylor University. Medical Center), 18, 244–258.\\n27. ^ _**a** _ _**b** _ _**c** _ _**d** _ Ruppert, Fox & Barnes (2004) , pp. 537\"\n },\n {\n \"doc_id\": 3972252304274929000,\n \"text\": \" inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.\\nA special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness\"\n },\n {\n \"doc_id\": 3440409639012102000,\n \"text\": \" a higher likelihood of producing twin offspring. In the case of the monozygotic twin calves in India, their existence could be connected to a new, positively selected adaptation of twinning attributed to species living under human care (Ward, 2014, p.7-11).\\nSpecies with small physicalities and quick reproductive cycles carry high twinning propensities as a result of increased predation and high mortality rates. As scientists continue to study the origin of dizygotic twinning in the animal kingdom, many have turned to species that demonstrated an increased output of twins during periods of evolutionary distress\"\n },\n {\n \"doc_id\": 1869446651260227300,\n \"text\": \" in particular shifts in allele frequency and adaptation. Macroevolution the outcome of long periods of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved. However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In\"\n },\n {\n \"doc_id\": 5173656170064837000,\n \"text\": \" to other parts of the system. These relationships involve the life history of the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.\\nNatural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as\"\n }\n]","isConversational":false}}},{"rowIdx":96,"cells":{"query":{"kind":"string","value":"I've been reading about vitamin C and came across some interesting points regarding its metabolism and excretion. Vitamin C, or ascorbic acid, is known to be highly concentrated in certain organs, which leads me to wonder how effective plasma vitamin C levels are as an indicator of the overall body status. Moreover, I read that the time it takes for symptoms of deficiency to appear can vary significantly among individuals, especially those on a diet very low in vitamin C. Additionally, ascorbic acid is excreted through urine, both as the unmetabolized form and as metabolites. The proportion of unmetabolized ascorbic acid in urine seems to increase with higher intake. Ascorbic acid also converts reversibly to dehydroascorbic acid (DHA), and from DHA, further non-reversible changes occur. \n\nCan someone elaborate on how the reversible conversion of ascorbic acid to DHA and the excretion patterns of vitamin C affect the assessment of vitamin C status in the human body? How reliable are plasma levels in diagnosing deficiency, and what other methods might be more accurate?"},"positive_passages":{"kind":"list like","value":[{"doc_id":1840698153413007000,"text":"al glands can exceed 2,000 µmol/L, and muscle is at 200–300 µmol/L. The known coenzymatic functions of ascorbic acid do not require such high concentrations, so there may be other, as yet unknown functions. A consequence of all this high concentration organ content is that plasma vitamin C is not a good indicator of whole-body status, and people may vary in the amount of time needed to show symptoms of deficiency when consuming a diet very low in vitamin C.\nExcretion (via urine) is as ascor"},{"doc_id":1078533498278672100,"text":"bic acid and metabolites. The fraction that is excreted as unmetabolized ascorbic acid increases as intake increases. In addition, ascorbic acid converts (reversibly) to DHA and from that compound non-reversibly to 2,3-diketogulonate and then oxalate. These three metabolites are also excreted via urine. During times of low dietary intake, vitamin C is reabsorbed by the kidneys rather than excreted. This salvage process delays onset of deficiency. Humans are better than guinea pigs"},{"doc_id":3456284582962325500,"text":" than in females.\nPlasma levels are considered saturated at about 65 μmol/L, achieved by intakes of 100 to 200 mg/day, which are well above the recommended intakes. Even higher oral intake does not further raise plasma nor tissue concentrations because absorption efficiency decreases and any excess that is absorbed is excreted in urine.\nVitamin C content in plasma is used to determine vitamin status. For research purposes, concentrations can be assessed in leukocytes and tissues, which are normally maintained at an order of magnitude higher than in plasma via an energy-dependent transport system, depleted slower"},{"doc_id":4745082524742403000,"text":" deficiency is common in low and middle-income countries, and not uncommon in high income countries. In the latter, prevalence is higher in males than in females.\nPlasma levels are considered saturated at about 65 μmol/L, achieved by intakes of 100 to 200 mg/day, which are well above the recommended intakes. Even higher oral intake does not further raise plasma nor tissue concentrations because absorption efficiency decreases and any excess that is absorbed is excreted in urine.\nDiagnostic testing[edit]\nVitamin C content in plasma is used to determine vitamin status. For research"},{"doc_id":3952197704319985000,"text":"-reversibly to 2,3-diketogulonate and then oxalate. These three metabolites are also excreted via urine. During times of low dietary intake, vitamin C is reabsorbed by the kidneys rather than excreted. This salvage process delays onset of deficiency. Humans are better than guinea pigs at converting DHA back to ascorbate, and thus take much longer to become vitamin C deficient.\nMost animals and plants are able to synthesize vitamin C through a sequence of enzyme-driven steps, which convert monosaccharides"},{"doc_id":7216188569036737000,"text":") form of vitamin C. The amount of DHA found in plasma and tissues under normal conditions is low, as cells rapidly reduce DHA to ascorbate.\nSVCTs are the predominant system for vitamin C transport within the body. In both vitamin C synthesizers (example: rat) and non-synthesizers (example: human) cells maintain ascorbic acid concentrations much higher than the approximately 50 micromoles/liter (µmol/L) found in plasma. For example, the ascorbic acid content of pituitary and adren"},{"doc_id":7598587754574724000,"text":" purposes, concentrations can be assessed in leukocytes and tissues, which are normally maintained at an order of magnitude higher than in plasma via an energy-dependent transport system, depleted slower than plasma concentrations during dietary deficiency and restored faster during dietary repletion, but these analysis are difficult to measure, and hence not part of standard diagnostic testing."},{"doc_id":2141155637567488000,"text":". In both vitamin C synthesizers (example: rat) and non-synthesizers (example: human) cells maintain ascorbic acid concentrations much higher than the approximately 50 micromoles/liter (µmol/L) found in plasma. For example, the ascorbic acid content of pituitary and adrenal glands can exceed 2,000 µmol/L, and muscle is at 200–300 µmol/L. The known coenzymatic functions of ascorbic acid do not require such high concentrations, so there"}],"string":"[\n {\n \"doc_id\": 1840698153413007000,\n \"text\": \"al glands can exceed 2,000 µmol/L, and muscle is at 200–300 µmol/L. The known coenzymatic functions of ascorbic acid do not require such high concentrations, so there may be other, as yet unknown functions. A consequence of all this high concentration organ content is that plasma vitamin C is not a good indicator of whole-body status, and people may vary in the amount of time needed to show symptoms of deficiency when consuming a diet very low in vitamin C.\\nExcretion (via urine) is as ascor\"\n },\n {\n \"doc_id\": 1078533498278672100,\n \"text\": \"bic acid and metabolites. The fraction that is excreted as unmetabolized ascorbic acid increases as intake increases. In addition, ascorbic acid converts (reversibly) to DHA and from that compound non-reversibly to 2,3-diketogulonate and then oxalate. These three metabolites are also excreted via urine. During times of low dietary intake, vitamin C is reabsorbed by the kidneys rather than excreted. This salvage process delays onset of deficiency. Humans are better than guinea pigs\"\n },\n {\n \"doc_id\": 3456284582962325500,\n \"text\": \" than in females.\\nPlasma levels are considered saturated at about 65 μmol/L, achieved by intakes of 100 to 200 mg/day, which are well above the recommended intakes. Even higher oral intake does not further raise plasma nor tissue concentrations because absorption efficiency decreases and any excess that is absorbed is excreted in urine.\\nVitamin C content in plasma is used to determine vitamin status. For research purposes, concentrations can be assessed in leukocytes and tissues, which are normally maintained at an order of magnitude higher than in plasma via an energy-dependent transport system, depleted slower\"\n },\n {\n \"doc_id\": 4745082524742403000,\n \"text\": \" deficiency is common in low and middle-income countries, and not uncommon in high income countries. In the latter, prevalence is higher in males than in females.\\nPlasma levels are considered saturated at about 65 μmol/L, achieved by intakes of 100 to 200 mg/day, which are well above the recommended intakes. Even higher oral intake does not further raise plasma nor tissue concentrations because absorption efficiency decreases and any excess that is absorbed is excreted in urine.\\nDiagnostic testing[edit]\\nVitamin C content in plasma is used to determine vitamin status. For research\"\n },\n {\n \"doc_id\": 3952197704319985000,\n \"text\": \"-reversibly to 2,3-diketogulonate and then oxalate. These three metabolites are also excreted via urine. During times of low dietary intake, vitamin C is reabsorbed by the kidneys rather than excreted. This salvage process delays onset of deficiency. Humans are better than guinea pigs at converting DHA back to ascorbate, and thus take much longer to become vitamin C deficient.\\nMost animals and plants are able to synthesize vitamin C through a sequence of enzyme-driven steps, which convert monosaccharides\"\n },\n {\n \"doc_id\": 7216188569036737000,\n \"text\": \") form of vitamin C. The amount of DHA found in plasma and tissues under normal conditions is low, as cells rapidly reduce DHA to ascorbate.\\nSVCTs are the predominant system for vitamin C transport within the body. In both vitamin C synthesizers (example: rat) and non-synthesizers (example: human) cells maintain ascorbic acid concentrations much higher than the approximately 50 micromoles/liter (µmol/L) found in plasma. For example, the ascorbic acid content of pituitary and adren\"\n },\n {\n \"doc_id\": 7598587754574724000,\n \"text\": \" purposes, concentrations can be assessed in leukocytes and tissues, which are normally maintained at an order of magnitude higher than in plasma via an energy-dependent transport system, depleted slower than plasma concentrations during dietary deficiency and restored faster during dietary repletion, but these analysis are difficult to measure, and hence not part of standard diagnostic testing.\"\n },\n {\n \"doc_id\": 2141155637567488000,\n \"text\": \". In both vitamin C synthesizers (example: rat) and non-synthesizers (example: human) cells maintain ascorbic acid concentrations much higher than the approximately 50 micromoles/liter (µmol/L) found in plasma. For example, the ascorbic acid content of pituitary and adrenal glands can exceed 2,000 µmol/L, and muscle is at 200–300 µmol/L. The known coenzymatic functions of ascorbic acid do not require such high concentrations, so there\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":3659852936339318300,"text":", but in solutions above pH 5 is predominantly found in the ionized form, ascorbate.\nNumerous analytical methods have been developed for ascorbic acid detection. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the volume of the sample required to decolorize a solution of dichlorophenolindophenol (DCPIP) and then calibrating the results by comparison with a known concentration of vitamin C.\nPlasma vitamin C is the most widely applied test for vitamin C status. Adequate levels are defined as"},{"doc_id":6546762869559694000,"text":" at converting DHA back to ascorbate, and thus take much longer to become vitamin C deficient."},{"doc_id":5832826501615092000,"text":"Deficiency[edit]\nPlasma vitamin C is the most widely applied test for vitamin C status. Adequate levels are defined as near 50 μmol/L. Hypovitaminosis of vitamin C is defined as less than 23 μmol/L, and deficiency as less than 11.4 μmol/L. For people 20 years of age or above, data from the US 2017-18 National Health and Nutrition Examination Survey showed mean serum concentrations of 53.4 μmol/L. The percent of people reported as deficient was 5.9%. Globally, vitamin C"},{"doc_id":7201258447329769000,"text":" that in humans and other primates the red blood cells have evolved a mechanism to more efficiently utilize the vitamin C present in the body by recycling oxidized l-dehydroascorbic acid (DHA) back into ascorbic acid for reuse by the body. The mechanism was not found to be present in mammals that synthesize their own vitamin C.\nHistory of large dose therapies[edit]\n\nFurther information: Vitamin C megadosage and Intravenous ascorbic acid\nVitamin C megadosage is a term describing the consumption or injection of vitamin C"},{"doc_id":1897329605376819700,"text":"Chemistry[edit]\nascorbic acid(reduced form)dehydroascorbic acid(oxidized form)\n\n\nMain article: Chemistry of ascorbic acid\nThe name \"vitamin C\" always refers to the l-enantiomer of ascorbic acid and its oxidized form, dehydroascorbate (DHA). Therefore, unless written otherwise, \"ascorbate\" and \"ascorbic acid\" refer in the nutritional literature to l-ascorbate and l-ascorbic acid respectively. Ascorbic"},{"doc_id":5547455179357877000,"text":"corbate may also provide antioxidant protection indirectly by regenerating other biological antioxidants such as α-tocopherol back to an active state. In addition, ascorbate also functions as a non-enzymatic reducing agent for mixed-function oxidases in the microsomal drug-metabolizing system that inactivates a wide variety of substrates such as drugs and environmental carcinogens.\nPharmacokinetics[edit]\nAscorbic acid is absorbed in the body by both simple diffusion and active transport. Approximately 70%–90% of vitamin C is absorbed at"},{"doc_id":2919147343159331000,"text":" C leads to a supraphysiological level of vitamin C followed by oxidative degradation to dehydroascorbic acid and hence to oxalate, increasing the risk of oxalate kidney stones and oxalate nephropathy. The risk may be higher in people with renal impairment, as kidneys efficiently excrete excess vitamin C. Second, treatment with high dose vitamin C should be avoided in patients with glucose-6-phosphate dehydrogenase deficiency as it can lead to acute hemolysis. Third, treatment might interfere with the accuracy of glucometer measurement of blood glucose"},{"doc_id":2599054069435418600,"text":"[ Ascorbate ](/wiki/Ascorbate \"Ascorbate\")\n( [ vitamin C ](/wiki/Vitamin_C \"Vitamin C\") )"},{"doc_id":7253695005352519000,"text":" Co-Transporter proteins (SVCTs) and Hexose Transporter proteins (GLUTs). SVCT1 and SVCT2 import ascorbate across plasma membranes. The Hexose Transporter proteins GLUT1, GLUT3 and GLUT4 transfer only the oxydized dehydroascorbic acid (DHA) form of vitamin C. The amount of DHA found in plasma and tissues under normal conditions is low, as cells rapidly reduce DHA to ascorbate.\nSVCTs are the predominant system for vitamin C transport within the body"},{"doc_id":1198937729232180200,"text":" levels, as both vitamin C and glucose have similar molecular structure, which could lead to false high blood glucose readings. Despite all these concerns, meta-analyses of patients in intensive care for sepsis, septic shock, COVID-19 and other acute conditions reported no increase in new-onset kidney stones, acute kidney injury or requirement for renal replacement therapy for patients receiving short-term, high-dose, intravenous vitamin C treatment. This suggests that intravenous vitamin C is safe under these short-term applications.\nScurvy was known to Hippocrates, described in book"},{"doc_id":8098761993998185000,"text":" moderate intakes of 30–180 mg/day. However, at doses above 1,000 mg/day, absorption falls to less than 50% as the active transport system becomes saturated. Active transport is managed by Sodium-Ascorbate Co-Transporter proteins (SVCTs) and Hexose Transporter proteins (GLUTs). SVCT1 and SVCT2 import ascorbate across plasma membranes. The Hexose Transporter proteins GLUT1, GLUT3 and GLUT4 transfer only the oxydized dehydroascorbic acid (DHA"},{"doc_id":2734320961715874000,"text":". Second, treatment with high dose vitamin C should be avoided in patients with glucose-6-phosphate dehydrogenase deficiency as it can lead to acute hemolysis. Third, treatment might interfere with the accuracy of glucometer measurement of blood glucose levels, as both vitamin C and glucose have similar molecular structure, which could lead to false high blood glucose readings. Despite all these concerns, meta-analyses of patients in intensive care for sepsis, septic shock, COVID-19 and other acute conditions reported no increase in new-onset kidney stones, acute kidney"},{"doc_id":5401560841183329000,"text":" near 50 μmol/L. Hypovitaminosis of vitamin C is defined as less than 23 μmol/L, and deficiency as less than 11.4 μmol/L. For people 20 years of age or above, data from the US 2017-18 National Health and Nutrition Examination Survey showed mean serum concentrations of 53.4 μmol/L. The percent of people reported as deficient was 5.9%. Globally, vitamin C deficiency is common in low and middle-income countries, and not uncommon in high income countries. In the latter, prevalence is higher in males"},{"doc_id":4165546165274181600,"text":"ymatic reducing agent for mixed-function oxidases in the microsomal drug-metabolizing system that inactivates a wide variety of substrates such as drugs and environmental carcinogens.\nAscorbic acid is absorbed in the body by both simple diffusion and active transport. Approximately 70%–90% of vitamin C is absorbed at moderate intakes of 30–180 mg/day. However, at doses above 1,000 mg/day, absorption falls to less than 50% as the active transport system becomes saturated. Active transport is managed by Sodium-Ascorbate"},{"doc_id":4029621959201428500,"text":" awarded the 1937 Nobel Prize in Physiology or Medicine.\n\nThe name \"vitamin C\" always refers to the l-enantiomer of ascorbic acid and its oxidized form, dehydroascorbate (DHA). Therefore, unless written otherwise, \"ascorbate\" and \"ascorbic acid\" refer in the nutritional literature to l-ascorbate and l-ascorbic acid respectively. Ascorbic acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH"},{"doc_id":4354579702268908500,"text":"hydroascorbic acid (DHA) back into ascorbic acid for reuse by the body. The mechanism was not found to be present in mammals that synthesize their own vitamin C.\nVitamin C megadosage is a term describing the consumption or injection of vitamin C in doses comparable to or higher than the amounts produced by the livers of mammals which are able to synthesize vitamin C. An argument for this, although not the actual term, was described in 1970 in an article by Linus Pauling. Briefly, his position was that for optimal health, humans"},{"doc_id":3364287394668921000,"text":" than plasma concentrations during dietary deficiency and restored faster during dietary repletion, but these analysis are difficult to measure, and hence not part of standard diagnostic testing.\nRecommendations for vitamin C intake by adults have been set by various national agencies:\nIn 2000, the chapter on Vitamin C in the North American Dietary Reference Intake was updated to give the Recommended Dietary Allowance (RDA) as 90 milligrams per day for adult men, 75 mg/day for adult women, and setting a Tolerable upper intake level (UL) for adults of 2,000 mg/day"},{"doc_id":2330525509765745000,"text":"Adverse effects[edit]\nOral intake as dietary supplements in excess of requirements are poorly absorbed, and excesses in the blood rapidly excreted in the urine, so it exhibits low acute toxicity. More than two to three grams, consumed orally, may cause nausea, abdominal cramps and diarrhea. These effects are attributed to the osmotic effect of unabsorbed vitamin C passing through the intestine. In theory, high vitamin C intake may cause excessive absorption of iron. A summary of reviews of supplementation in healthy subjects did not report this problem, but left as untested the possibility that"},{"doc_id":3326576835879253500,"text":" l-gulonolactone oxidase (GULO), which catalyzes the last step in the biosynthesis, is highly mutated and non-functional.\nThere is some information on serum vitamin C concentrations maintained in animal species that are able to synthesize vitamin C. One study of several breeds of dogs reported an average of 35.9 μmol/L. A report on goats, sheep and cattle reported ranges of 100–110, 265–270 and 160–350 μmol/L, respectively.\nThe biosynthesis of ascorbic acid in vertebrates"},{"doc_id":1870739135480758000,"text":" vitamin C concentrations were reported in people with cognitive impairment and Alzheimer's disease compared to people with normal cognition.\nHigher dietary intake of vitamin C was associated with lower risk of age-related cataracts. Vitamin C supplementation did not prevent age-related macular degeneration.\nLow intake and low serum concentration were associated with greater progression of periodontal disease.\nOral intake as dietary supplements in excess of requirements are poorly absorbed, and excesses in the blood rapidly excreted in the urine, so it exhibits low acute toxicity. More than two to three grams, consumed orally, may"},{"doc_id":878213562614537200,"text":" has led to the suggestion that, in higher primates, uric acid has taken over some of the functions of ascorbate.\nPlant synthesis[edit]\nVitamin C biosynthesis in plants\nThere are many different biosynthesis pathways to ascorbic acid in plants. Most proceed through products of glycolysis and other metabolic pathways. For example, one pathway utilizes plant cell wall polymers. The principal plant ascorbic acid biosynthesis pathway seems to be via l-galactose. The enzyme l-galactose dehydrogenase catalyzes the overall"},{"doc_id":6893234181156601000,"text":"Pharmacology[edit]\n\nSee also: Chemistry of ascorbic acid\nPharmacodynamics is the study of how the drug – in this instance vitamin C – affects the organism, whereas pharmacokinetics is the study of how an organism affects the drug.\nPharmacodynamics[edit]\nPharmacodynamics includes enzymes for which vitamin C is a cofactor, with function potentially compromised in a deficiency state, and any enzyme cofactor or other physiological function affected by administration of vitamin C, orally or injected, in excess of normal requirements. At normal physiological concentrations, vitamin C serves"},{"doc_id":7873948095790947000,"text":" extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process. In humans and in animals that cannot synthesize vitamin C, the enzyme l-gulonolactone oxidase (GULO), which catalyzes the last step in the biosynthesis, is highly mutated and non-functional.\nAnimal synthesis[edit]\nThere is some information on serum vitamin C concentrations maintained in animal species that are able to synthesize vitamin C. One study of several breeds of dogs reported an average of 35.9 μmol/L. A report on"},{"doc_id":5015729327251342000,"text":" veterinary manual states that daily intake of vitamin C at 3–6 mg/kg prevents scurvy in non-human primates. By way of comparison, across several countries, the recommended dietary intake for adult humans is in the range of 1–2 mg/kg.\nEvolution of animal synthesis[edit]\nAscorbic acid is a common enzymatic cofactor in mammals used in the synthesis of collagen, as well as a powerful reducing agent capable of rapidly scavenging a number of reactive oxygen species (ROS). Given that ascorbate has these important functions"},{"doc_id":2715709140712976000,"text":" acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH, but in solutions above pH 5 is predominantly found in the ionized form, ascorbate.\nNumerous analytical methods have been developed for ascorbic acid detection. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the volume of the sample required to decolorize a solution of dichlorophenolindophenol (DCPIP) and then calibrating the results by comparison with a known"},{"doc_id":3325132733794705000,"text":"corbic acid concentrations increase as intake increases until it plateaus at about 190 to 200 micromoles per liter (µmol/L) once consumption exceeds 1,250 milligrams. As noted, government recommendations are a range of 40 to 110 mg/day and normal plasma is approximately 50 µmol/L, so 'normal' is about 25% of what can be achieved when oral consumption is in the proposed megadose range.\nPauling popularized the concept of high dose vitamin C as prevention and treatment of the common cold in 1970. A few years"},{"doc_id":884264079800409700,"text":" body. Approximately 60%\nof ingested F by healthy adults are excreted in the urine, but only 45% for\nchildren, with the rest re-circulating into the plasma or deposited into the\nbone [ 18 ]. As a consequence, plasma and urinary excretion reflect a\nphysiologic homeostasis determined by previous F intake, rate of F uptake and\nremoval from bone, and the efficiency with which the kidneys excrete F."},{"doc_id":3158263817555756500,"text":" renal disease\". A review states that \"data from epidemiological studies do not support an association between excess ascorbic acid intake and kidney stone formation in apparently healthy individuals\", although one large, multi-year trial did report a nearly two-fold increase in kidney stones in men who regularly consumed a vitamin C supplement.\nThere is extensive research on the purported benefits of intravenous vitamin C for treatment of sepsis, severe COVID-19 and cancer. Reviews list trials with doses as high as 24 grams per day. Concerns about possible adverse effects are that intravenous high-dose vitamin"},{"doc_id":4383411552392145400,"text":" 10.1016/j.tibs.2006.11.006 ](https://doi.org/10.1016%2Fj.tibs.2006.11.006). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 17161604 ](https://pubmed.ncbi.nlm.nih.gov/17161604).\n13. ** ^ ** Murray, Robert F. (2003). _Harper's illustrated biochemistry_. New York"},{"doc_id":4967566730723076000,"text":"See also[edit]\nVitamin C (ascorbic acid)\nOxidoreductase\nScurvy"},{"doc_id":274634719536155260,"text":" goats, sheep and cattle reported ranges of 100–110, 265–270 and 160–350 μmol/L, respectively.\nThe biosynthesis of ascorbic acid in vertebrates starts with the formation of UDP-glucuronic acid. UDP-glucuronic acid is formed when UDP-glucose undergoes two oxidations catalyzed by the enzyme UDP-glucose 6-dehydrogenase. UDP-glucose 6-dehydrogenase uses the co-factor NAD as the electron acceptor. The transferase UDP-gluc"},{"doc_id":7388942664155367000,"text":"153-4 ](https://doi.org/10.1016%2FS0021-9258%2818%2967153-4). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 3528161 ](https://pubmed.ncbi.nlm.nih.gov/3528161).\n16. ** ^ ** Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert (2007). _Biochemistry_ (6th"},{"doc_id":9045961011940932000,"text":"Consequences of loss[edit]\nIt is likely that some level of adaptation occurred after the loss of the GULO gene by primates. Erythrocyte Glut1 and associated dehydroascorbic acid uptake modulated by stomatin switch are unique traits of humans and the few other mammals that have lost the ability to synthesize ascorbic acid from glucose. As GLUT transporters and stomatin are ubiquitously distributed in different human cell types and tissues, similar interactions may occur in human cells other than erythrocytes.\nLinus Pauling observed that after the loss of endogenous ascorbate production, apo(a) and Lp(a) were greatly favored by evolution, acting as ascorbate surrogate, since the frequency of occurrence of elevated Lp(a) plasma levels in species that had lost the ability to synthesize ascorbate is great. Also, only primates share regulation of CAMP gene expression by vitamin D, which occurred after the loss of GULO gene.\nJohnson et al. have hypothesized that the mutation of the GULOP pseudogene so that it stopped producing GULO may have been of benefit to early primates by increasing uric acid levels and enhancing fructose effects on weight gain and fat accumulation. With a shortage of food supplies this gave mutants a survival advantage."},{"doc_id":6284642480449593000,"text":"Gulonolactone oxidase deficiency[edit]\nThe non-functional gulonolactone oxidase pseudogene (GULOP) was mapped to human chromosome 8p21, which corresponds to an evolutionarily conserved segment on either porcine chromosome 4 (SSC4) or 14 (SSC14). GULO produces the precursor to ascorbic acid, which spontaneously converts to the vitamin itself.\nThe loss of activity of the gene encoding L-gulonolactone oxidase (GULO) has occurred separately in the history of several species. GULO activity has been lost in some species of bats, but others retain it. The loss of this enzyme activity is responsible for the inability of guinea pigs to enzymatically synthesize vitamin C. Both these events happened independently of the loss in the haplorrhine suborder of primates, which includes humans.\nThe remnant of this non-functional gene with many mutations is still present in the genomes of guinea pigs and humans. It is unknown if remains of the gene exist in the bats who lack GULO activity. The function of GULO appears to have been lost several times, and possibly re-acquired, in several lines of passerine birds, where ability to make vitamin C varies from species to species.\nLoss of GULO activity in the primate order occurred about 63 million years ago, at about the time it split into the suborders Haplorhini (which lost the enzyme activity) and Strepsirrhini (which retained it). The haplorhine (\"simple-nosed\") primates, which cannot make vitamin C enzymatically, include the tarsiers and the simians (apes, monkeys and humans). The strepsirrhine (\"bent-nosed\" or \"wet-nosed\") primates, which can still make vitamin C enzymatically, include lorises, galagos, pottos, and, to some extent, lemurs.\nL-Gulonolactone oxidase deficiency has been called \"hypoascorbemia\" and is described by OMIM (Online Mendelian Inheritance in Man) as \"a public inborn error of metabolism\", as it affects all humans. There exists a wide discrepancy between the amounts of ascorbic acid other primates consume and what are recommended as \"reference intakes\" for humans. In its patently pathological form, the effects of ascorbate deficiency are manifested as scurvy."},{"doc_id":7318715996946053000,"text":"Urine is the only biomarker capable of measuring F excretion. However, urine\nmay not be the most pertinent biomarker for the estimation of TDFI especially\nin children due to F accumulation during bone growth and mineralization.\nChildren can retain more F in their skeleton (~50%) than adults (approximately\n36%), with inverse retention in bone with age of the children and with the\nexcess of F excreted in urine [ 67 ]. The absence of correlation between\nDUFE and TDFI suggests that there is a variability but a non-neglig"},{"doc_id":2477233756556740600,"text":"obalamin \"Methylcobalamin\") (B 12 )\n* [ Ascorbic acid ](/wiki/Vitamin_C \"Vitamin C\") (C)\n* [ Phylloquinone ](/wiki/Phytomenadione \"Phytomenadione\") (K 1 ), [ Menaquinone ](/wiki/Vitamin_K \"Vitamin K\") (K 2 )\n* [ Coenzyme F420 ](/wiki/Coenzyme_F420 \"Coenzyme F420"},{"doc_id":2997268058314230300,"text":" the drug – in this instance vitamin C – affects the organism, whereas pharmacokinetics is the study of how an organism affects the drug.\nPharmacodynamics includes enzymes for which vitamin C is a cofactor, with function potentially compromised in a deficiency state, and any enzyme cofactor or other physiological function affected by administration of vitamin C, orally or injected, in excess of normal requirements. At normal physiological concentrations, vitamin C serves as an enzyme substrate or cofactor and an electron donor antioxidant. The enzymatic functions include the synthesis of collagen, carnitine, and neurotransmitters; the"},{"doc_id":8158331714726235000,"text":"[ Article ](https://doi.org/10.1093%2Fajcn%2F77.1.128) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=12499331)\n[ CAS ](/articles/cas-redirect/1:CAS:528:DC%2BD3sXhtFyntg%3D%3D) [ Google\nSch"},{"doc_id":104978552204890530,"text":" expression peaks in the morning to supporting biosynthesis for when mid-day sunlight intensity demands high ascorbic acid concentrations. Minor pathways may be specific to certain parts of plants; these can be either identical to the vertebrate pathway (including the GLO enzyme), or start with inositol and get to ascorbic acid via l-galactonic acid to l-galactonolactone.\nIndustrial synthesis[edit]\n\nMain article: Chemistry of ascorbic acid\nVitamin C can be produced from glucose by two main routes. The no longer"},{"doc_id":3683835109007077000,"text":" cause nausea, abdominal cramps and diarrhea. These effects are attributed to the osmotic effect of unabsorbed vitamin C passing through the intestine. In theory, high vitamin C intake may cause excessive absorption of iron. A summary of reviews of supplementation in healthy subjects did not report this problem, but left as untested the possibility that individuals with hereditary hemochromatosis might be adversely affected.\nThere is a longstanding belief among the mainstream medical community that vitamin C increases risk of kidney stones. \"Reports of kidney stone formation associated with excess ascorbic acid intake are limited to individuals with"},{"doc_id":6232058006680044000,"text":" see Unitary pseudogenes) and the cofactor FAD+. This reaction produces 2-oxogulonolactone (2-keto-gulonolactone), which spontaneously undergoes enolization to form ascorbic acid. Reptiles and older orders of birds make ascorbic acid in their kidneys. Recent orders of birds and most mammals make ascorbic acid in their liver.\nSome mammals have lost the ability to synthesize vitamin C, including simians and tarsiers, which together make up one of two major primate sub"},{"doc_id":742021988745971200,"text":" C-free diet, whereas in the earlier British study, six to eight months were required, possibly due to the pre-loading of this group with a 70 mg/day supplement for six weeks before the scorbutic diet was fed. Men in both studies had blood levels of ascorbic acid too low to be accurately measured by the time they developed signs of scurvy. These studies both reported that all obvious symptoms of scurvy could be completely reversed by supplementation of only 10 mg a day. Treatment of scurvy can be with vitamin C-containing foods or"},{"doc_id":4527458718898450400,"text":" the 1980s. Men in the prison study developed the first signs of scurvy about four weeks after starting the vitamin C-free diet, whereas in the earlier British study, six to eight months were required, possibly due to the pre-loading of this group with a 70 mg/day supplement for six weeks before the scorbutic diet was fed. Men in both studies had blood levels of ascorbic acid too low to be accurately measured by the time they developed signs of scurvy. These studies both reported that all obvious symptoms of scurvy could be completely reversed by"},{"doc_id":5975079325498478000,"text":"[ Mayo Clinic Laboratories ](https://www.mayocliniclabs.com)"},{"doc_id":3180349578149413400,"text":" C in either the kidney or the liver. In all of the cases where genomic analysis was done on an ascorbic acid auxotroph, the origin of the change was found to be a result of loss-of-function mutations in the gene that encodes L-gulono-γ-lactone oxidase, the enzyme that catalyzes the last step of the ascorbic acid pathway outlined above. One explanation for the repeated loss of the ability to synthesize vitamin C is that it was the result of genetic drift; assuming that the diet was rich in vitamin "},{"doc_id":8228831586340699000,"text":"* [ Pentose phosphate pathway ](/wiki/Pentose_phosphate_pathway \"Pentose phosphate pathway\")\n* [ Fructolysis ](/wiki/Fructolysis \"Fructolysis\")\n* [ Polyol pathway ](/wiki/Polyol_pathway \"Polyol pathway\")\n* [ Galactolysis ](/wiki/Galactolysis \"Galactolysis\")\n* [ Leloir pathway ](/wiki/Leloir_pathway \"Leloir pathway\")"},{"doc_id":4310867272975448600,"text":" ages 14–18 years)\n\n75\n\n\nRDA (adult female)\n\n75\n\n\nRDA (adult male)\n\n90\n\n\nRDA (pregnancy)\n\n85\n\n\nRDA (lactation)\n\n120\n\n\nUL (adult female)\n\n2,000\n\n\nUL (adult male)\n\n2,000\nIn 2000, the chapter on Vitamin C in the North American Dietary Reference Intake was updated to give the Recommended Dietary Allowance (RDA) as 90 milligrams per day for adult men, 75 "},{"doc_id":1226072014279458600,"text":" marker points (63 to 58 million years ago).\nIt has also been noted that the loss of the ability to synthesize ascorbate strikingly parallels the inability to break down uric acid, also a characteristic of primates. Uric acid and ascorbate are both strong reducing agents. This has led to the suggestion that, in higher primates, uric acid has taken over some of the functions of ascorbate.\nThere are many different biosynthesis pathways to ascorbic acid in plants. Most proceed through products of glycolysis and other metabolic pathways. For"},{"doc_id":2050089849119220200,"text":" extensive research on the purported benefits of intravenous vitamin C for treatment of sepsis, severe COVID-19 and cancer. Reviews list trials with doses as high as 24 grams per day. Concerns about possible adverse effects are that intravenous high-dose vitamin C leads to a supraphysiological level of vitamin C followed by oxidative degradation to dehydroascorbic acid and hence to oxalate, increasing the risk of oxalate kidney stones and oxalate nephropathy. The risk may be higher in people with renal impairment, as kidneys efficiently excrete excess vitamin C"},{"doc_id":8735146826395335000,"text":"ate residue from their c-terminal glycine residues. This increases peptide hormone stability and activity.\nAs an antioxidant, ascorbate scavenges reactive oxygen and nitrogen compounds, thus neutralizing the potential tissue damage of these free radical compounds. Dehydroascorbate, the oxidized form, is then recycled back to ascorbate by endogenous antioxidants such as glutathione. In the eye, ascorbate is thought to protect against photolytically generated free-radical damage; higher plasma ascorbate is associated with lower risk of cateracts. As"},{"doc_id":3946738525946685000,"text":" individuals with hereditary hemochromatosis might be adversely affected.\nThere is a longstanding belief among the mainstream medical community that vitamin C increases risk of kidney stones. \"Reports of kidney stone formation associated with excess ascorbic acid intake are limited to individuals with renal disease\". A review states that \"data from epidemiological studies do not support an association between excess ascorbic acid intake and kidney stone formation in apparently healthy individuals\", although one large, multi-year trial did report a nearly two-fold increase in kidney stones in men who regularly consumed a vitamin C supplement.\nThere is"},{"doc_id":8808915870447512000,"text":"_[ Pentose\nphosphate\npathway ](/wiki/Pentose_phosphate_pathway \"Pentose phosphate pathway\") _"},{"doc_id":1686582037625913300,"text":"[ Article ](https://doi.org/10.1016%2FS0303-8467%2899%2900035-9) [ CAS\n](/articles/cas-redirect/1:STN:280:DC%2BD3c%2FgsFaqtg%3D%3D) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=10536902)"},{"doc_id":4049941878204482600,"text":"olic acid\"), [ DHFA / H 2 FA ](/wiki/Dihydrofolic_acid \"Dihydrofolic acid\"), [ MTHF ](/wiki/5,10-Methylenetetrahydrofolate \"5,10-Methylenetetrahydrofolate\") (B 9 )\n* [ AdoCbl ](/wiki/Adenosylcobalamin \"Adenosylcobalamin\"), [ MeCbl ](/wiki/Methylc"},{"doc_id":8808874834402231000,"text":"• Validity of data and methods: F intake, F excretion (urine collection), and\nF analytical method, or not reported."},{"doc_id":3719440369263880700,"text":"Animal models[edit]\nStudies of human diseases have benefited from the availability of small laboratory animal models. However, the tissues of animal models with a GULO gene generally have high levels of ascorbic acid and so are often only slightly influenced by exogenous vitamin C. This is a major handicap for studies involving the endogenous redox systems of primates and other animals that lack this gene.\nGuinea pigs are a popular human model. They lost the ability to make GULO 20 million years ago.\nIn 1999, Maeda et al. genetically engineered mice with inactivated GULO gene. The mutant mice, like humans, entirely depend on dietary vitamin C, and they show changes indicating that the integrity of their vasculature is compromised. GULO mice have been used as a human model in multiple subsequent studies.\nThere have been successful attempts to activate lost enzymatic function in different animal species. Various GULO mutants were also identified."},{"doc_id":4143018406264960000,"text":"ifier\\)\") : [ 10.1016/j.abb.2006.05.001 ](https://doi.org/10.1016%2Fj.abb.2006.05.001). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 16806044 ](https://pubmed.ncbi.nlm.nih.gov/16806044).\n33. ** ^ ** Kamenetsky, M.; Middelhaufe, S.; Bank,"},{"doc_id":8098842388776744000,"text":" synthesis and catabolism of tyrosine; and the metabolism of microsomes. In nonenzymatic functions it acts as a reducing agent, donating electrons to oxidized molecules and preventing oxidation in order to keep iron and copper atoms in their reduced states. At non-physiological concentrations achieved by intravenous dosing, vitamin C may function as a pro-oxidant, with therapeutic toxicity against cancer cells.\nVitamin C functions as a cofactor for the following enzymes:\nAs an antioxidant, ascorbate scavenges reactive oxygen and nitrogen compounds, thus neutralizing the"},{"doc_id":170279123303973920,"text":"alaf MR. Historical and recent biological markers of exposure to fluoride. Monogr Oral Sci 2011;22:52-65. [ [ PubMed abstract ](https://pubmed.ncbi.nlm.nih.gov/21701191/) ]\n5. Rango T, Vengosh A, Jeuland M, Whitford GM, Tekle-Haimanot R. Biomarkers of chronic fluoride exposure in groundwater in a highly exposed population. Sci Total Environ 2017;596-597:1-11. [ [ PubMed abstract ](https://"},{"doc_id":3000185096634474000,"text":"[ α-Keto-\nglutarate ](/wiki/Alpha-Ketoglutaric_acid \"Alpha-Ketoglutaric acid\")"},{"doc_id":3253288000330283500,"text":"2 mg/kg.\nAscorbic acid is a common enzymatic cofactor in mammals used in the synthesis of collagen, as well as a powerful reducing agent capable of rapidly scavenging a number of reactive oxygen species (ROS). Given that ascorbate has these important functions, it is surprising that the ability to synthesize this molecule has not always been conserved. In fact, anthropoid primates, Cavia porcellus (guinea pigs), teleost fishes, most bats, and some passerine birds have all independently lost the ability to internally synthesize vitamin"},{"doc_id":498406216741366300,"text":" should be consuming at least 2,300 mg/day to compensate for the inability to synthesize vitamin C. The recommendation also fell into the consumption range for gorillas – a non-synthesizing near-relative to humans. A second argument for high intake is that serum ascorbic acid concentrations increase as intake increases until it plateaus at about 190 to 200 micromoles per liter (µmol/L) once consumption exceeds 1,250 milligrams. As noted, government recommendations are a range of 40 to 110 mg/day and normal plasma is approximately"},{"doc_id":2820596826049959000,"text":"---|---\n[ Nucleotide\nmetabolism ](/wiki/Nucleic_acid_metabolism \"Nucleic acid metabolism\") |"},{"doc_id":4118230098960824000,"text":"-arginine 1, and there is also evidence cited that vitamin C directly enhances the biological activity of nitric oxide, a vasodilator.\nType 2 diabetes[edit]\nThere are contradictory reviews. From one, vitamin C supplementation cannot be recommended for management of type 2 diabetes. However, another reported that supplementation with high doses of vitamin C can decrease blood glucose, insulin and hemoglobin A1c.\nIron deficiency[edit]\nOne of the causes of iron-deficiency anemia is reduced absorption of iron. Iron absorption can be enhanced through ingestion of vitamin C alongside"},{"doc_id":7472875436405453000,"text":", it is surprising that the ability to synthesize this molecule has not always been conserved. In fact, anthropoid primates, Cavia porcellus (guinea pigs), teleost fishes, most bats, and some passerine birds have all independently lost the ability to internally synthesize vitamin C in either the kidney or the liver. In all of the cases where genomic analysis was done on an ascorbic acid auxotroph, the origin of the change was found to be a result of loss-of-function mutations in the gene that encodes L-gulono-γ"},{"doc_id":5628233220328595000,"text":"_**[ Gluconeo-\ngenesis ](/wiki/Gluconeogenesis \"Gluconeogenesis\") ** _"},{"doc_id":6407680627101928000,"text":" that half the women and more than half the men are not consuming the RDA for vitamin C. The same survey stated that about 30% of adults reported they consumed a vitamin C dietary supplement or a multi-vitamin/mineral supplement that included vitamin C, and that for these people total consumption was between 300 and 400 mg/d.\nTolerable upper intake level[edit]\nIn 2000, the Institute of Medicine of the US National Academy of Sciences set a Tolerable upper intake level (UL) for adults of 2,000 mg/day. The amount was"},{"doc_id":4161775671760185300,"text":"[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)"},{"doc_id":7325009641872769000,"text":" to have been lost and then later re-acquired in at least two cases. The ability to synthesize vitamin C has also been lost in about 96% of extant fish (the teleosts).\nOn a milligram consumed per kilogram of body weight basis, simian non-synthesizer species consume the vitamin in amounts 10 to 20 times higher than what is recommended by governments for humans. This discrepancy constituted some of the basis of the controversy on human recommended dietary allowances being set too low. However, simian consumption does not indicate simian requirements. Merck's"},{"doc_id":3669686666216705500,"text":" is one of the most labile vitamins in foods. Its main loss during processing and storage is from oxidation, which is accelerated by light, oxygen, heat, increased pH, high moisture content (water activity), and the presence of copper or ferrous salts. To reduce oxidation, the vitamin C used in commodity fortification is coated with ethyl cellulose (2.5 percent). Oxidative losses also occur during food processing and preparation, and additional vitamin C may be lost if it dissolves into cooking liquid and is then discarded.\"\nAscorbic acid and some of its salts and"},{"doc_id":3309188676753162000,"text":"[ Article ](https://doi.org/10.1016%2Fj.clinph.2006.01.017) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=16581292)\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865224) [ CAS\n](/articles/cas-red"},{"doc_id":538595360612986940,"text":"cine \"Glycine\")\n* [ GSNO ](/wiki/S-Nitrosoglutathione \"S-Nitrosoglutathione\")\n* [ GSSG ](/wiki/Oxidized_glutathione \"Oxidized glutathione\")\n* [ Kynurenic acid ](/wiki/Kynurenic_acid \"Kynurenic acid\")\n* [ NAA ](/wiki/N-Acetylaspartic_acid \"N-Acetylaspartic acid\")"},{"doc_id":3142707818421335600,"text":" to vitamin C. Yeasts do not make l-ascorbic acid but rather its stereoisomer, erythorbic acid. In plants, synthesis is accomplished through the conversion of mannose or galactose to ascorbic acid. In animals, the starting material is glucose. In some species that synthesize ascorbate in the liver (including mammals and perching birds), the glucose is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process. In humans and in animals that cannot synthesize vitamin C, the enzyme"},{"doc_id":478117815120135600,"text":"[ Article ](https://doi.org/10.1093%2Fajcn%2F85.2.426) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=17284739)\n[ CAS ](/articles/cas-redirect/1:CAS:528:DC%2BD2sXhvFWhu7g%3D) [ Google\n"},{"doc_id":8948586324244455000,"text":"Further investigation combining measures of F in plasma and urine could be\ninformative on the bioavailability of F and its interactions with different\norgans. Once absorbed, F reaches peak serum concentrations after 20–60 min,\nand then returns to the baseline after approximately 15 h suggesting that part\nof the oral F passes through systemic route [ 56 , 57 , 68 , 69 ].\nThis is probably the reason why a relation has been reported between\nsupplement use or the amount of toothpaste used for brushing and child's\nfluorosis scores ["},{"doc_id":7298847495697952000,"text":"* [ Metal metabolism ](/wiki/Bioinorganic_chemistry \"Bioinorganic chemistry\")\n* [ Iron metabolism ](/wiki/Human_iron_metabolism \"Human iron metabolism\")\n* [ Ethanol metabolism ](/wiki/Ethanol_metabolism \"Ethanol metabolism\")\n* [ Phospagen system (ATP-PCr) ](/wiki/Phosphagen \"Phosphagen\")"},{"doc_id":3600014978857611000,"text":" States National Academy of Sciences\n100 mg/day: Japan National Institute of Health and Nutrition\n110 mg/day (males) and 95 mg/day (females): European Food Safety Authority\nUS vitamin C recommendations (mg per day)\n\n\nRDA (children ages 1–3 years)\n\n15\n\n\nRDA (children ages 4–8 years)\n\n25\n\n\nRDA (children ages 9–13 years)\n\n45\n\n\nRDA (girls ages 14–18 years)\n\n65\n\n\nRDA (boys"},{"doc_id":7484065216331891000,"text":" 1. Biochemistry, L. Stryer (W.H. Freeman and Co, San Francisco, 1975). \n 2. The Cambridge Guide to the Material World, Rodney Cotterill (Cambridge University Press, Cambridge, 1985) \n\n## Contributors"},{"doc_id":2506392599316147700,"text":"[ Acidity ](/wiki/Acid_dissociation_constant \"Acid dissociation constant\") (p\n_K_ a ) | 0.9, 1.4, 3.8, 6.5\n[ UV-vis ](/wiki/Ultraviolet%E2%80%93visible_spectroscopy \"Ultraviolet–visible\nspectroscopy\") (λ max ) | 259 nm [1]\n[ Absorbance ](/wiki/Absorbance \"Absorbance\") | "},{"doc_id":2743938332044641000,"text":"* [ Purine metabolism ](/wiki/Purine_metabolism \"Purine metabolism\")\n* [ Nucleotide salvage ](/wiki/Nucleotide_salvage \"Nucleotide salvage\")\n* [ Pyrimidine metabolism ](/wiki/Pyrimidine_metabolism \"Pyrimidine metabolism\")\n* [ Purine nucleotide cycle ](/wiki/Purine_nucleotide_cycle \"Purine nucleotide cycle\")"},{"doc_id":4282846855766989300,"text":" as an enzyme substrate or cofactor and an electron donor antioxidant. The enzymatic functions include the synthesis of collagen, carnitine, and neurotransmitters; the synthesis and catabolism of tyrosine; and the metabolism of microsomes. In nonenzymatic functions it acts as a reducing agent, donating electrons to oxidized molecules and preventing oxidation in order to keep iron and copper atoms in their reduced states. At non-physiological concentrations achieved by intravenous dosing, vitamin C may function as a pro-oxidant, with therapeutic toxicity against cancer cells.\n"},{"doc_id":2085103195239381000,"text":" (along with minerals and other vitamins). Supplemental rations of these highly fortified, blended foods are provided to refugees and displaced persons in camps and to beneficiaries of development feeding programs that are targeted largely toward mothers and children. The report adds: \"The stability of vitamin C (L-ascorbic acid) is of concern because this is one of the most labile vitamins in foods. Its main loss during processing and storage is from oxidation, which is accelerated by light, oxygen, heat, increased pH, high moisture content (water activity), and the presence of copper or ferrous salts. To reduce"},{"doc_id":8894272547326739000,"text":" potential tissue damage of these free radical compounds. Dehydroascorbate, the oxidized form, is then recycled back to ascorbate by endogenous antioxidants such as glutathione. In the eye, ascorbate is thought to protect against photolytically generated free-radical damage; higher plasma ascorbate is associated with lower risk of cateracts. Ascorbate may also provide antioxidant protection indirectly by regenerating other biological antioxidants such as α-tocopherol back to an active state. In addition, ascorbate also functions as a non-enz"},{"doc_id":7736402839752618000,"text":" mechanism is that it functions as an antioxidant, neutralizing free radicals from sunlight exposure, air pollutants or normal metabolic processes. The efficacy of topical treatment, as opposed to oral intake is poorly understood. The clinical trial literature is characterized as insufficient to support health claims, one reason being put forward was that \"All the studies used vitamin C in combination with other ingredients or therapeutic mechanisms, thereby complicating any specific conclusions regarding the efficacy of vitamin C.\" More research is needed.\nCognitive impairment and Alzheimer's disease[edit]\nLower plasma vitamin C concentrations were reported in people with cognitive impairment and Alzheimer's"},{"doc_id":4066839141633327000,"text":"LOAEL), meaning that other adverse effects were observed at even higher intakes. ULs are progressively lower for younger and younger children. In 2006, the European Food Safety Authority (EFSA) also pointed out the disturbances at that dose level, but reached the conclusion that there was not sufficient evidence to set a UL for vitamin C, as did the Japan National Institute of Health and Nutrition in 2010.\nFor US food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For vitamin C labeling purposes, 100% of the Daily Value was"},{"doc_id":450371710151089660,"text":" adults reported they consumed a vitamin C dietary supplement or a multi-vitamin/mineral supplement that included vitamin C, and that for these people total consumption was between 300 and 400 mg/d.\nIn 2000, the Institute of Medicine of the US National Academy of Sciences set a Tolerable upper intake level (UL) for adults of 2,000 mg/day. The amount was chosen because human trials had reported diarrhea and other gastrointestinal disturbances at intakes of greater than 3,000 mg/day. This was the Lowest-Observed-Adverse-Effect Level ("},{"doc_id":2172318637421551600,"text":"Synthesis[edit]\nMost animals and plants are able to synthesize vitamin C through a sequence of enzyme-driven steps, which convert monosaccharides to vitamin C. Yeasts do not make l-ascorbic acid but rather its stereoisomer, erythorbic acid. In plants, synthesis is accomplished through the conversion of mannose or galactose to ascorbic acid. In animals, the starting material is glucose. In some species that synthesize ascorbate in the liver (including mammals and perching birds), the glucose is"},{"doc_id":959887416059251200,"text":"[ CAS ](/articles/cas-redirect/1:STN:280:DC%2BD3c7jt1OgtA%3D%3D) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=10669252)\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7976324)"},{"doc_id":766047156750821100,"text":"[ Article ](https://doi.org/10.1152%2Fajpregu.00197.2002) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=12529281)\n[ CAS ](/articles/cas-redirect/1:CAS:528:DC%2BD3sXhtlOqsLs%3D) [ Google\nScholar\n"},{"doc_id":6828994514780543000,"text":"mg / 100g)\n\n\nGreen bell pepper/capsicum\n80\n\n\nBrussels sprouts\n80\n\n\nLoganberry, redcurrant\n80\n\n\nCloudberry, elderberry\n60\n\n\nStrawberry\n60\n\n\nPapaya\n60\n\n\nOrange, lemon\n53\n\n\nCauliflower\n48\n\n\nPineapple\n48\n\n\nCantaloupe\n40\n\n\nPassion fruit, raspberry\n30\n\n\nGrapefruit, lime\n30\n\n\nCabbage, spinach\n30\nRaw plant source\n\n"},{"doc_id":7671617941354681000,"text":" dietary supplements or injection.\nPeople in sepsis may have micronutrient deficiencies, including low levels of vitamin C. An intake of 3.0 g/day, which requires intravenous administration, appears to be needed to maintain normal plasma concentrations in people with sepsis or severe burn injury. Sepsis mortality is reduced with administration of intravenous vitamin C.\nResearch on vitamin C in the common cold has been divided into effects on prevention, duration, and severity. Oral intakes of more than 200 mg/day taken on a regular basis was not effective in prevention of the"}],"string":"[\n {\n \"doc_id\": 3659852936339318300,\n \"text\": \", but in solutions above pH 5 is predominantly found in the ionized form, ascorbate.\\nNumerous analytical methods have been developed for ascorbic acid detection. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the volume of the sample required to decolorize a solution of dichlorophenolindophenol (DCPIP) and then calibrating the results by comparison with a known concentration of vitamin C.\\nPlasma vitamin C is the most widely applied test for vitamin C status. Adequate levels are defined as\"\n },\n {\n \"doc_id\": 6546762869559694000,\n \"text\": \" at converting DHA back to ascorbate, and thus take much longer to become vitamin C deficient.\"\n },\n {\n \"doc_id\": 5832826501615092000,\n \"text\": \"Deficiency[edit]\\nPlasma vitamin C is the most widely applied test for vitamin C status. Adequate levels are defined as near 50 μmol/L. Hypovitaminosis of vitamin C is defined as less than 23 μmol/L, and deficiency as less than 11.4 μmol/L. For people 20 years of age or above, data from the US 2017-18 National Health and Nutrition Examination Survey showed mean serum concentrations of 53.4 μmol/L. The percent of people reported as deficient was 5.9%. Globally, vitamin C\"\n },\n {\n \"doc_id\": 7201258447329769000,\n \"text\": \" that in humans and other primates the red blood cells have evolved a mechanism to more efficiently utilize the vitamin C present in the body by recycling oxidized l-dehydroascorbic acid (DHA) back into ascorbic acid for reuse by the body. The mechanism was not found to be present in mammals that synthesize their own vitamin C.\\nHistory of large dose therapies[edit]\\n\\nFurther information: Vitamin C megadosage and Intravenous ascorbic acid\\nVitamin C megadosage is a term describing the consumption or injection of vitamin C\"\n },\n {\n \"doc_id\": 1897329605376819700,\n \"text\": \"Chemistry[edit]\\nascorbic acid(reduced form)dehydroascorbic acid(oxidized form)\\n\\n\\nMain article: Chemistry of ascorbic acid\\nThe name \\\"vitamin C\\\" always refers to the l-enantiomer of ascorbic acid and its oxidized form, dehydroascorbate (DHA). Therefore, unless written otherwise, \\\"ascorbate\\\" and \\\"ascorbic acid\\\" refer in the nutritional literature to l-ascorbate and l-ascorbic acid respectively. Ascorbic\"\n },\n {\n \"doc_id\": 5547455179357877000,\n \"text\": \"corbate may also provide antioxidant protection indirectly by regenerating other biological antioxidants such as α-tocopherol back to an active state. In addition, ascorbate also functions as a non-enzymatic reducing agent for mixed-function oxidases in the microsomal drug-metabolizing system that inactivates a wide variety of substrates such as drugs and environmental carcinogens.\\nPharmacokinetics[edit]\\nAscorbic acid is absorbed in the body by both simple diffusion and active transport. Approximately 70%–90% of vitamin C is absorbed at\"\n },\n {\n \"doc_id\": 2919147343159331000,\n \"text\": \" C leads to a supraphysiological level of vitamin C followed by oxidative degradation to dehydroascorbic acid and hence to oxalate, increasing the risk of oxalate kidney stones and oxalate nephropathy. The risk may be higher in people with renal impairment, as kidneys efficiently excrete excess vitamin C. Second, treatment with high dose vitamin C should be avoided in patients with glucose-6-phosphate dehydrogenase deficiency as it can lead to acute hemolysis. Third, treatment might interfere with the accuracy of glucometer measurement of blood glucose\"\n },\n {\n \"doc_id\": 2599054069435418600,\n \"text\": \"[ Ascorbate ](/wiki/Ascorbate \\\"Ascorbate\\\")\\n( [ vitamin C ](/wiki/Vitamin_C \\\"Vitamin C\\\") )\"\n },\n {\n \"doc_id\": 7253695005352519000,\n \"text\": \" Co-Transporter proteins (SVCTs) and Hexose Transporter proteins (GLUTs). SVCT1 and SVCT2 import ascorbate across plasma membranes. The Hexose Transporter proteins GLUT1, GLUT3 and GLUT4 transfer only the oxydized dehydroascorbic acid (DHA) form of vitamin C. The amount of DHA found in plasma and tissues under normal conditions is low, as cells rapidly reduce DHA to ascorbate.\\nSVCTs are the predominant system for vitamin C transport within the body\"\n },\n {\n \"doc_id\": 1198937729232180200,\n \"text\": \" levels, as both vitamin C and glucose have similar molecular structure, which could lead to false high blood glucose readings. Despite all these concerns, meta-analyses of patients in intensive care for sepsis, septic shock, COVID-19 and other acute conditions reported no increase in new-onset kidney stones, acute kidney injury or requirement for renal replacement therapy for patients receiving short-term, high-dose, intravenous vitamin C treatment. This suggests that intravenous vitamin C is safe under these short-term applications.\\nScurvy was known to Hippocrates, described in book\"\n },\n {\n \"doc_id\": 8098761993998185000,\n \"text\": \" moderate intakes of 30–180 mg/day. However, at doses above 1,000 mg/day, absorption falls to less than 50% as the active transport system becomes saturated. Active transport is managed by Sodium-Ascorbate Co-Transporter proteins (SVCTs) and Hexose Transporter proteins (GLUTs). SVCT1 and SVCT2 import ascorbate across plasma membranes. The Hexose Transporter proteins GLUT1, GLUT3 and GLUT4 transfer only the oxydized dehydroascorbic acid (DHA\"\n },\n {\n \"doc_id\": 2734320961715874000,\n \"text\": \". Second, treatment with high dose vitamin C should be avoided in patients with glucose-6-phosphate dehydrogenase deficiency as it can lead to acute hemolysis. Third, treatment might interfere with the accuracy of glucometer measurement of blood glucose levels, as both vitamin C and glucose have similar molecular structure, which could lead to false high blood glucose readings. Despite all these concerns, meta-analyses of patients in intensive care for sepsis, septic shock, COVID-19 and other acute conditions reported no increase in new-onset kidney stones, acute kidney\"\n },\n {\n \"doc_id\": 5401560841183329000,\n \"text\": \" near 50 μmol/L. Hypovitaminosis of vitamin C is defined as less than 23 μmol/L, and deficiency as less than 11.4 μmol/L. For people 20 years of age or above, data from the US 2017-18 National Health and Nutrition Examination Survey showed mean serum concentrations of 53.4 μmol/L. The percent of people reported as deficient was 5.9%. Globally, vitamin C deficiency is common in low and middle-income countries, and not uncommon in high income countries. In the latter, prevalence is higher in males\"\n },\n {\n \"doc_id\": 4165546165274181600,\n \"text\": \"ymatic reducing agent for mixed-function oxidases in the microsomal drug-metabolizing system that inactivates a wide variety of substrates such as drugs and environmental carcinogens.\\nAscorbic acid is absorbed in the body by both simple diffusion and active transport. Approximately 70%–90% of vitamin C is absorbed at moderate intakes of 30–180 mg/day. However, at doses above 1,000 mg/day, absorption falls to less than 50% as the active transport system becomes saturated. Active transport is managed by Sodium-Ascorbate\"\n },\n {\n \"doc_id\": 4029621959201428500,\n \"text\": \" awarded the 1937 Nobel Prize in Physiology or Medicine.\\n\\nThe name \\\"vitamin C\\\" always refers to the l-enantiomer of ascorbic acid and its oxidized form, dehydroascorbate (DHA). Therefore, unless written otherwise, \\\"ascorbate\\\" and \\\"ascorbic acid\\\" refer in the nutritional literature to l-ascorbate and l-ascorbic acid respectively. Ascorbic acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH\"\n },\n {\n \"doc_id\": 4354579702268908500,\n \"text\": \"hydroascorbic acid (DHA) back into ascorbic acid for reuse by the body. The mechanism was not found to be present in mammals that synthesize their own vitamin C.\\nVitamin C megadosage is a term describing the consumption or injection of vitamin C in doses comparable to or higher than the amounts produced by the livers of mammals which are able to synthesize vitamin C. An argument for this, although not the actual term, was described in 1970 in an article by Linus Pauling. Briefly, his position was that for optimal health, humans\"\n },\n {\n \"doc_id\": 3364287394668921000,\n \"text\": \" than plasma concentrations during dietary deficiency and restored faster during dietary repletion, but these analysis are difficult to measure, and hence not part of standard diagnostic testing.\\nRecommendations for vitamin C intake by adults have been set by various national agencies:\\nIn 2000, the chapter on Vitamin C in the North American Dietary Reference Intake was updated to give the Recommended Dietary Allowance (RDA) as 90 milligrams per day for adult men, 75 mg/day for adult women, and setting a Tolerable upper intake level (UL) for adults of 2,000 mg/day\"\n },\n {\n \"doc_id\": 2330525509765745000,\n \"text\": \"Adverse effects[edit]\\nOral intake as dietary supplements in excess of requirements are poorly absorbed, and excesses in the blood rapidly excreted in the urine, so it exhibits low acute toxicity. More than two to three grams, consumed orally, may cause nausea, abdominal cramps and diarrhea. These effects are attributed to the osmotic effect of unabsorbed vitamin C passing through the intestine. In theory, high vitamin C intake may cause excessive absorption of iron. A summary of reviews of supplementation in healthy subjects did not report this problem, but left as untested the possibility that\"\n },\n {\n \"doc_id\": 3326576835879253500,\n \"text\": \" l-gulonolactone oxidase (GULO), which catalyzes the last step in the biosynthesis, is highly mutated and non-functional.\\nThere is some information on serum vitamin C concentrations maintained in animal species that are able to synthesize vitamin C. One study of several breeds of dogs reported an average of 35.9 μmol/L. A report on goats, sheep and cattle reported ranges of 100–110, 265–270 and 160–350 μmol/L, respectively.\\nThe biosynthesis of ascorbic acid in vertebrates\"\n },\n {\n \"doc_id\": 1870739135480758000,\n \"text\": \" vitamin C concentrations were reported in people with cognitive impairment and Alzheimer's disease compared to people with normal cognition.\\nHigher dietary intake of vitamin C was associated with lower risk of age-related cataracts. Vitamin C supplementation did not prevent age-related macular degeneration.\\nLow intake and low serum concentration were associated with greater progression of periodontal disease.\\nOral intake as dietary supplements in excess of requirements are poorly absorbed, and excesses in the blood rapidly excreted in the urine, so it exhibits low acute toxicity. More than two to three grams, consumed orally, may\"\n },\n {\n \"doc_id\": 878213562614537200,\n \"text\": \" has led to the suggestion that, in higher primates, uric acid has taken over some of the functions of ascorbate.\\nPlant synthesis[edit]\\nVitamin C biosynthesis in plants\\nThere are many different biosynthesis pathways to ascorbic acid in plants. Most proceed through products of glycolysis and other metabolic pathways. For example, one pathway utilizes plant cell wall polymers. The principal plant ascorbic acid biosynthesis pathway seems to be via l-galactose. The enzyme l-galactose dehydrogenase catalyzes the overall\"\n },\n {\n \"doc_id\": 6893234181156601000,\n \"text\": \"Pharmacology[edit]\\n\\nSee also: Chemistry of ascorbic acid\\nPharmacodynamics is the study of how the drug – in this instance vitamin C – affects the organism, whereas pharmacokinetics is the study of how an organism affects the drug.\\nPharmacodynamics[edit]\\nPharmacodynamics includes enzymes for which vitamin C is a cofactor, with function potentially compromised in a deficiency state, and any enzyme cofactor or other physiological function affected by administration of vitamin C, orally or injected, in excess of normal requirements. At normal physiological concentrations, vitamin C serves\"\n },\n {\n \"doc_id\": 7873948095790947000,\n \"text\": \" extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process. In humans and in animals that cannot synthesize vitamin C, the enzyme l-gulonolactone oxidase (GULO), which catalyzes the last step in the biosynthesis, is highly mutated and non-functional.\\nAnimal synthesis[edit]\\nThere is some information on serum vitamin C concentrations maintained in animal species that are able to synthesize vitamin C. One study of several breeds of dogs reported an average of 35.9 μmol/L. A report on\"\n },\n {\n \"doc_id\": 5015729327251342000,\n \"text\": \" veterinary manual states that daily intake of vitamin C at 3–6 mg/kg prevents scurvy in non-human primates. By way of comparison, across several countries, the recommended dietary intake for adult humans is in the range of 1–2 mg/kg.\\nEvolution of animal synthesis[edit]\\nAscorbic acid is a common enzymatic cofactor in mammals used in the synthesis of collagen, as well as a powerful reducing agent capable of rapidly scavenging a number of reactive oxygen species (ROS). Given that ascorbate has these important functions\"\n },\n {\n \"doc_id\": 2715709140712976000,\n \"text\": \" acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH, but in solutions above pH 5 is predominantly found in the ionized form, ascorbate.\\nNumerous analytical methods have been developed for ascorbic acid detection. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the volume of the sample required to decolorize a solution of dichlorophenolindophenol (DCPIP) and then calibrating the results by comparison with a known\"\n },\n {\n \"doc_id\": 3325132733794705000,\n \"text\": \"corbic acid concentrations increase as intake increases until it plateaus at about 190 to 200 micromoles per liter (µmol/L) once consumption exceeds 1,250 milligrams. As noted, government recommendations are a range of 40 to 110 mg/day and normal plasma is approximately 50 µmol/L, so 'normal' is about 25% of what can be achieved when oral consumption is in the proposed megadose range.\\nPauling popularized the concept of high dose vitamin C as prevention and treatment of the common cold in 1970. A few years\"\n },\n {\n \"doc_id\": 884264079800409700,\n \"text\": \" body. Approximately 60%\\nof ingested F by healthy adults are excreted in the urine, but only 45% for\\nchildren, with the rest re-circulating into the plasma or deposited into the\\nbone [ 18 ]. As a consequence, plasma and urinary excretion reflect a\\nphysiologic homeostasis determined by previous F intake, rate of F uptake and\\nremoval from bone, and the efficiency with which the kidneys excrete F.\"\n },\n {\n \"doc_id\": 3158263817555756500,\n \"text\": \" renal disease\\\". A review states that \\\"data from epidemiological studies do not support an association between excess ascorbic acid intake and kidney stone formation in apparently healthy individuals\\\", although one large, multi-year trial did report a nearly two-fold increase in kidney stones in men who regularly consumed a vitamin C supplement.\\nThere is extensive research on the purported benefits of intravenous vitamin C for treatment of sepsis, severe COVID-19 and cancer. Reviews list trials with doses as high as 24 grams per day. Concerns about possible adverse effects are that intravenous high-dose vitamin\"\n },\n {\n \"doc_id\": 4383411552392145400,\n \"text\": \" 10.1016/j.tibs.2006.11.006 ](https://doi.org/10.1016%2Fj.tibs.2006.11.006). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 17161604 ](https://pubmed.ncbi.nlm.nih.gov/17161604).\\n13. ** ^ ** Murray, Robert F. (2003). _Harper's illustrated biochemistry_. New York\"\n },\n {\n \"doc_id\": 4967566730723076000,\n \"text\": \"See also[edit]\\nVitamin C (ascorbic acid)\\nOxidoreductase\\nScurvy\"\n },\n {\n \"doc_id\": 274634719536155260,\n \"text\": \" goats, sheep and cattle reported ranges of 100–110, 265–270 and 160–350 μmol/L, respectively.\\nThe biosynthesis of ascorbic acid in vertebrates starts with the formation of UDP-glucuronic acid. UDP-glucuronic acid is formed when UDP-glucose undergoes two oxidations catalyzed by the enzyme UDP-glucose 6-dehydrogenase. UDP-glucose 6-dehydrogenase uses the co-factor NAD as the electron acceptor. The transferase UDP-gluc\"\n },\n {\n \"doc_id\": 7388942664155367000,\n \"text\": \"153-4 ](https://doi.org/10.1016%2FS0021-9258%2818%2967153-4). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 3528161 ](https://pubmed.ncbi.nlm.nih.gov/3528161).\\n16. ** ^ ** Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert (2007). _Biochemistry_ (6th\"\n },\n {\n \"doc_id\": 9045961011940932000,\n \"text\": \"Consequences of loss[edit]\\nIt is likely that some level of adaptation occurred after the loss of the GULO gene by primates. Erythrocyte Glut1 and associated dehydroascorbic acid uptake modulated by stomatin switch are unique traits of humans and the few other mammals that have lost the ability to synthesize ascorbic acid from glucose. As GLUT transporters and stomatin are ubiquitously distributed in different human cell types and tissues, similar interactions may occur in human cells other than erythrocytes.\\nLinus Pauling observed that after the loss of endogenous ascorbate production, apo(a) and Lp(a) were greatly favored by evolution, acting as ascorbate surrogate, since the frequency of occurrence of elevated Lp(a) plasma levels in species that had lost the ability to synthesize ascorbate is great. Also, only primates share regulation of CAMP gene expression by vitamin D, which occurred after the loss of GULO gene.\\nJohnson et al. have hypothesized that the mutation of the GULOP pseudogene so that it stopped producing GULO may have been of benefit to early primates by increasing uric acid levels and enhancing fructose effects on weight gain and fat accumulation. With a shortage of food supplies this gave mutants a survival advantage.\"\n },\n {\n \"doc_id\": 6284642480449593000,\n \"text\": \"Gulonolactone oxidase deficiency[edit]\\nThe non-functional gulonolactone oxidase pseudogene (GULOP) was mapped to human chromosome 8p21, which corresponds to an evolutionarily conserved segment on either porcine chromosome 4 (SSC4) or 14 (SSC14). GULO produces the precursor to ascorbic acid, which spontaneously converts to the vitamin itself.\\nThe loss of activity of the gene encoding L-gulonolactone oxidase (GULO) has occurred separately in the history of several species. GULO activity has been lost in some species of bats, but others retain it. The loss of this enzyme activity is responsible for the inability of guinea pigs to enzymatically synthesize vitamin C. Both these events happened independently of the loss in the haplorrhine suborder of primates, which includes humans.\\nThe remnant of this non-functional gene with many mutations is still present in the genomes of guinea pigs and humans. It is unknown if remains of the gene exist in the bats who lack GULO activity. The function of GULO appears to have been lost several times, and possibly re-acquired, in several lines of passerine birds, where ability to make vitamin C varies from species to species.\\nLoss of GULO activity in the primate order occurred about 63 million years ago, at about the time it split into the suborders Haplorhini (which lost the enzyme activity) and Strepsirrhini (which retained it). The haplorhine (\\\"simple-nosed\\\") primates, which cannot make vitamin C enzymatically, include the tarsiers and the simians (apes, monkeys and humans). The strepsirrhine (\\\"bent-nosed\\\" or \\\"wet-nosed\\\") primates, which can still make vitamin C enzymatically, include lorises, galagos, pottos, and, to some extent, lemurs.\\nL-Gulonolactone oxidase deficiency has been called \\\"hypoascorbemia\\\" and is described by OMIM (Online Mendelian Inheritance in Man) as \\\"a public inborn error of metabolism\\\", as it affects all humans. There exists a wide discrepancy between the amounts of ascorbic acid other primates consume and what are recommended as \\\"reference intakes\\\" for humans. In its patently pathological form, the effects of ascorbate deficiency are manifested as scurvy.\"\n },\n {\n \"doc_id\": 7318715996946053000,\n \"text\": \"Urine is the only biomarker capable of measuring F excretion. However, urine\\nmay not be the most pertinent biomarker for the estimation of TDFI especially\\nin children due to F accumulation during bone growth and mineralization.\\nChildren can retain more F in their skeleton (~50%) than adults (approximately\\n36%), with inverse retention in bone with age of the children and with the\\nexcess of F excreted in urine [ 67 ]. The absence of correlation between\\nDUFE and TDFI suggests that there is a variability but a non-neglig\"\n },\n {\n \"doc_id\": 2477233756556740600,\n \"text\": \"obalamin \\\"Methylcobalamin\\\") (B 12 )\\n* [ Ascorbic acid ](/wiki/Vitamin_C \\\"Vitamin C\\\") (C)\\n* [ Phylloquinone ](/wiki/Phytomenadione \\\"Phytomenadione\\\") (K 1 ), [ Menaquinone ](/wiki/Vitamin_K \\\"Vitamin K\\\") (K 2 )\\n* [ Coenzyme F420 ](/wiki/Coenzyme_F420 \\\"Coenzyme F420\"\n },\n {\n \"doc_id\": 2997268058314230300,\n \"text\": \" the drug – in this instance vitamin C – affects the organism, whereas pharmacokinetics is the study of how an organism affects the drug.\\nPharmacodynamics includes enzymes for which vitamin C is a cofactor, with function potentially compromised in a deficiency state, and any enzyme cofactor or other physiological function affected by administration of vitamin C, orally or injected, in excess of normal requirements. At normal physiological concentrations, vitamin C serves as an enzyme substrate or cofactor and an electron donor antioxidant. The enzymatic functions include the synthesis of collagen, carnitine, and neurotransmitters; the\"\n },\n {\n \"doc_id\": 8158331714726235000,\n \"text\": \"[ Article ](https://doi.org/10.1093%2Fajcn%2F77.1.128) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=12499331)\\n[ CAS ](/articles/cas-redirect/1:CAS:528:DC%2BD3sXhtFyntg%3D%3D) [ Google\\nSch\"\n },\n {\n \"doc_id\": 104978552204890530,\n \"text\": \" expression peaks in the morning to supporting biosynthesis for when mid-day sunlight intensity demands high ascorbic acid concentrations. Minor pathways may be specific to certain parts of plants; these can be either identical to the vertebrate pathway (including the GLO enzyme), or start with inositol and get to ascorbic acid via l-galactonic acid to l-galactonolactone.\\nIndustrial synthesis[edit]\\n\\nMain article: Chemistry of ascorbic acid\\nVitamin C can be produced from glucose by two main routes. The no longer\"\n },\n {\n \"doc_id\": 3683835109007077000,\n \"text\": \" cause nausea, abdominal cramps and diarrhea. These effects are attributed to the osmotic effect of unabsorbed vitamin C passing through the intestine. In theory, high vitamin C intake may cause excessive absorption of iron. A summary of reviews of supplementation in healthy subjects did not report this problem, but left as untested the possibility that individuals with hereditary hemochromatosis might be adversely affected.\\nThere is a longstanding belief among the mainstream medical community that vitamin C increases risk of kidney stones. \\\"Reports of kidney stone formation associated with excess ascorbic acid intake are limited to individuals with\"\n },\n {\n \"doc_id\": 6232058006680044000,\n \"text\": \" see Unitary pseudogenes) and the cofactor FAD+. This reaction produces 2-oxogulonolactone (2-keto-gulonolactone), which spontaneously undergoes enolization to form ascorbic acid. Reptiles and older orders of birds make ascorbic acid in their kidneys. Recent orders of birds and most mammals make ascorbic acid in their liver.\\nSome mammals have lost the ability to synthesize vitamin C, including simians and tarsiers, which together make up one of two major primate sub\"\n },\n {\n \"doc_id\": 742021988745971200,\n \"text\": \" C-free diet, whereas in the earlier British study, six to eight months were required, possibly due to the pre-loading of this group with a 70 mg/day supplement for six weeks before the scorbutic diet was fed. Men in both studies had blood levels of ascorbic acid too low to be accurately measured by the time they developed signs of scurvy. These studies both reported that all obvious symptoms of scurvy could be completely reversed by supplementation of only 10 mg a day. Treatment of scurvy can be with vitamin C-containing foods or\"\n },\n {\n \"doc_id\": 4527458718898450400,\n \"text\": \" the 1980s. Men in the prison study developed the first signs of scurvy about four weeks after starting the vitamin C-free diet, whereas in the earlier British study, six to eight months were required, possibly due to the pre-loading of this group with a 70 mg/day supplement for six weeks before the scorbutic diet was fed. Men in both studies had blood levels of ascorbic acid too low to be accurately measured by the time they developed signs of scurvy. These studies both reported that all obvious symptoms of scurvy could be completely reversed by\"\n },\n {\n \"doc_id\": 5975079325498478000,\n \"text\": \"[ Mayo Clinic Laboratories ](https://www.mayocliniclabs.com)\"\n },\n {\n \"doc_id\": 3180349578149413400,\n \"text\": \" C in either the kidney or the liver. In all of the cases where genomic analysis was done on an ascorbic acid auxotroph, the origin of the change was found to be a result of loss-of-function mutations in the gene that encodes L-gulono-γ-lactone oxidase, the enzyme that catalyzes the last step of the ascorbic acid pathway outlined above. One explanation for the repeated loss of the ability to synthesize vitamin C is that it was the result of genetic drift; assuming that the diet was rich in vitamin \"\n },\n {\n \"doc_id\": 8228831586340699000,\n \"text\": \"* [ Pentose phosphate pathway ](/wiki/Pentose_phosphate_pathway \\\"Pentose phosphate pathway\\\")\\n* [ Fructolysis ](/wiki/Fructolysis \\\"Fructolysis\\\")\\n* [ Polyol pathway ](/wiki/Polyol_pathway \\\"Polyol pathway\\\")\\n* [ Galactolysis ](/wiki/Galactolysis \\\"Galactolysis\\\")\\n* [ Leloir pathway ](/wiki/Leloir_pathway \\\"Leloir pathway\\\")\"\n },\n {\n \"doc_id\": 4310867272975448600,\n \"text\": \" ages 14–18 years)\\n\\n75\\n\\n\\nRDA (adult female)\\n\\n75\\n\\n\\nRDA (adult male)\\n\\n90\\n\\n\\nRDA (pregnancy)\\n\\n85\\n\\n\\nRDA (lactation)\\n\\n120\\n\\n\\nUL (adult female)\\n\\n2,000\\n\\n\\nUL (adult male)\\n\\n2,000\\nIn 2000, the chapter on Vitamin C in the North American Dietary Reference Intake was updated to give the Recommended Dietary Allowance (RDA) as 90 milligrams per day for adult men, 75 \"\n },\n {\n \"doc_id\": 1226072014279458600,\n \"text\": \" marker points (63 to 58 million years ago).\\nIt has also been noted that the loss of the ability to synthesize ascorbate strikingly parallels the inability to break down uric acid, also a characteristic of primates. Uric acid and ascorbate are both strong reducing agents. This has led to the suggestion that, in higher primates, uric acid has taken over some of the functions of ascorbate.\\nThere are many different biosynthesis pathways to ascorbic acid in plants. Most proceed through products of glycolysis and other metabolic pathways. For\"\n },\n {\n \"doc_id\": 2050089849119220200,\n \"text\": \" extensive research on the purported benefits of intravenous vitamin C for treatment of sepsis, severe COVID-19 and cancer. Reviews list trials with doses as high as 24 grams per day. Concerns about possible adverse effects are that intravenous high-dose vitamin C leads to a supraphysiological level of vitamin C followed by oxidative degradation to dehydroascorbic acid and hence to oxalate, increasing the risk of oxalate kidney stones and oxalate nephropathy. The risk may be higher in people with renal impairment, as kidneys efficiently excrete excess vitamin C\"\n },\n {\n \"doc_id\": 8735146826395335000,\n \"text\": \"ate residue from their c-terminal glycine residues. This increases peptide hormone stability and activity.\\nAs an antioxidant, ascorbate scavenges reactive oxygen and nitrogen compounds, thus neutralizing the potential tissue damage of these free radical compounds. Dehydroascorbate, the oxidized form, is then recycled back to ascorbate by endogenous antioxidants such as glutathione. In the eye, ascorbate is thought to protect against photolytically generated free-radical damage; higher plasma ascorbate is associated with lower risk of cateracts. As\"\n },\n {\n \"doc_id\": 3946738525946685000,\n \"text\": \" individuals with hereditary hemochromatosis might be adversely affected.\\nThere is a longstanding belief among the mainstream medical community that vitamin C increases risk of kidney stones. \\\"Reports of kidney stone formation associated with excess ascorbic acid intake are limited to individuals with renal disease\\\". A review states that \\\"data from epidemiological studies do not support an association between excess ascorbic acid intake and kidney stone formation in apparently healthy individuals\\\", although one large, multi-year trial did report a nearly two-fold increase in kidney stones in men who regularly consumed a vitamin C supplement.\\nThere is\"\n },\n {\n \"doc_id\": 8808915870447512000,\n \"text\": \"_[ Pentose\\nphosphate\\npathway ](/wiki/Pentose_phosphate_pathway \\\"Pentose phosphate pathway\\\") _\"\n },\n {\n \"doc_id\": 1686582037625913300,\n \"text\": \"[ Article ](https://doi.org/10.1016%2FS0303-8467%2899%2900035-9) [ CAS\\n](/articles/cas-redirect/1:STN:280:DC%2BD3c%2FgsFaqtg%3D%3D) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=10536902)\"\n },\n {\n \"doc_id\": 4049941878204482600,\n \"text\": \"olic acid\\\"), [ DHFA / H 2 FA ](/wiki/Dihydrofolic_acid \\\"Dihydrofolic acid\\\"), [ MTHF ](/wiki/5,10-Methylenetetrahydrofolate \\\"5,10-Methylenetetrahydrofolate\\\") (B 9 )\\n* [ AdoCbl ](/wiki/Adenosylcobalamin \\\"Adenosylcobalamin\\\"), [ MeCbl ](/wiki/Methylc\"\n },\n {\n \"doc_id\": 8808874834402231000,\n \"text\": \"• Validity of data and methods: F intake, F excretion (urine collection), and\\nF analytical method, or not reported.\"\n },\n {\n \"doc_id\": 3719440369263880700,\n \"text\": \"Animal models[edit]\\nStudies of human diseases have benefited from the availability of small laboratory animal models. However, the tissues of animal models with a GULO gene generally have high levels of ascorbic acid and so are often only slightly influenced by exogenous vitamin C. This is a major handicap for studies involving the endogenous redox systems of primates and other animals that lack this gene.\\nGuinea pigs are a popular human model. They lost the ability to make GULO 20 million years ago.\\nIn 1999, Maeda et al. genetically engineered mice with inactivated GULO gene. The mutant mice, like humans, entirely depend on dietary vitamin C, and they show changes indicating that the integrity of their vasculature is compromised. GULO mice have been used as a human model in multiple subsequent studies.\\nThere have been successful attempts to activate lost enzymatic function in different animal species. Various GULO mutants were also identified.\"\n },\n {\n \"doc_id\": 4143018406264960000,\n \"text\": \"ifier\\\\)\\\") : [ 10.1016/j.abb.2006.05.001 ](https://doi.org/10.1016%2Fj.abb.2006.05.001). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 16806044 ](https://pubmed.ncbi.nlm.nih.gov/16806044).\\n33. ** ^ ** Kamenetsky, M.; Middelhaufe, S.; Bank,\"\n },\n {\n \"doc_id\": 8098842388776744000,\n \"text\": \" synthesis and catabolism of tyrosine; and the metabolism of microsomes. In nonenzymatic functions it acts as a reducing agent, donating electrons to oxidized molecules and preventing oxidation in order to keep iron and copper atoms in their reduced states. At non-physiological concentrations achieved by intravenous dosing, vitamin C may function as a pro-oxidant, with therapeutic toxicity against cancer cells.\\nVitamin C functions as a cofactor for the following enzymes:\\nAs an antioxidant, ascorbate scavenges reactive oxygen and nitrogen compounds, thus neutralizing the\"\n },\n {\n \"doc_id\": 170279123303973920,\n \"text\": \"alaf MR. Historical and recent biological markers of exposure to fluoride. Monogr Oral Sci 2011;22:52-65. [ [ PubMed abstract ](https://pubmed.ncbi.nlm.nih.gov/21701191/) ]\\n5. Rango T, Vengosh A, Jeuland M, Whitford GM, Tekle-Haimanot R. Biomarkers of chronic fluoride exposure in groundwater in a highly exposed population. Sci Total Environ 2017;596-597:1-11. [ [ PubMed abstract ](https://\"\n },\n {\n \"doc_id\": 3000185096634474000,\n \"text\": \"[ α-Keto-\\nglutarate ](/wiki/Alpha-Ketoglutaric_acid \\\"Alpha-Ketoglutaric acid\\\")\"\n },\n {\n \"doc_id\": 3253288000330283500,\n \"text\": \"2 mg/kg.\\nAscorbic acid is a common enzymatic cofactor in mammals used in the synthesis of collagen, as well as a powerful reducing agent capable of rapidly scavenging a number of reactive oxygen species (ROS). Given that ascorbate has these important functions, it is surprising that the ability to synthesize this molecule has not always been conserved. In fact, anthropoid primates, Cavia porcellus (guinea pigs), teleost fishes, most bats, and some passerine birds have all independently lost the ability to internally synthesize vitamin\"\n },\n {\n \"doc_id\": 498406216741366300,\n \"text\": \" should be consuming at least 2,300 mg/day to compensate for the inability to synthesize vitamin C. The recommendation also fell into the consumption range for gorillas – a non-synthesizing near-relative to humans. A second argument for high intake is that serum ascorbic acid concentrations increase as intake increases until it plateaus at about 190 to 200 micromoles per liter (µmol/L) once consumption exceeds 1,250 milligrams. As noted, government recommendations are a range of 40 to 110 mg/day and normal plasma is approximately\"\n },\n {\n \"doc_id\": 2820596826049959000,\n \"text\": \"---|---\\n[ Nucleotide\\nmetabolism ](/wiki/Nucleic_acid_metabolism \\\"Nucleic acid metabolism\\\") |\"\n },\n {\n \"doc_id\": 4118230098960824000,\n \"text\": \"-arginine 1, and there is also evidence cited that vitamin C directly enhances the biological activity of nitric oxide, a vasodilator.\\nType 2 diabetes[edit]\\nThere are contradictory reviews. From one, vitamin C supplementation cannot be recommended for management of type 2 diabetes. However, another reported that supplementation with high doses of vitamin C can decrease blood glucose, insulin and hemoglobin A1c.\\nIron deficiency[edit]\\nOne of the causes of iron-deficiency anemia is reduced absorption of iron. Iron absorption can be enhanced through ingestion of vitamin C alongside\"\n },\n {\n \"doc_id\": 7472875436405453000,\n \"text\": \", it is surprising that the ability to synthesize this molecule has not always been conserved. In fact, anthropoid primates, Cavia porcellus (guinea pigs), teleost fishes, most bats, and some passerine birds have all independently lost the ability to internally synthesize vitamin C in either the kidney or the liver. In all of the cases where genomic analysis was done on an ascorbic acid auxotroph, the origin of the change was found to be a result of loss-of-function mutations in the gene that encodes L-gulono-γ\"\n },\n {\n \"doc_id\": 5628233220328595000,\n \"text\": \"_**[ Gluconeo-\\ngenesis ](/wiki/Gluconeogenesis \\\"Gluconeogenesis\\\") ** _\"\n },\n {\n \"doc_id\": 6407680627101928000,\n \"text\": \" that half the women and more than half the men are not consuming the RDA for vitamin C. The same survey stated that about 30% of adults reported they consumed a vitamin C dietary supplement or a multi-vitamin/mineral supplement that included vitamin C, and that for these people total consumption was between 300 and 400 mg/d.\\nTolerable upper intake level[edit]\\nIn 2000, the Institute of Medicine of the US National Academy of Sciences set a Tolerable upper intake level (UL) for adults of 2,000 mg/day. The amount was\"\n },\n {\n \"doc_id\": 4161775671760185300,\n \"text\": \"[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)\"\n },\n {\n \"doc_id\": 7325009641872769000,\n \"text\": \" to have been lost and then later re-acquired in at least two cases. The ability to synthesize vitamin C has also been lost in about 96% of extant fish (the teleosts).\\nOn a milligram consumed per kilogram of body weight basis, simian non-synthesizer species consume the vitamin in amounts 10 to 20 times higher than what is recommended by governments for humans. This discrepancy constituted some of the basis of the controversy on human recommended dietary allowances being set too low. However, simian consumption does not indicate simian requirements. Merck's\"\n },\n {\n \"doc_id\": 3669686666216705500,\n \"text\": \" is one of the most labile vitamins in foods. Its main loss during processing and storage is from oxidation, which is accelerated by light, oxygen, heat, increased pH, high moisture content (water activity), and the presence of copper or ferrous salts. To reduce oxidation, the vitamin C used in commodity fortification is coated with ethyl cellulose (2.5 percent). Oxidative losses also occur during food processing and preparation, and additional vitamin C may be lost if it dissolves into cooking liquid and is then discarded.\\\"\\nAscorbic acid and some of its salts and\"\n },\n {\n \"doc_id\": 3309188676753162000,\n \"text\": \"[ Article ](https://doi.org/10.1016%2Fj.clinph.2006.01.017) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=16581292)\\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2865224) [ CAS\\n](/articles/cas-red\"\n },\n {\n \"doc_id\": 538595360612986940,\n \"text\": \"cine \\\"Glycine\\\")\\n* [ GSNO ](/wiki/S-Nitrosoglutathione \\\"S-Nitrosoglutathione\\\")\\n* [ GSSG ](/wiki/Oxidized_glutathione \\\"Oxidized glutathione\\\")\\n* [ Kynurenic acid ](/wiki/Kynurenic_acid \\\"Kynurenic acid\\\")\\n* [ NAA ](/wiki/N-Acetylaspartic_acid \\\"N-Acetylaspartic acid\\\")\"\n },\n {\n \"doc_id\": 3142707818421335600,\n \"text\": \" to vitamin C. Yeasts do not make l-ascorbic acid but rather its stereoisomer, erythorbic acid. In plants, synthesis is accomplished through the conversion of mannose or galactose to ascorbic acid. In animals, the starting material is glucose. In some species that synthesize ascorbate in the liver (including mammals and perching birds), the glucose is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process. In humans and in animals that cannot synthesize vitamin C, the enzyme\"\n },\n {\n \"doc_id\": 478117815120135600,\n \"text\": \"[ Article ](https://doi.org/10.1093%2Fajcn%2F85.2.426) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=17284739)\\n[ CAS ](/articles/cas-redirect/1:CAS:528:DC%2BD2sXhvFWhu7g%3D) [ Google\\n\"\n },\n {\n \"doc_id\": 8948586324244455000,\n \"text\": \"Further investigation combining measures of F in plasma and urine could be\\ninformative on the bioavailability of F and its interactions with different\\norgans. Once absorbed, F reaches peak serum concentrations after 20–60 min,\\nand then returns to the baseline after approximately 15 h suggesting that part\\nof the oral F passes through systemic route [ 56 , 57 , 68 , 69 ].\\nThis is probably the reason why a relation has been reported between\\nsupplement use or the amount of toothpaste used for brushing and child's\\nfluorosis scores [\"\n },\n {\n \"doc_id\": 7298847495697952000,\n \"text\": \"* [ Metal metabolism ](/wiki/Bioinorganic_chemistry \\\"Bioinorganic chemistry\\\")\\n* [ Iron metabolism ](/wiki/Human_iron_metabolism \\\"Human iron metabolism\\\")\\n* [ Ethanol metabolism ](/wiki/Ethanol_metabolism \\\"Ethanol metabolism\\\")\\n* [ Phospagen system (ATP-PCr) ](/wiki/Phosphagen \\\"Phosphagen\\\")\"\n },\n {\n \"doc_id\": 3600014978857611000,\n \"text\": \" States National Academy of Sciences\\n100 mg/day: Japan National Institute of Health and Nutrition\\n110 mg/day (males) and 95 mg/day (females): European Food Safety Authority\\nUS vitamin C recommendations (mg per day)\\n\\n\\nRDA (children ages 1–3 years)\\n\\n15\\n\\n\\nRDA (children ages 4–8 years)\\n\\n25\\n\\n\\nRDA (children ages 9–13 years)\\n\\n45\\n\\n\\nRDA (girls ages 14–18 years)\\n\\n65\\n\\n\\nRDA (boys\"\n },\n {\n \"doc_id\": 7484065216331891000,\n \"text\": \" 1. Biochemistry, L. Stryer (W.H. Freeman and Co, San Francisco, 1975). \\n 2. The Cambridge Guide to the Material World, Rodney Cotterill (Cambridge University Press, Cambridge, 1985) \\n\\n## Contributors\"\n },\n {\n \"doc_id\": 2506392599316147700,\n \"text\": \"[ Acidity ](/wiki/Acid_dissociation_constant \\\"Acid dissociation constant\\\") (p\\n_K_ a ) | 0.9, 1.4, 3.8, 6.5\\n[ UV-vis ](/wiki/Ultraviolet%E2%80%93visible_spectroscopy \\\"Ultraviolet–visible\\nspectroscopy\\\") (λ max ) | 259 nm [1]\\n[ Absorbance ](/wiki/Absorbance \\\"Absorbance\\\") | \"\n },\n {\n \"doc_id\": 2743938332044641000,\n \"text\": \"* [ Purine metabolism ](/wiki/Purine_metabolism \\\"Purine metabolism\\\")\\n* [ Nucleotide salvage ](/wiki/Nucleotide_salvage \\\"Nucleotide salvage\\\")\\n* [ Pyrimidine metabolism ](/wiki/Pyrimidine_metabolism \\\"Pyrimidine metabolism\\\")\\n* [ Purine nucleotide cycle ](/wiki/Purine_nucleotide_cycle \\\"Purine nucleotide cycle\\\")\"\n },\n {\n \"doc_id\": 4282846855766989300,\n \"text\": \" as an enzyme substrate or cofactor and an electron donor antioxidant. The enzymatic functions include the synthesis of collagen, carnitine, and neurotransmitters; the synthesis and catabolism of tyrosine; and the metabolism of microsomes. In nonenzymatic functions it acts as a reducing agent, donating electrons to oxidized molecules and preventing oxidation in order to keep iron and copper atoms in their reduced states. At non-physiological concentrations achieved by intravenous dosing, vitamin C may function as a pro-oxidant, with therapeutic toxicity against cancer cells.\\n\"\n },\n {\n \"doc_id\": 2085103195239381000,\n \"text\": \" (along with minerals and other vitamins). Supplemental rations of these highly fortified, blended foods are provided to refugees and displaced persons in camps and to beneficiaries of development feeding programs that are targeted largely toward mothers and children. The report adds: \\\"The stability of vitamin C (L-ascorbic acid) is of concern because this is one of the most labile vitamins in foods. Its main loss during processing and storage is from oxidation, which is accelerated by light, oxygen, heat, increased pH, high moisture content (water activity), and the presence of copper or ferrous salts. To reduce\"\n },\n {\n \"doc_id\": 8894272547326739000,\n \"text\": \" potential tissue damage of these free radical compounds. Dehydroascorbate, the oxidized form, is then recycled back to ascorbate by endogenous antioxidants such as glutathione. In the eye, ascorbate is thought to protect against photolytically generated free-radical damage; higher plasma ascorbate is associated with lower risk of cateracts. Ascorbate may also provide antioxidant protection indirectly by regenerating other biological antioxidants such as α-tocopherol back to an active state. In addition, ascorbate also functions as a non-enz\"\n },\n {\n \"doc_id\": 7736402839752618000,\n \"text\": \" mechanism is that it functions as an antioxidant, neutralizing free radicals from sunlight exposure, air pollutants or normal metabolic processes. The efficacy of topical treatment, as opposed to oral intake is poorly understood. The clinical trial literature is characterized as insufficient to support health claims, one reason being put forward was that \\\"All the studies used vitamin C in combination with other ingredients or therapeutic mechanisms, thereby complicating any specific conclusions regarding the efficacy of vitamin C.\\\" More research is needed.\\nCognitive impairment and Alzheimer's disease[edit]\\nLower plasma vitamin C concentrations were reported in people with cognitive impairment and Alzheimer's\"\n },\n {\n \"doc_id\": 4066839141633327000,\n \"text\": \"LOAEL), meaning that other adverse effects were observed at even higher intakes. ULs are progressively lower for younger and younger children. In 2006, the European Food Safety Authority (EFSA) also pointed out the disturbances at that dose level, but reached the conclusion that there was not sufficient evidence to set a UL for vitamin C, as did the Japan National Institute of Health and Nutrition in 2010.\\nFor US food and dietary supplement labeling purposes, the amount in a serving is expressed as a percent of Daily Value (%DV). For vitamin C labeling purposes, 100% of the Daily Value was\"\n },\n {\n \"doc_id\": 450371710151089660,\n \"text\": \" adults reported they consumed a vitamin C dietary supplement or a multi-vitamin/mineral supplement that included vitamin C, and that for these people total consumption was between 300 and 400 mg/d.\\nIn 2000, the Institute of Medicine of the US National Academy of Sciences set a Tolerable upper intake level (UL) for adults of 2,000 mg/day. The amount was chosen because human trials had reported diarrhea and other gastrointestinal disturbances at intakes of greater than 3,000 mg/day. This was the Lowest-Observed-Adverse-Effect Level (\"\n },\n {\n \"doc_id\": 2172318637421551600,\n \"text\": \"Synthesis[edit]\\nMost animals and plants are able to synthesize vitamin C through a sequence of enzyme-driven steps, which convert monosaccharides to vitamin C. Yeasts do not make l-ascorbic acid but rather its stereoisomer, erythorbic acid. In plants, synthesis is accomplished through the conversion of mannose or galactose to ascorbic acid. In animals, the starting material is glucose. In some species that synthesize ascorbate in the liver (including mammals and perching birds), the glucose is\"\n },\n {\n \"doc_id\": 959887416059251200,\n \"text\": \"[ CAS ](/articles/cas-redirect/1:STN:280:DC%2BD3c7jt1OgtA%3D%3D) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=10669252)\\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7976324)\"\n },\n {\n \"doc_id\": 766047156750821100,\n \"text\": \"[ Article ](https://doi.org/10.1152%2Fajpregu.00197.2002) [ PubMed\\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=12529281)\\n[ CAS ](/articles/cas-redirect/1:CAS:528:DC%2BD3sXhtlOqsLs%3D) [ Google\\nScholar\\n\"\n },\n {\n \"doc_id\": 6828994514780543000,\n \"text\": \"mg / 100g)\\n\\n\\nGreen bell pepper/capsicum\\n80\\n\\n\\nBrussels sprouts\\n80\\n\\n\\nLoganberry, redcurrant\\n80\\n\\n\\nCloudberry, elderberry\\n60\\n\\n\\nStrawberry\\n60\\n\\n\\nPapaya\\n60\\n\\n\\nOrange, lemon\\n53\\n\\n\\nCauliflower\\n48\\n\\n\\nPineapple\\n48\\n\\n\\nCantaloupe\\n40\\n\\n\\nPassion fruit, raspberry\\n30\\n\\n\\nGrapefruit, lime\\n30\\n\\n\\nCabbage, spinach\\n30\\nRaw plant source\\n\\n\"\n },\n {\n \"doc_id\": 7671617941354681000,\n \"text\": \" dietary supplements or injection.\\nPeople in sepsis may have micronutrient deficiencies, including low levels of vitamin C. An intake of 3.0 g/day, which requires intravenous administration, appears to be needed to maintain normal plasma concentrations in people with sepsis or severe burn injury. Sepsis mortality is reduced with administration of intravenous vitamin C.\\nResearch on vitamin C in the common cold has been divided into effects on prevention, duration, and severity. Oral intakes of more than 200 mg/day taken on a regular basis was not effective in prevention of the\"\n }\n]","isConversational":false}}},{"rowIdx":97,"cells":{"query":{"kind":"string","value":"I'm studying the concept of adaptedness in organisms and how adaptive traits contribute to survival and reproduction. Could someone provide a detailed explanation of how adaptation can lead to both the gain of new features and the loss of ancestral ones? For instance, how does bacterial adaptation to antibiotic selection illustrate these two types of changes? What specific genetic mechanisms are involved in modifying the target of the drug or increasing the activity of transporters to achieve antibiotic resistance? Thanks in advance for any insights or examples!"},"positive_passages":{"kind":"list like","value":[{"doc_id":1521164811212119300,"text":" cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment, Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil"},{"doc_id":5214900912710051000,"text":"porters that pump the drug out of the cell. Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment, Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms'"},{"doc_id":5903057290488301000,"text":" had different functions. Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.\nAn area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. It is also"}],"string":"[\n {\n \"doc_id\": 1521164811212119300,\n \"text\": \" cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment, Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil\"\n },\n {\n \"doc_id\": 5214900912710051000,\n \"text\": \"porters that pump the drug out of the cell. Other striking examples are the bacteria Escherichia coli evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment, Flavobacterium evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms'\"\n },\n {\n \"doc_id\": 5903057290488301000,\n \"text\": \" had different functions. Another example is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.\\nAn area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. It is also\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":7011957731131189000,"text":" bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).\nAdaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and"},{"doc_id":5332197148514614000,"text":"1. [ Stabilizing Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-1)\n2. [ Directional Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-2)\n3. [ Diversifying Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-3"},{"doc_id":6676066334518801000,"text":" ](https://slcc.pressbooks.pub/collegebiology1/chapter/variation-adaptation/)"},{"doc_id":7557534906429007000,"text":"Natural outcomes\nA visual demonstration of rapid antibiotic resistance evolution by E. coli growing across a plate with increasing concentrations of trimethoprim\nEvolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups"},{"doc_id":8846767774428441000,"text":"3. [ 3.3 Adaptive Evolution ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/)"},{"doc_id":6381177455009793000,"text":" proteins, capsules, flagella, and pili, among others. However, some studies suggest that when phage resistance is caused by mutations in phage receptors, this might result in fitness costs to the resistance bacterium, which will ultimately become less virulent. Moreover, it has been shown that the evolution of bacterial resistance to phage attack changes the efflux pump mechanism, causing increased sensitivity to drugs from several antibiotic classes. Therefore, it is conceivable to think that phage therapy that uses phages that exert selection for multidrug-resistant bacteria to become antibiotic-sensitive could potentially reduce"},{"doc_id":1010349355485385100,"text":" be bacterial adaptations. [20]\n[51]"},{"doc_id":1544282805858573600,"text":"les within a population of interbreeding organisms, for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, and mutation bias.\nEvolution by natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles:\nMore offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass"},{"doc_id":1914211970812137500,"text":" having a less beneficial or deleterious allele results in this allele likely becoming rarer—they are \"selected against.\"\nImportantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form. However, a re-activation of dormant genes, as long as they have not been eliminated from the genome and were only suppressed perhaps for hundreds of"},{"doc_id":8349157190851547000,"text":"ichia coli strain MG1655: Growth\ndefects and apparent cross-regulation of gene expression. J. Bacteriol. 185,\n5611–5626 (2003).\n14. Oxman, E., Alon, U. & Dekel, E. Defined order of evolutionary adaptations:\nexperimental evidence. Evolution 62, 1547–1554 (2008).\n15. Mortimer, R. K. Evolution and variation of the yeast (Saccharomyces) genome.\nGenome Res. 10, 403–409 (2000).\n16"},{"doc_id":2504545380939800600,"text":" of gene function and thus pseudo-genes. More commonly, single amino acid changes have limited consequences although some can change protein function substantially, especially in enzymes. For instance, many enzymes can change their substrate specificity by one or a few mutations. Changes in substrate specificity are facilitated by substrate promiscuity, i.e. the ability of many enzymes to bind and process multiple substrates. When mutations occur, the specificity of an enzyme can increase (or decrease) and thus its enzymatic activity. Thus, bacteria (or other organisms) can adapt to different food sources, including unnatural substrates"},{"doc_id":4951364018294375000,"text":"©2009 Macmillan Publishers Limited. All rights reserved\n5. Kussell, E. & Leibler, S. Phenotypic diversity, population growth, and information in\nfluctuating environments. Science 309, 2075–2078 (2005).\n6. Acar, M., Mettetal, J. T. & van Oudenaarden, A. Stochastic switching as a survival\nstrategy in fluctuating environments. Nature Genet. 40, 471–475 (2008).\n7. Balaban, N. Q. et al"},{"doc_id":1984461514986374700,"text":" beneficial or deleterious allele results in this allele likely becoming rarer—they are \"selected against.\"\nImportantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form. However, a re-activation of dormant genes, as long as they have not been eliminated from the genome and were only suppressed perhaps for hundreds of generations, can"},{"doc_id":1220129963295904800,"text":" of mutation and selection (for example modified enzymes and new antibodies) in a process called directed evolution.\nUnderstanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human genetic disorders. For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves. This helped identify genes required for vision and pigmentation.\n"},{"doc_id":2111678933010816500,"text":"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H"},{"doc_id":5513803758095037000,"text":" possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles. It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.\nInteractions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a"},{"doc_id":5819475469232278000,"text":" evolvability).\nA baleen whale skeleton. Letters a and b label flipper bones, which were adapted from front leg bones, while c indicates vestigial leg bones, both suggesting an adaptation from land to sea.\nAdaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all"},{"doc_id":991332541439228800,"text":" where genes involved in a particular growth-limiting process have an increased mutation rate.\nSome bacteria transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment in a process called transformation. Many bacteria can naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA. The development of competence in nature is usually associated with stressful environmental conditions and seems to be an adaptation for facilitating repair of DNA damage in recipient cells. Second, bacteriophages can integrate into the bacterial chromosome"},{"doc_id":4071155514373589000,"text":"1.00\nNormalized promoter activity\nWild-type strain\nAdded sugar:\nLactose\nMaltose\nNone\na\n0\n0.25\n0.50\n0.75\n1.00\nGFP-fused promoter\nb Evolved strain\nFigure 2 | Conditioned response in E. coli sugar metabolism. Promoter\nactivity in wild type (a) and the evolved strain (b). Colours mark the added\nsugar. Error bars denote standard deviation of four repeats. Promoter\nactivities of four out of five maltose operons in wild type are significantly\nhigher under lactose relative to the untreated culture (t-test, P , 0.01\naccording to a Bonferroni adjustment). In contrast, none of the promoters\nshow increased activity in the evolved strain. The experiment was done in\nM9-Glu medium and was repeated using glycerol as a background carbon\nsource (Supplementary Fig. 3) and further verified using quantitative\npolymerase chain reaction with reverse transcription (RT–PCR)\n(Supplementary Figs 2 and 4).\nEvolved\n0.98\n0.99\n1.00\n1.01\n1.02\n1.03\n1.04\nFitness advantage\nObserved effect:\nDirectionality\nSpecificity\nExtinction\nLac\nMal\nMal\nLac\nGal\nMal\nSuc\nMal\nLac\nMal\nWild type\nFigure 3 | Fitness in an alternating sugar environment. Two isogenic\npopulations were grown on M9-Gly either supplemented with a sugar as S1\n(treated) or not (untreated). An equal amount of cells was diluted into media\ncontaining the second sugar (S2) as a sole carbon source. The ratio treated/\nuntreated (after 2 h) represents the fitness advantage originating from preexposure to S1. The error bars denote standard deviation of three repeats. All\nratios are normalized to the ratio measured in a control experiment in which\nS2 was glycerol (Methods). A significant fitness advantage was observed only\nin the transfer of wild-type cells from lactose to maltose (P 5 0.02, t-test).\nNATURE| Vol 460| 9 July 2009 ARTICLES\n221\n©2009 Macmillan Publishers Limited. All rights reserved\nobserved in the wild type due to exposure to lactose before maltose is\nlost in the evolved strain that features extinction (Fig. 3).\nFinally, as required by the specificity criterion, we ensured that\npre-exposure to other sugars does not improve growth of cells upon\ntransfer to maltose. As can be seen in Fig. 3, two alternative related\ncarbon sources, galactose and sucrose, cannot substitute lactose as a\npreceding signal before maltose. This indicates that the observed\nanticipation evolved in response to the specific stimuli pair that is\npresented to the organism in its ecological niche. In summary, we\nconclude that anticipation of subsequent carbon source changes is an\nadaptive trait that has been selected by the natural ecology of E. coli.\nThe shift from fermentation to respiration in yeast\nWe next proceeded to a more complicated case that may also manifest environmental change anticipation. We examined how environmental anticipation is encoded in the regulatory network of\nS. cerevisiae, and whether this response strategy has been selected\nfor because of a net fitness advantage it entails. We examined conditions that emulate some of the stresses that occur during the process\nof alcoholic brew production, such as wine, by yeast15: Fig. 4a\ndescribes this process, which starts at a highly osmotic and low pH\nenvironment, followed by potentially lethal temperature raise due to\nvigorous fermentation and accumulation of ethanol, culminating in\nthe diauxic shift—the exhaustion of fermentable sugars (for example,\nglucose) and the switch to oxidative respiration16–19. Respiration is\nchallenging to the cell because it eventually generates oxygen radicals20. We checked whether yeast cells can cope better with these\nstresses provided that they appear in their natural temporal order,\nthat is, whether yeasts use early stresses as predictive signals for the\nlikely arrival of later stresses. Such behaviour would indicate that\nyeast cells are preparing for later challenges even before they arise.\nTo address these questions we systematically studied the survival\nof S. cerevisiae under stresses that comprise the switch from its\npreferred catabolic state, fermentation, to the alternative one, respiration (Fig. 4). As a potential manifestation of anticipation, we\nfocused on the cross-protection phenotype between different stress\npairs21,22. A cross-protection phenotype between two stresses exists if\npre-exposure to one stress improves the survival of cells under a\nsubsequent stress. Under our hypothesis we expect an asymmetrical\ncross-protection phenotype between stress pairs. Particularly, we\npredict that directionality of cross-protection will be in accordance\nwith the order of events during the transition from fermentation to\nrespiration. For example, because heat shock and ethanol accumulation precede oxidative stress in the ecology, we expect improved\nsurvival under oxidative stress if it is preceded by either heat shock\nor ethanol stress, yet we do not expect cross-protection if the stresses\nare applied in the reverse order. Figure 4a shows the measured protection phenotype systematically tested for stresses that occur before\nand after the diauxic shift. In accordance with our hypothesis, we\nobserve that the cross-protection phenotype is indeed extremely\nasymmetric and, most importantly, that the directionality of protection fits the natural temporal context. Specifically, for each stress pair\nprotection is always stronger in the direction found in the natural\nenvironment, thus fulfilling the asymmetric fitness advantage criterion. In a more faithful emulation of the natural habitat we have\nalso applied the three stresses, heat shock, ethanol and oxidative\nstress, sequentially and observed an equally high cross-protection\nactivity (Supplementary Information). Note that activation of the\n"},{"doc_id":4882298945857637000,"text":" experimental evolution and the observation of evolution and adaptation in real time.\nAdaptation is the process that makes organisms better suited to their habitat. Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection. The following definitions are due to Theodosius Dobzhansky:\nAdaptation may"},{"doc_id":2822051962951010000,"text":" the incidence of antibiotic-resistant infections.\nBesides the prevention of phage adsorption by loss or modification of bacterial receptors, phage insensitivity can be caused by: (i) prevention of phage DNA entry by superinfection exclusion systems; (ii) degradation of phage DNA by restriction-modification systems or by CRISPR-Cas systems; and (iii) use of abortive infection systems that block phage replication, transcription, or translation, usually in conjunction with suicide of the host cell. Altogether, these mechanisms promote a quick adaptation of bacteria to phage attack"},{"doc_id":2214854763095047700,"text":" concerns usually associated with phage therapy is the emergence of phage-insensitive mutants (BIMs) that could hinder the success of this therapy. In fact, several in vitro studies have reported a fast emergence of BIMs within a short period of time after phage treatment. The emergence of BIMs has also been observed in vivo using different animal models, although this usually occurs later than in vitro (reviewed in ). This fast adaptation of bacteria to phage attack is usually caused by mutations on genes encoding phage receptors, which include lipopolysaccharides (LPS), outer membrane"},{"doc_id":4904674638133632000,"text":" predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution. An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly"},{"doc_id":4719842190235495000,"text":"itable variations that may underlay some of the mechanics in developmental plasticity and canalisation. Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.\nFrom a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alle"},{"doc_id":1718319608984949000,"text":" subsequent generations that carry an organism's genes. For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.\nIf an allele increases fitness more than the other alleles of that gene, then with each generation this allele has a higher probability of becoming common within the population. These traits are said to be \"selected for.\" Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by having a less"},{"doc_id":1871963658792519400,"text":".\nAdaptation is the process that makes organisms better suited to their habitat. Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection. The following definitions are due to Theodosius Dobzhansky:\nAdaptation is the evolutionary process whereby an organism becomes better able to live in"},{"doc_id":5560836972122283000,"text":" mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.\nSeveral studies report that the mutations implicated in adaptation reflect common mutation biases though others dispute this interpretation.\nGenetic hitchhiking\n\nFurther information: Genetic hitchhiking, Hill–Robertson effect, and Selective sweep\nRecombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome"},{"doc_id":4276364739988023000,"text":" the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.\nA common"},{"doc_id":7653446544395892000,"text":" as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors.\nConcepts and models used in evolutionary biology, such as natural selection, have many applications.\nArtificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the domestication of plants and animals. More recently, such selection has become a vital part of genetic engineering, with selectable markers such as antibiotic resistance genes being used to manipulate DNA. Proteins with valuable properties have evolved by repeated rounds"},{"doc_id":7471452766319398000,"text":"Evolutionary forces\nMutation followed by natural selection results in a population with darker colouration.\nFrom a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms, for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, and mutation bias.\nNatural selection\n\nMain article: Natural selection\n\nSee also: Dollo's law of irreversibility\nEvolution by natural selection is the"},{"doc_id":2758089624746548000,"text":" and new antibodies) in a process called directed evolution.\nUnderstanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human genetic disorders. For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves. This helped identify genes required for vision and pigmentation.\nEvolutionary theory has many applications in medicine"},{"doc_id":2372858552183286000,"text":" the same for every gene in the same population.\nIt is usually difficult to measure the relative importance of selection and neutral processes, including drift. The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.\nMutation bias\nMutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to the idea of developmental bias. Haldane and Fisher argued that, because mutation is a weak pressure"},{"doc_id":4918649367885492000,"text":" generations, can lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. \"Throwbacks\" such as these are known as atavisms.\nNatural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Secondly, disruptive selection is"},{"doc_id":8746751609829276000,"text":". Bacterial persistence as a phenotypic switch. Science 305,\n1622–1625 (2004).\n8. Pavlov, I. P. Conditioned Reflexes (Oxford Univ. Press, 1927).\n9. Tagkopoulos, I., Liu, Y. C. & Tavazoie, S. Predictive behavior within microbial\ngenetic networks. Science 320, 1313–1317 (2008).\n10. Savageau, M. A. Escherichia coli habitats, cell-types, and molecular mechanisms of\ngene control. Am"},{"doc_id":5758082833741322000,"text":" shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions. For example, polyketide synthases are large enzymes that make antibiotics; they contain up to 100 independent domains that each catalyse one step in the overall process, like a step in an assembly line.\nOne example of mutation is wild boar piglets. They are camouflage coloured and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in the melanocortin 1 receptor (MC1R) disrupt the pattern."},{"doc_id":218121489928662980,"text":" lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. \"Throwbacks\" such as these are known as atavisms.\nThese charts depict the different types of genetic selection. On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the number of organisms. Group A is the original population and Group B is the population after selection. · Graph 1 shows directional selection, in which a single extreme"},{"doc_id":3206778632482066400,"text":" exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Within cells, molecular machines such as the bacterial flagella and protein sorting machinery evolved by the recruitment of several pre-existing proteins that previously"},{"doc_id":8134024385157509000,"text":". For example, polyketide synthases are large enzymes that make antibiotics; they contain up to 100 independent domains that each catalyse one step in the overall process, like a step in an assembly line.\nOne example of mutation is wild boar piglets. They are camouflage coloured and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in the melanocortin 1 receptor (MC1R) disrupt the pattern. The majority of pig breeds carry MC1R mutations disrupting wild-type colour and different mutations causing dominant black colouring.\nIn asexual organisms,"},{"doc_id":1796167171500132000,"text":" second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution. An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.\nNot all co-evolved interactions between species involve conflict. Many cases of"},{"doc_id":7033329867091455000,"text":"Adaptivity and senescence[edit]\nAn antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in their old age. An example of this is testosterone levels in male humans. Higher levels of this hormone lead to increased fitness in early life, while causing decreased fitness in later life due to a higher risk for prostate cancer. This is an example of antagonistic pleiotropy being an explanation for senescence. Senescence is the act of ageing in individuals; it's the failure over time of the individual's life processes by natural causes. Williams's theory has been the motivation for many of the experimental studies on the reasons for aging in the last 25 years. However, there is more than one theory out there for aging. The competing model to explain senescence is Medawar's \"mutation accumulation\" hypothesis, saying that \"over evolutionary time, late-acting mutations will accumulate at a much faster rate than early-acting mutation. These late-acting mutations will thus lead to declining viability and/or fertility as an organism ages.\" Medawar's theory is based around the older concept of selection shadow that had been discussed throughout the early 1900s and led to Medawar's theory after discussions with J. B. S. Haldane in the 1940s."},{"doc_id":4136637634019466000,"text":" process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles:\nVariation exists within populations of organisms with respect to morphology, physiology and behaviour (phenotypic variation).\nDifferent traits confer different rates of survival and reproduction (differential fitness).\nThese traits can be passed from generation to generation (heritability of fitness).\nMore offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits"},{"doc_id":8617192067338247000,"text":"Department of Molecular Genetics, Weizmann Institute of Science Rehovot 76100, Israel. 2\nDepartment of Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv\n69978, Israel. 3\nDepartment of Molecular Cell Biology, Weizmann Institute of Science Rehovot 76100, Israel. 4\nDepartment of Systems Biology, Harvard Medical School, Boston,\nMassachusetts 02115, USA.\n*These authors contributed equally to this work.\nS1 S2\nR1 R2\nDirect\nregulation\nS1 S2\nR1 R2\nStochastic\nswitching\nR1 R2\nS1 S2\nR1 R2\nSymmetrical\nanticipatory\nregulation\nS1 S2\nR1 R2\nAsymmetrical\nanticipatory\nregulation\nFigure 1 | Four possible regulation strategies in response to environmental\nstimuli. Under direct regulation, each of the stimuli, S1 and S2, activates\nexclusively the responses R1 and R2, respectively. Under stochastic\nswitching, cells randomly sample either R1 or R2 in response to either S1 or\nS2. Under symmetrical anticipatory regulation, each of the stimuli activates\nboth responses. Under asymmetrical anticipatory regulation, the stimulus\nthat usually appears first in the ecology activates both responses, whereas the\nstimulus that appears later induces only the second response.\nVol 460| 9 July 2009| doi:10.1038/nature08112\n220\n©2009 Macmillan Publishers Limited. All rights reserved\ncost of preparation: pre-induction of genes needed to cope with S2 is\ncostly and not beneficial during S1. This indicates that early induction\nis preserved due to a future benefit that is expected to exceed the cost,\nupon encounter with S2. Third, specificity: the conditioned response is\nspecific to S1 and not to other unrelated stimuli, suggesting that anticipation evolved in response to the specific conditions of the ecological niche.\nE. coli carbon source switch\nDuring its life cycle E. coli alternates between two principal habitats,\nintestines of mammals and water, sediment and soil10 (Supplementary Information). Focusing on the intestinal ecology reveals a predictable metabolic environment. Specifically, during passage along the\ndigestive tract, exposure to lactose precedes exposure to another sugar,\nmaltose11. We thus expect that this environment can select for asymmetric anticipation, so that bacteria that link the presence of lactose\nwith future exposure to maltose are expected to activate the maltose\ngenes already upon encounter with lactose. Such pre-induction is\nexpected to allow cells to better use maltose upon its encounter.\nWe start by examination of anticipatory behaviour at the molecular level. To test systematically the promoter activity of all relevant\noperons in the presence of maltose and lactose we used an E. coli\npromoter-fused green fluorescent protein (GFP) library previously\ndescribed12. Our experiments revealed the unidirectional regulation\npattern expected under a conditioned response strategy (Fig. 2a and\nSupplementary Figs 1–4). Maltose operons, which are induced by\nmaltose, are also induced, but to a lower level, by lactose. We note\nthat the basic responsiveness of some of the maltose operons to\nlactose is not restricted only to the E. coli strain used in our study13.\nIn contrast, and as expected by our hypothesis, the lactose operon\nshows no response to maltose (Fig. 2a and Supplementary Figs 1–4).\nThe crucial question is whether the observed asymmetric response\nstrategy that implies anticipation is indeed adaptive—that is, whether\nit could be selected for during evolution. Following the criteria outlined previously, we first validated that the conditioned response\nindeed provides an asymmetric fitness advantage in an alternating\nsugar environment. We monitored cell growth on maltose after an\ninitial growth phase on lactose. Reassuringly, we observed that the\nwild-type strain displays a fitness advantage if growth on maltose was\npreceded by growth on lactose (Fig. 3). We ensured that this advantage does not originate from the metabolic contribution of lactose as\na carbon source (Supplementary Information). In addition, and as\nrequired by our first principle, we found that pre-exposure to maltose\nbefore growth on lactose does not improve fitness (Fig. 3).\nOur second criterion for adaptiveness requires that pre-induction\nof the maltose genes is maladaptive or neutral during growth on\nlactose. This criterion is crucial to rule out the possibility that the\nmaltose genes are induced during exposure to lactose simply because\nthey are needed during that phase. Built into the classical conditioning paradigm from the cognitive context is the possible extinction of\nthe association. To put in present case terms, it is expected that\nrepeated exposure to lactose without consequent arrival of maltose\nwould select for weakening of the conditioned response due to the\nfutile cost of preparation. Accordingly, we have examined laboratory-evolved strains of E. coli, which grew for 500 generations on\nhigh levels of lactose yet without exposure to maltose14. We measured\nthe promoter activity of relevant operons in three lineages that\nevolved independently under various lactose concentrations.\nNotably, we found that, in all three lines, the maltose operons show\nalmost no activity in response to lactose (Fig. 2b and Supplementary\nFig. 5). Reassuringly, the strains seem to have preserved the ability to\nactivate the maltose genes in response to maltose itself (Supplementary Fig. 6), indicating that only the asymmetrical cross talk between\nthe two pathways was removed during this laboratory evolution\nperiod. These results imply that the conditioning observed in the\nwild-type strain is costly and that without a subsequent benefit that\nshould exceed this cost, purifying selection acts to eliminate this cross\ntalk. Consistent with the second criterion, the growth advantage LacZ MalE MalK MalP MalS MalZ\nLacZ MalE MalK MalP MalS MalZ\n0\n0.25\n0.50\n0.75\n"},{"doc_id":449019984534268200,"text":" a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character"},{"doc_id":4510932946447888400,"text":" on their traits to the next generation than those with traits that do not confer an advantage. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Consequences of selection include nonrandom mating and genetic hitchhiking.\nThe central concept of natural selection is the evolutionary fitness of an organism. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, fitness is not the same as the total number of offspring: instead fitness is indicated by"},{"doc_id":6806517297278293000,"text":" to the next generation than those with traits that do not confer an advantage. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Consequences of selection include nonrandom mating and genetic hitchhiking.\nThe central concept of natural selection is the evolutionary fitness of an organism. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of"},{"doc_id":5481420576488986000,"text":" bacteria, these are much rarer than in eukaryotes.\nBacteria, as asexual organisms, inherit an identical copy of the parent's genome and are clonal. However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations arise from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. Genetic changes in bacterial genomes emerge from either random mutation during replication or \"stress-directed mutation\","},{"doc_id":8197887054600267000,"text":" to composition in diploid organisms such as mammals and (2) bacterial genomes frequently have AT-biased mutation.\nContemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work showed that the original \"pressures\" theory assumes that evolution is based on standing variation: when evolution depends on events of mutation that introduce new alleles, mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.\nSeveral studies report that the mutations implicated in adaptation reflect"},{"doc_id":241119448779526140,"text":", but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Within cells, molecular machines such as the bacterial flagella and protein sorting machinery evolved by the recruitment of several pre-existing proteins that previously had different functions. Another example is the recruitment of"},{"doc_id":3685274842462513000,"text":"Benefits and costs[edit]\nAntagonistic pleiotropy has several negative consequences. It results in delayed adaptation, an altered path of evolution, and reduced adaptation of other traits. In addition, the overall benefit of alleles is cut down significantly (by about half) by pleiotropy. Still, antagonistic pleiotropy has some evolutionary benefits. In fact, the conservation of genes is directly related to the pleiotropic character of an organism. This implies that genes that control for multiple traits, even if the traits have different implications for the organism's fitness, have more staying power in an evolutionary context."},{"doc_id":3731771800428118500,"text":" trait works in some cases, most traits are influenced by multiple genes in a quantitative or epistatic manner.\nEvolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is very similar among all individuals of that species. However, discoveries in the field of evolutionary developmental biology have demonstrated that even relatively small differences in genotype can lead to dramatic differences in phenotype both within"},{"doc_id":4002705701123597300,"text":"Text adapted from OpenStax Biology 2e and used under a [ Creative Commons\nAttribution License 4.0 ](https://creativecommons.org/licenses/by/4.0/)."},{"doc_id":6342130594967206000,"text":" as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population,"},{"doc_id":7335190644482435000,"text":" reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles. It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.\nCoevolution\n\nFurther information: Coevolution\nThe common garter snake has evolved resistance to the defensive substance tetrodotoxin in its amphibian prey.\nInteractions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a"},{"doc_id":221831125176784060,"text":"As a constraint on perfection[edit]\nAntagonistic pleiotropy is one of the several reasons evolutionary biologists give for organisms never being able to reach perfection through natural selection. Antagonistically pleiotropic genes are the explanation for fitness trade-offs. This means that genes that are pleiotropic control for some beneficial traits and some detrimental traits; thus, if they happen to persist through natural selection, this will prevent organisms from reaching perfection because if they possess the benefits of the gene, they must also possess the imperfections or faults. An example of this would be female rodents that live in a nest with other females and may end up feeding young that are not theirs due to their intense parental drive. This strong parental drive will be selected for, but the organisms will still make the mistake of feeding young that are not theirs and misallocating their resources."},{"doc_id":2348511018954172400,"text":" and between species.\nAn individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely.\nMutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms. When mutations occur,"},{"doc_id":53925992549567576,"text":"Go to:\nAcknowledgments\nWe thank Ford Doolittle and Susan Rosenberg for organizing this series of papers on “How Microbes ‘Jeopardize’ the Modern Synthesis.”"},{"doc_id":4519866947457956400,"text":" genotype can lead to dramatic differences in phenotype both within and between species.\nAn individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely.\nMutation\n\nMain article: Mutation\nDuplication of part of a chromosome"},{"doc_id":2202892938114897200,"text":"ogenic strains of human flu\nViruses undergo genetic change by several mechanisms. These include a process called antigenic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are \"silent\"—they do not change the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs. Antigenic shift occurs when there is a major change in the genome of the virus. This can be a result of recombination or reassortment. When this happens with influenza viruses, pandemics might result. RNA viruses often"},{"doc_id":2482187478625555500,"text":" and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity and canalisation. Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis."},{"doc_id":7456392705571888000,"text":" coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\nGene flow\n\nFurther information: Gene flow\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the"},{"doc_id":4659267878746529000,"text":"ARTICLES\nAdaptive prediction of environmental\nchanges by microorganisms\nAmir Mitchell1\n, Gal H. Romano2\n, Bella Groisman1\n, Avihu Yona1\n, Erez Dekel3\n, Martin Kupiec2\n, Orna Dahan1\n*\n& Yitzhak Pilpel1,4*\nNatural habitats of some microorganisms may fluctuate erratically, whereas others, which are more predictable, offer the\nopportunity to prepare in advance for the next environmental change. In analogy to classical Pavlovian conditioning,\nmicroorganisms may have evolved to anticipate environmental stimuli by adapting to their temporal order of appearance.\nHere we present evidence for environmental change anticipation in two model microorganisms, Escherichia coli and\nSaccharomyces cerevisiae. We show that anticipation is an adaptive trait, because pre-exposure to the stimulus that typically\nappears early in the ecology improves the organism’s fitness when encountered with a second stimulus. Additionally, we\nobserve loss of the conditioned response in E. coli strains that were repeatedly exposed in a laboratory evolution experiment\nonly to the first stimulus. Focusing on the molecular level reveals that the natural temporal order of stimuli is embedded in the\nwiring of the regulatory network—early stimuli pre-induce genes that would be needed for later ones, yet later stimuli only\ninduce genes needed to cope with them. Our work indicates that environmental anticipation is an adaptive trait that was\nrepeatedly selected for during evolution and thus may be ubiquitous in biology.\nMicroorganisms are constantly faced with environmental stimuli and\nstresses. The cellular response to such challenges has been intensively\nstudied in several model organisms1–4. The simplest response strategy\nto a stimulus is to monitor the environment and to respond directly\nto it using designated mechanisms (Fig. 1). The environmental stress\nresponse in yeast represents a more complicated strategy in which the\nresponses to many stresses are partially overlapping1,2. Theoretical\nwork has shown that when a population of microorganisms evolves\nunder erratic environmental fluctuations, cells may not effectively\nmonitor the environment, but rather use stochasticity to randomly\nalternate between potential states5 (Fig. 1). Stochastic switching\nmight thus ensure that a portion of the population is prepared in\nadvance for the unpredicted challenge6,7. However, other, more predictable, environments offer organisms the opportunity to adopt an\nalternative regulation strategy of anticipating an environmental\nchange based on a preceding signal. The capacity of some complex\nmulticellular eukaryotes to capture the statistics that govern the temporal connection between events in their environment, known as\nclassical Pavlovian conditioning, serves as a central paradigm in the\nstudy of learning8\n. Here we ask whether genetic regulatory networks\nof microorganisms adaptively evolved to capture the temporal connections between subsequent stimuli in their environment. Most\nrecently, ‘anticipatory regulation’ was discovered9 (Fig. 1)—an association between environmental changes in bacteria. Specifically, this\nstudy investigated the response of E. coli to temperature increase that\nis followed by a drop in oxygen availability upon its entry to the\ndigestive tract. Interestingly, these two signals show a symmetrical\nassociative regulation pattern—each signal affects the expression of\ngenes needed to cope with both (Fig. 1). Remarkably, the authors\nsuccessfully decoupled the two responses during a laboratory evolution experiment in which the two signals were presented out-ofphase from one another. The ability to decouple the two responses\nis an indication that the coupling seen in the wild type is not a trivial\ncombined response to the two stresses.\nHere we show that biological systems that react to a unidirectional\ntemporal order of environmental changes may manifest a more elaborate predictive capacity. This capacity is reflected in a corresponding\nasymmetric response strategy between subsequent stimuli, denoted S1\nand S2 hereafter, and their designated responses R1 and R2, respectively. The first stimulus, S1, activates both responses, R1 and R2, yet\nbecause the second stimulus, S2, does not predict the appearance of S1,\nit only activates its own response (Fig. 1). We propose three criteria to\ndetermine whether the observed cross-regulation pattern forms an\nadaptive anticipatory response strategy that could be selected for by\nevolution. First, asymmetric fitness advantage: pre-exposure to S1\nincreases the fitness under S2, yet pre-exposure to S2 should not\nenhance fitness upon subsequent growth on S1. This ensures that\nthe natural order of stimuli was captured during evolution. Second,\n1"},{"doc_id":2404171571940034000,"text":"Mechanism of action[edit]\n(a) The survival rate within a population decreases with age, while the reproduction rate remains constant. (b) The reproduction probability peaks early in life, at sexual maturity, and then steadily decreases as an individual ages, with the remaining share of the population decreasing with age as they enter the selection shadow.\nAmongst almost all populations, the likelihood that an individual will reproduce is related directly to their age. Starting at 0 at birth, the probability increases to its maximum in young adulthood once sexual maturity has been reached, before gradually decreasing with age. This decrease is caused by the increasing likelihood of death due to external pressures such as predation or illness, as well as the internal pressures inherent to organisms that experience senescence. In such cases deleterious mutations which are expressed early on are strongly selected against due to their major impact on the number of offspring produced by that individual. Mutations that present later in life, by contrast, are relatively unaffected by selective pressure, as their carriers have already passed on their genes, assuming they survive long enough for the mutation to be expressed at all. The result, as predicted by Medawar, is that deleterious late-life mutations will accumulate and result in the evolution of aging as it is known colloquially. This concept is portrayed graphically by Medawar through the concept of a \"selection shadow\". The shaded region represents the 'shadow' of time during which selective pressure has no effect. Mutations that are expressed within this selection shadow will remain as long as reproductive probability within that age range remains low."},{"doc_id":6014319786151096000,"text":"Applications\n\nMain articles: Applications of evolution, Selective breeding, and Evolutionary computation\nConcepts and models used in evolutionary biology, such as natural selection, have many applications.\nArtificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the domestication of plants and animals. More recently, such selection has become a vital part of genetic engineering, with selectable markers such as antibiotic resistance genes being used to manipulate DNA. Proteins with valuable properties have evolved by repeated rounds of mutation and selection (for example modified enzymes"},{"doc_id":6223078474715175000,"text":" enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.\nAn area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. It is also possible for structures that have been lost in evolution to"},{"doc_id":4385609516791664600,"text":"26. Braun, E. & Brenner, N. Transient responses and adaptation to steady state in a\neukaryotic gene regulation system. Phys. Biol. 1, 67–76 (2004).\n27. Winzeler, E. A. et al. Functional characterization of the S. cerevisiae genome by\ngene deletion and parallel analysis. Science 285, 901–906 (1999).\nSupplementary Information is linked to the online version of the paper at\nwww.nature.com/nature.\nAcknowledgements We thank S. Trattner"},{"doc_id":8650248059159904000,"text":" the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.\nNatural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection, may allow the evolution of cooperation.\nGenetic drift"},{"doc_id":6174758067960444000,"text":" the proportion of subsequent generations that carry an organism's genes. For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.\nIf an allele increases fitness more than the other alleles of that gene, then with each generation this allele has a higher probability of becoming common within the population. These traits are said to be \"selected for.\" Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by"},{"doc_id":9021716622346336000,"text":" between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.\nIn the 1920s and 1930s, the modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that included random genetic drift, mutation, and gene flow. This new version of evolutionary theory focused on changes in allele frequencies in population. It explained patterns observed across species in populations, through fossil transitions in palaeontology.\nSince then, further syntheses have extended evolution's explanatory power in the light of numerous discoveries, to cover biological phenomena across the whole"},{"doc_id":9181138567613823000,"text":"-adaptive forces in driving evolutionary change is an area of current research.\nMutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to the idea of developmental bias. Haldane and Fisher argued that, because mutation is a weak pressure easily overcome by selection, tendencies of mutation would be ineffectual except under conditions of neutral evolution or extraordinarily high mutation rates. This opposing-pressures argument was long used to dismiss the possibility of internal"},{"doc_id":17671705224215012,"text":"Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome. Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.\nNew genes can be generated from an ancestral gene when a duplicate"},{"doc_id":1139347458278782600,"text":" a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.\nFinally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences"},{"doc_id":3997471738175323600,"text":" increase or decrease in genome size.\nHowever, mutational hypotheses for the evolution of composition suffered a reduction in scope when it was discovered that (1) GC-biased gene conversion makes an important contribution to composition in diploid organisms such as mammals and (2) bacterial genomes frequently have AT-biased mutation.\nContemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work showed that the original \"pressures\" theory assumes that evolution is based on standing variation: when evolution depends on events of mutation that introduce new alleles,"},{"doc_id":8660851524417328000,"text":"Role in disease[edit]\nThe survival of many serious genetic disorders in our long evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are defined by the existence of deleterious alleles, then natural selection acting over evolutionary time would result in a lower frequency of mutations than are currently observed. In a recent article, Carter and Nguyen identify several genetic disorders, arguing that far from being a rare phenomenon, antagonistic pleiotropy might be a fundamental mechanism for the survival of these non-optimal alleles.\nIn one of these studies, 99 individuals with Laron syndrome (a rare form of dwarfism) were monitored alongside their non-dwarf kin for a period of ten years. Patients with Laron syndrome possess one of three genotypes for the growth hormone receptor gene (GHR). Most patients have an A->G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin. This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.\nAnother instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. Incidentally, the high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.\nAdditionally, it was found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. Yet, it was hypothesized that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.\nSickle cell anemia, Beta-thalassemia, and cystic fibrosis are some other examples of the role antagonistic pleiotropy may play in genetic disorders."},{"doc_id":832615000926436200,"text":"We then tested whether induction of the conditioned gene set is\nneutral in heat shock, as required by the second criterion. Towards\nthis aim we collected 29 strains, each of which is deleted for one of the\ngenes from the cluster described above, and tested their sensitivity to\nheat shock and oxidative stress (Methods). Analysis of survival ratios\nreveals that whereas 65% of the strains show increased sensitivity to\noxidative stress, none shows a significantly increased sensitivity to\nheat shock relative to the wild-type strain (Fig. 5b and Supplementary\nFig. 7). This is a direct indication that the expression of many of the\nheat-induced genes is at least neutral, if not maladaptive, under heat\nshock. Taken together with the fact that these genes are essential\nunder oxidative stress, this experiment corroborates the claim that\nan early preparation to oxidative stress during heat shock is adaptive\nin this system.\nFuture outlook\nAn open question is how prevalent anticipatory regulation is in the\nworld of microorganisms. One way to address this issue is to develop\na mathematical framework that estimates the potential advantage of a\nconditioned response in a given ecology. Intuitively, the added value\nfrom anticipation depends both on environmental and on biological\nparameters. Two cellular parameters, gain and cost, should be considered. A conditioned response is beneficial provided that benefit\ngained from anticipation exceeds the cost of early preparation. These\nparameters are in turn dependent on the typical time constants of the\nenvironment, for example, the time gap between the appearances of\nthe two stimuli. Additionally, predictable environments in which\nthe stimuli are often sequentially coupled promote anticipation. A\nsimple equation captures the relationship between these variables\nand their effect on the net fitness:\nDF 5 p(B(Dt) 2 C(Dt)) 2 (1 2 p)(C(tS1)),\nwhere DF is the difference in fitness between an anticipating cell and a\ncell that adheres to direct regulation (Fig. 1), p is the probability that\nS2 will occur given that S1 occurred, and B and C are functions\ndescribing the benefit and cost of early preparation, respectively. Dt\nis the length of the time interval between the appearance of S1 and S2,\nand tS1 is the duration of the first stimulus. The dependence of fitness\non the interplay between the key factors is shown in Supplementary\nFig. 8. We note that quantitative predictions from the above phenomenological equation can be drawn by incorporating into the\nmodel biologically meaningful and realistic parameters, such as measurable gain, cost and timescales (A.M. et al. in preparation)."},{"doc_id":4246999504903770000,"text":" be a strategy to circumvent phage resistance."},{"doc_id":7326570392395027000,"text":" sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might"},{"doc_id":8383252946560158000,"text":"The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls more than one phenotypic trait in an organism. A gene is considered to possess antagonistic pleiotropy if it controls more than one trait, where at least one of these traits is beneficial to the organism's fitness early on in life and at least one is detrimental to the organism's fitness later on due to a decline in the force of natural selection. The theme of G. C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs."},{"doc_id":8435702021988683000,"text":"Drug induced[edit]\nNeural adaptation can occur for other than natural means. Antidepressant drugs, such as those that cause down regulation of β-adrenergic receptors, can cause rapid neural adaptations in the brain. By creating a quick adaptation in the regulation of these receptors, it is possible for drugs to reduce the effects of stress on those taking the medication."},{"doc_id":527421555664797440,"text":"To conclude, as we observed that anticipation at the cellular level\nappears to be adaptive both in prokaryotes (E. coli) and eukaryotes\n(S. cerevisiae), and under remote systems such as sugar metabolism\nand stress response, we propose that it may be prevalent in a diverse\nrange of species. Asymmetric anticipation, along with associative\nlearning9\n, memory25 and physiological adaptation26, represent a\npowerful set of tools used by microorganisms to achieve a sophisticated environmental response.\nMETHODS SUMMARY\nStrains. E. coli MG1655 was used for most experiments. An E. coli GFP reporter\nlibrary12 was used to monitor operon expression. The evolved E. coli strains were\ndescribed previously14\nS. cerevisiae BY4741 (MATa; his3D1; leu2D0; met15D0; ura3D0) was used in all\nyeast experiments. All deletion strains were obtained from the Saccharomyces\nGenome Deletion Project27.\nS. cerevisiae cross-protection. An overnight culture was diluted into fresh YPD\nmedium and grown to a concentration of 2 3 106 cells per ml. Cells were diluted\n1:2 into the first stress (S1). Transfer into the second stress (S2) was done by\nadding treated medium after centrifugation. Stresses were calibrated to achieve a\nmild effect (,50% survival) for S1 and a severe effect (,0.5% survival) for S2\n(Supplementary Table 1). Samples were taken from cultures at the end of each\ntreatment and plated. Each experiment was carried out in three repeats. The fold\nprotection was calculated as described in Supplementary Equation (1).\nE. coli fitness advantage. Overnight cultures were diluted into M9-Gly plus S1\n(for example, lactose; treated) or M9-Gly (untreated). After 3 h, population size\nwas determined and used to dilute (,1:100) an equal amount of cells into a new\ngrowth medium containing low levels of S2 (for example, maltose) as a sole\ncarbon source. To account for a potential metabolic effect of residual S1, an\nidentical trace amount of S1 was added to the untreated growth medium. The\nratio treated/untreated after 2 h of growth was used as an indication of fitness\nadvantage originating from pre-exposure to S1. To rule out the possibility that\nthe growth advantage observed due to early exposure to lactose originates from\nits metabolic value rather than its rule as a conditioning signal, we preformed a\ncontrol experiment (Supplementary Information).\nFull Methods and any associated references are available in the online version of\nthe paper at www.nature.com/nature.\nReceived 25 March; accepted 7 May 2009.\nPublished online 17 June 2009.\n1. Causton, H. C. et al. Remodeling of yeast genome expression in response to\nenvironmental changes. Mol. Biol. Cell 12, 323–337 (2001).\n2. Gasch, A. P. et al. Genomic expression programs in the response of yeast cells to\nenvironmental changes. Mol. Biol. Cell 11, 4241–4257 (2000).\n3. Cheung, K. J. et al. A microarray-based antibiotic screen identifies a regulatory\nrole for supercoiling in the osmotic stress response of Escherichia coli. Genome Res.\n13, 206–215 (2003).\n4. Tirosh, I., Weinberger, A., Carmi, M. & Barkai, N. A genetic signature of\ninterspecies variations in gene expression. Nature Genet. 38, 830–834 (2006).\nNATURE| Vol 460| 9 July 2009 ARTICLES\n223\n"},{"doc_id":1736052534461021000,"text":" the European Research Council ‘Ideas Program’ and the Ben May\nFoundation for grant support. M.K. was supported from grants from the Israel\nScience Foundation and the Israeli Ministry of Science and Technology.\nAuthor Contributions A.M. raised the original idea and performed all the\nexperiments; G.R., B.G. and A.Y. participated in experiments; E.D. evolved the E. coli\nstrain; A.M., O.D. and Y.P. designed the experiments; A.M., M.K., O"},{"doc_id":2662230032262301000,"text":" the transfer of heritable variations along the germline. To explain how new variants originate, de Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.\nThe'modern synthesis'\n\n"},{"doc_id":3717223793901211600,"text":"Sources of variation\n\nMain article: Genetic variation\n\nFurther information: Genetic diversity and Population genetics\nWhite peppered mothBlack morph in peppered moth evolution\nEvolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is very similar among all individuals of that species. However, discoveries in the field of evolutionary developmental biology have demonstrated that even relatively small differences in"},{"doc_id":887670927609664300,"text":" effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.\nSexual selection\n\nFurther information: Sexual selection\nMale moor frogs become blue during the height of mating season. Blue reflectance may be a form of intersexual communication. It is hypothesised that males with brighter blue coloration may signal greater sexual and genetic fitness.\nA special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the"},{"doc_id":8389227544347302000,"text":" selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilising selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. This would, for example, cause organisms to eventually have a similar height.\nNatural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. \"Nature\" in this sense refers"},{"doc_id":3428675185126628000,"text":". Many human diseases are not static phenomena, but capable of evolution. Viruses, bacteria, fungi and cancers evolve to be resistant to host immune defences, as well as to pharmaceutical drugs. These same problems occur in agriculture with pesticide and herbicide resistance. It is possible that we are facing the end of the effective life of most of available antibiotics and predicting the evolution and evolvability of our pathogens and devising strategies to slow or circumvent it is requiring deeper knowledge of the complex forces driving evolution at the molecular level.\nIn computer science, simulations of evolution using evolutionary algorithms and artificial life started"},{"doc_id":3972252304274929000,"text":" inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.\nA special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness"},{"doc_id":5937972175565269000,"text":" biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes, enzymes, and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetic and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of Escherichia coli metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineer bacteria"},{"doc_id":2469823221813689300,"text":" that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced"},{"doc_id":8940875874985204000,"text":". Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.\nNew genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system;"},{"doc_id":1185125279276353300,"text":", introducing foreign DNA in a process known as transduction. Many types of bacteriophage exist; some infect and lyse their host bacteria, while others insert into the bacterial chromosome. Bacteria resist phage infection through restriction modification systems that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. Third, bacteria can transfer genetic material through direct cell contact via conjugation.\nIn ordinary circumstances, transduction"},{"doc_id":5136899540305229000,"text":" one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA, a phenomenon termed de novo gene birth.\nThe generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions (exon shuffling). When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions"},{"doc_id":1560288069817212400,"text":" phenotype is favoured. · Graph 2 depicts stabilizing selection, where the intermediate phenotype is favoured over the extreme traits. · Graph 3 shows disruptive selection, in which the extreme phenotypes are favoured over the intermediate.\nNatural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most"},{"doc_id":8072226226985974000,"text":"les of the same gene.\nMHC allelic diversity has challenged evolutionary biologists for explanation. Most posit balancing selection (see polymorphism (biology)), which is any natural selection process whereby no single allele is absolutely most fit, such as frequency-dependent selection and heterozygote advantage. Pathogenic coevolution, as a type of balancing selection, posits that common alleles are under greatest pathogenic pressure, driving positive selection of uncommon alleles—moving targets, so to say, for pathogens. As pathogenic pressure on the previously common alleles decreases, their frequency in the population stabilizes"},{"doc_id":6176450517828379000,"text":" seem to fit some genetic variation seen in nature. A better-supported version of this model is the nearly neutral theory, according to which a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population. Other theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft. Another concept is constructive neutral evolution (CNE), which explains that complex systems can emerge and spread into a population through neutral transitions due to the principles of excess capacity, presuppression, and ratcheting, and"},{"doc_id":7684727185464362000,"text":" edits, from single nucleotide base pairs to whole genome or gene networks simultaneously with results in a matter of days.\nMAGE experiments can be divided into three classes, characterized by varying degrees of scale and complexity: (i) many target sites, single genetic mutations; (ii) single target site, many genetic mutations; and (iii) many target sites, many genetic mutations. An example of class three was reflected in 2009, where Church and colleagues were able to program Escherichia coli to produce five times the normal amount of lycopene, an antioxidant normally found in tomato seeds and"},{"doc_id":2356343650515559400,"text":" copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA, a phenomenon termed de novo gene birth.\nThe generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions (exon shuffling). When new genes are assembled from"}],"string":"[\n {\n \"doc_id\": 7011957731131189000,\n \"text\": \" bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' evolvability).\\nAdaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and\"\n },\n {\n \"doc_id\": 5332197148514614000,\n \"text\": \"1. [ Stabilizing Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-1)\\n2. [ Directional Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-2)\\n3. [ Diversifying Selection ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/#chapter-1608-section-3\"\n },\n {\n \"doc_id\": 6676066334518801000,\n \"text\": \" ](https://slcc.pressbooks.pub/collegebiology1/chapter/variation-adaptation/)\"\n },\n {\n \"doc_id\": 7557534906429007000,\n \"text\": \"Natural outcomes\\nA visual demonstration of rapid antibiotic resistance evolution by E. coli growing across a plate with increasing concentrations of trimethoprim\\nEvolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups\"\n },\n {\n \"doc_id\": 8846767774428441000,\n \"text\": \"3. [ 3.3 Adaptive Evolution ](https://slcc.pressbooks.pub/collegebiology1/chapter/adaptive-evolution/)\"\n },\n {\n \"doc_id\": 6381177455009793000,\n \"text\": \" proteins, capsules, flagella, and pili, among others. However, some studies suggest that when phage resistance is caused by mutations in phage receptors, this might result in fitness costs to the resistance bacterium, which will ultimately become less virulent. Moreover, it has been shown that the evolution of bacterial resistance to phage attack changes the efflux pump mechanism, causing increased sensitivity to drugs from several antibiotic classes. Therefore, it is conceivable to think that phage therapy that uses phages that exert selection for multidrug-resistant bacteria to become antibiotic-sensitive could potentially reduce\"\n },\n {\n \"doc_id\": 1010349355485385100,\n \"text\": \" be bacterial adaptations. [20]\\n[51]\"\n },\n {\n \"doc_id\": 1544282805858573600,\n \"text\": \"les within a population of interbreeding organisms, for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, and mutation bias.\\nEvolution by natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles:\\nMore offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass\"\n },\n {\n \"doc_id\": 1914211970812137500,\n \"text\": \" having a less beneficial or deleterious allele results in this allele likely becoming rarer—they are \\\"selected against.\\\"\\nImportantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form. However, a re-activation of dormant genes, as long as they have not been eliminated from the genome and were only suppressed perhaps for hundreds of\"\n },\n {\n \"doc_id\": 8349157190851547000,\n \"text\": \"ichia coli strain MG1655: Growth\\ndefects and apparent cross-regulation of gene expression. J. Bacteriol. 185,\\n5611–5626 (2003).\\n14. Oxman, E., Alon, U. & Dekel, E. Defined order of evolutionary adaptations:\\nexperimental evidence. Evolution 62, 1547–1554 (2008).\\n15. Mortimer, R. K. Evolution and variation of the yeast (Saccharomyces) genome.\\nGenome Res. 10, 403–409 (2000).\\n16\"\n },\n {\n \"doc_id\": 2504545380939800600,\n \"text\": \" of gene function and thus pseudo-genes. More commonly, single amino acid changes have limited consequences although some can change protein function substantially, especially in enzymes. For instance, many enzymes can change their substrate specificity by one or a few mutations. Changes in substrate specificity are facilitated by substrate promiscuity, i.e. the ability of many enzymes to bind and process multiple substrates. When mutations occur, the specificity of an enzyme can increase (or decrease) and thus its enzymatic activity. Thus, bacteria (or other organisms) can adapt to different food sources, including unnatural substrates\"\n },\n {\n \"doc_id\": 4951364018294375000,\n \"text\": \"©2009 Macmillan Publishers Limited. All rights reserved\\n5. Kussell, E. & Leibler, S. Phenotypic diversity, population growth, and information in\\nfluctuating environments. Science 309, 2075–2078 (2005).\\n6. Acar, M., Mettetal, J. T. & van Oudenaarden, A. Stochastic switching as a survival\\nstrategy in fluctuating environments. Nature Genet. 40, 471–475 (2008).\\n7. Balaban, N. Q. et al\"\n },\n {\n \"doc_id\": 1984461514986374700,\n \"text\": \" beneficial or deleterious allele results in this allele likely becoming rarer—they are \\\"selected against.\\\"\\nImportantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form. However, a re-activation of dormant genes, as long as they have not been eliminated from the genome and were only suppressed perhaps for hundreds of generations, can\"\n },\n {\n \"doc_id\": 1220129963295904800,\n \"text\": \" of mutation and selection (for example modified enzymes and new antibodies) in a process called directed evolution.\\nUnderstanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human genetic disorders. For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves. This helped identify genes required for vision and pigmentation.\\n\"\n },\n {\n \"doc_id\": 2111678933010816500,\n \"text\": \"248.2012.01814.x ](https://doi.org/10.1111%2Fj.1461-0248.2012.01814.x). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 22690742 ](https://pubmed.ncbi.nlm.nih.gov/22690742).\\n20. ^ _**a** _ _**b** _ _**c** _ Michod, R. E.; Bernstein, H\"\n },\n {\n \"doc_id\": 5513803758095037000,\n \"text\": \" possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles. It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.\\nInteractions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a\"\n },\n {\n \"doc_id\": 5819475469232278000,\n \"text\": \" evolvability).\\nA baleen whale skeleton. Letters a and b label flipper bones, which were adapted from front leg bones, while c indicates vestigial leg bones, both suggesting an adaptation from land to sea.\\nAdaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organisation may have different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are very similar to those in mice feet and primate hands, due to the descent of all\"\n },\n {\n \"doc_id\": 991332541439228800,\n \"text\": \" where genes involved in a particular growth-limiting process have an increased mutation rate.\\nSome bacteria transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment in a process called transformation. Many bacteria can naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA. The development of competence in nature is usually associated with stressful environmental conditions and seems to be an adaptation for facilitating repair of DNA damage in recipient cells. Second, bacteriophages can integrate into the bacterial chromosome\"\n },\n {\n \"doc_id\": 4071155514373589000,\n \"text\": \"1.00\\nNormalized promoter activity\\nWild-type strain\\nAdded sugar:\\nLactose\\nMaltose\\nNone\\na\\n0\\n0.25\\n0.50\\n0.75\\n1.00\\nGFP-fused promoter\\nb Evolved strain\\nFigure 2 | Conditioned response in E. coli sugar metabolism. Promoter\\nactivity in wild type (a) and the evolved strain (b). Colours mark the added\\nsugar. Error bars denote standard deviation of four repeats. Promoter\\nactivities of four out of five maltose operons in wild type are significantly\\nhigher under lactose relative to the untreated culture (t-test, P , 0.01\\naccording to a Bonferroni adjustment). In contrast, none of the promoters\\nshow increased activity in the evolved strain. The experiment was done in\\nM9-Glu medium and was repeated using glycerol as a background carbon\\nsource (Supplementary Fig. 3) and further verified using quantitative\\npolymerase chain reaction with reverse transcription (RT–PCR)\\n(Supplementary Figs 2 and 4).\\nEvolved\\n0.98\\n0.99\\n1.00\\n1.01\\n1.02\\n1.03\\n1.04\\nFitness advantage\\nObserved effect:\\nDirectionality\\nSpecificity\\nExtinction\\nLac\\nMal\\nMal\\nLac\\nGal\\nMal\\nSuc\\nMal\\nLac\\nMal\\nWild type\\nFigure 3 | Fitness in an alternating sugar environment. Two isogenic\\npopulations were grown on M9-Gly either supplemented with a sugar as S1\\n(treated) or not (untreated). An equal amount of cells was diluted into media\\ncontaining the second sugar (S2) as a sole carbon source. The ratio treated/\\nuntreated (after 2 h) represents the fitness advantage originating from preexposure to S1. The error bars denote standard deviation of three repeats. All\\nratios are normalized to the ratio measured in a control experiment in which\\nS2 was glycerol (Methods). A significant fitness advantage was observed only\\nin the transfer of wild-type cells from lactose to maltose (P 5 0.02, t-test).\\nNATURE| Vol 460| 9 July 2009 ARTICLES\\n221\\n©2009 Macmillan Publishers Limited. All rights reserved\\nobserved in the wild type due to exposure to lactose before maltose is\\nlost in the evolved strain that features extinction (Fig. 3).\\nFinally, as required by the specificity criterion, we ensured that\\npre-exposure to other sugars does not improve growth of cells upon\\ntransfer to maltose. As can be seen in Fig. 3, two alternative related\\ncarbon sources, galactose and sucrose, cannot substitute lactose as a\\npreceding signal before maltose. This indicates that the observed\\nanticipation evolved in response to the specific stimuli pair that is\\npresented to the organism in its ecological niche. In summary, we\\nconclude that anticipation of subsequent carbon source changes is an\\nadaptive trait that has been selected by the natural ecology of E. coli.\\nThe shift from fermentation to respiration in yeast\\nWe next proceeded to a more complicated case that may also manifest environmental change anticipation. We examined how environmental anticipation is encoded in the regulatory network of\\nS. cerevisiae, and whether this response strategy has been selected\\nfor because of a net fitness advantage it entails. We examined conditions that emulate some of the stresses that occur during the process\\nof alcoholic brew production, such as wine, by yeast15: Fig. 4a\\ndescribes this process, which starts at a highly osmotic and low pH\\nenvironment, followed by potentially lethal temperature raise due to\\nvigorous fermentation and accumulation of ethanol, culminating in\\nthe diauxic shift—the exhaustion of fermentable sugars (for example,\\nglucose) and the switch to oxidative respiration16–19. Respiration is\\nchallenging to the cell because it eventually generates oxygen radicals20. We checked whether yeast cells can cope better with these\\nstresses provided that they appear in their natural temporal order,\\nthat is, whether yeasts use early stresses as predictive signals for the\\nlikely arrival of later stresses. Such behaviour would indicate that\\nyeast cells are preparing for later challenges even before they arise.\\nTo address these questions we systematically studied the survival\\nof S. cerevisiae under stresses that comprise the switch from its\\npreferred catabolic state, fermentation, to the alternative one, respiration (Fig. 4). As a potential manifestation of anticipation, we\\nfocused on the cross-protection phenotype between different stress\\npairs21,22. A cross-protection phenotype between two stresses exists if\\npre-exposure to one stress improves the survival of cells under a\\nsubsequent stress. Under our hypothesis we expect an asymmetrical\\ncross-protection phenotype between stress pairs. Particularly, we\\npredict that directionality of cross-protection will be in accordance\\nwith the order of events during the transition from fermentation to\\nrespiration. For example, because heat shock and ethanol accumulation precede oxidative stress in the ecology, we expect improved\\nsurvival under oxidative stress if it is preceded by either heat shock\\nor ethanol stress, yet we do not expect cross-protection if the stresses\\nare applied in the reverse order. Figure 4a shows the measured protection phenotype systematically tested for stresses that occur before\\nand after the diauxic shift. In accordance with our hypothesis, we\\nobserve that the cross-protection phenotype is indeed extremely\\nasymmetric and, most importantly, that the directionality of protection fits the natural temporal context. Specifically, for each stress pair\\nprotection is always stronger in the direction found in the natural\\nenvironment, thus fulfilling the asymmetric fitness advantage criterion. In a more faithful emulation of the natural habitat we have\\nalso applied the three stresses, heat shock, ethanol and oxidative\\nstress, sequentially and observed an equally high cross-protection\\nactivity (Supplementary Information). Note that activation of the\\n\"\n },\n {\n \"doc_id\": 4882298945857637000,\n \"text\": \" experimental evolution and the observation of evolution and adaptation in real time.\\nAdaptation is the process that makes organisms better suited to their habitat. Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection. The following definitions are due to Theodosius Dobzhansky:\\nAdaptation may\"\n },\n {\n \"doc_id\": 2822051962951010000,\n \"text\": \" the incidence of antibiotic-resistant infections.\\nBesides the prevention of phage adsorption by loss or modification of bacterial receptors, phage insensitivity can be caused by: (i) prevention of phage DNA entry by superinfection exclusion systems; (ii) degradation of phage DNA by restriction-modification systems or by CRISPR-Cas systems; and (iii) use of abortive infection systems that block phage replication, transcription, or translation, usually in conjunction with suicide of the host cell. Altogether, these mechanisms promote a quick adaptation of bacteria to phage attack\"\n },\n {\n \"doc_id\": 2214854763095047700,\n \"text\": \" concerns usually associated with phage therapy is the emergence of phage-insensitive mutants (BIMs) that could hinder the success of this therapy. In fact, several in vitro studies have reported a fast emergence of BIMs within a short period of time after phage treatment. The emergence of BIMs has also been observed in vivo using different animal models, although this usually occurs later than in vitro (reviewed in ). This fast adaptation of bacteria to phage attack is usually caused by mutations on genes encoding phage receptors, which include lipopolysaccharides (LPS), outer membrane\"\n },\n {\n \"doc_id\": 4904674638133632000,\n \"text\": \" predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution. An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly\"\n },\n {\n \"doc_id\": 4719842190235495000,\n \"text\": \"itable variations that may underlay some of the mechanics in developmental plasticity and canalisation. Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.\\nFrom a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alle\"\n },\n {\n \"doc_id\": 1718319608984949000,\n \"text\": \" subsequent generations that carry an organism's genes. For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.\\nIf an allele increases fitness more than the other alleles of that gene, then with each generation this allele has a higher probability of becoming common within the population. These traits are said to be \\\"selected for.\\\" Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by having a less\"\n },\n {\n \"doc_id\": 1871963658792519400,\n \"text\": \".\\nAdaptation is the process that makes organisms better suited to their habitat. Also, the term adaptation may refer to a trait that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass. By using the term adaptation for the evolutionary process and adaptive trait for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by natural selection. The following definitions are due to Theodosius Dobzhansky:\\nAdaptation is the evolutionary process whereby an organism becomes better able to live in\"\n },\n {\n \"doc_id\": 5560836972122283000,\n \"text\": \" mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.\\nSeveral studies report that the mutations implicated in adaptation reflect common mutation biases though others dispute this interpretation.\\nGenetic hitchhiking\\n\\nFurther information: Genetic hitchhiking, Hill–Robertson effect, and Selective sweep\\nRecombination allows alleles on the same strand of DNA to become separated. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome\"\n },\n {\n \"doc_id\": 4276364739988023000,\n \"text\": \" the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.\\nA common\"\n },\n {\n \"doc_id\": 7653446544395892000,\n \"text\": \" as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of rapid evolution and speciation in survivors.\\nConcepts and models used in evolutionary biology, such as natural selection, have many applications.\\nArtificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the domestication of plants and animals. More recently, such selection has become a vital part of genetic engineering, with selectable markers such as antibiotic resistance genes being used to manipulate DNA. Proteins with valuable properties have evolved by repeated rounds\"\n },\n {\n \"doc_id\": 7471452766319398000,\n \"text\": \"Evolutionary forces\\nMutation followed by natural selection results in a population with darker colouration.\\nFrom a neo-Darwinian perspective, evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms, for example, the allele for black colour in a population of moths becoming more common. Mechanisms that can lead to changes in allele frequencies include natural selection, genetic drift, and mutation bias.\\nNatural selection\\n\\nMain article: Natural selection\\n\\nSee also: Dollo's law of irreversibility\\nEvolution by natural selection is the\"\n },\n {\n \"doc_id\": 2758089624746548000,\n \"text\": \" and new antibodies) in a process called directed evolution.\\nUnderstanding the changes that have occurred during an organism's evolution can reveal the genes needed to construct parts of the body, genes which may be involved in human genetic disorders. For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. Breeding together different populations of this blind fish produced some offspring with functional eyes, since different mutations had occurred in the isolated populations that had evolved in different caves. This helped identify genes required for vision and pigmentation.\\nEvolutionary theory has many applications in medicine\"\n },\n {\n \"doc_id\": 2372858552183286000,\n \"text\": \" the same for every gene in the same population.\\nIt is usually difficult to measure the relative importance of selection and neutral processes, including drift. The comparative importance of adaptive and non-adaptive forces in driving evolutionary change is an area of current research.\\nMutation bias\\nMutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to the idea of developmental bias. Haldane and Fisher argued that, because mutation is a weak pressure\"\n },\n {\n \"doc_id\": 4918649367885492000,\n \"text\": \" generations, can lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. \\\"Throwbacks\\\" such as these are known as atavisms.\\nNatural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Secondly, disruptive selection is\"\n },\n {\n \"doc_id\": 8746751609829276000,\n \"text\": \". Bacterial persistence as a phenotypic switch. Science 305,\\n1622–1625 (2004).\\n8. Pavlov, I. P. Conditioned Reflexes (Oxford Univ. Press, 1927).\\n9. Tagkopoulos, I., Liu, Y. C. & Tavazoie, S. Predictive behavior within microbial\\ngenetic networks. Science 320, 1313–1317 (2008).\\n10. Savageau, M. A. Escherichia coli habitats, cell-types, and molecular mechanisms of\\ngene control. Am\"\n },\n {\n \"doc_id\": 5758082833741322000,\n \"text\": \" shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions. For example, polyketide synthases are large enzymes that make antibiotics; they contain up to 100 independent domains that each catalyse one step in the overall process, like a step in an assembly line.\\nOne example of mutation is wild boar piglets. They are camouflage coloured and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in the melanocortin 1 receptor (MC1R) disrupt the pattern.\"\n },\n {\n \"doc_id\": 218121489928662980,\n \"text\": \" lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. \\\"Throwbacks\\\" such as these are known as atavisms.\\nThese charts depict the different types of genetic selection. On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the number of organisms. Group A is the original population and Group B is the population after selection. · Graph 1 shows directional selection, in which a single extreme\"\n },\n {\n \"doc_id\": 3206778632482066400,\n \"text\": \" exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Within cells, molecular machines such as the bacterial flagella and protein sorting machinery evolved by the recruitment of several pre-existing proteins that previously\"\n },\n {\n \"doc_id\": 8134024385157509000,\n \"text\": \". For example, polyketide synthases are large enzymes that make antibiotics; they contain up to 100 independent domains that each catalyse one step in the overall process, like a step in an assembly line.\\nOne example of mutation is wild boar piglets. They are camouflage coloured and show a characteristic pattern of dark and light longitudinal stripes. However, mutations in the melanocortin 1 receptor (MC1R) disrupt the pattern. The majority of pig breeds carry MC1R mutations disrupting wild-type colour and different mutations causing dominant black colouring.\\nIn asexual organisms,\"\n },\n {\n \"doc_id\": 1796167171500132000,\n \"text\": \" second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called coevolution. An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of toxin resistance in the snake.\\nNot all co-evolved interactions between species involve conflict. Many cases of\"\n },\n {\n \"doc_id\": 7033329867091455000,\n \"text\": \"Adaptivity and senescence[edit]\\nAn antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in their old age. An example of this is testosterone levels in male humans. Higher levels of this hormone lead to increased fitness in early life, while causing decreased fitness in later life due to a higher risk for prostate cancer. This is an example of antagonistic pleiotropy being an explanation for senescence. Senescence is the act of ageing in individuals; it's the failure over time of the individual's life processes by natural causes. Williams's theory has been the motivation for many of the experimental studies on the reasons for aging in the last 25 years. However, there is more than one theory out there for aging. The competing model to explain senescence is Medawar's \\\"mutation accumulation\\\" hypothesis, saying that \\\"over evolutionary time, late-acting mutations will accumulate at a much faster rate than early-acting mutation. These late-acting mutations will thus lead to declining viability and/or fertility as an organism ages.\\\" Medawar's theory is based around the older concept of selection shadow that had been discussed throughout the early 1900s and led to Medawar's theory after discussions with J. B. S. Haldane in the 1940s.\"\n },\n {\n \"doc_id\": 4136637634019466000,\n \"text\": \" process by which traits that enhance survival and reproduction become more common in successive generations of a population. It embodies three principles:\\nVariation exists within populations of organisms with respect to morphology, physiology and behaviour (phenotypic variation).\\nDifferent traits confer different rates of survival and reproduction (differential fitness).\\nThese traits can be passed from generation to generation (heritability of fitness).\\nMore offspring are produced than can possibly survive, and these conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors are more likely to pass on their traits\"\n },\n {\n \"doc_id\": 8617192067338247000,\n \"text\": \"Department of Molecular Genetics, Weizmann Institute of Science Rehovot 76100, Israel. 2\\nDepartment of Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv\\n69978, Israel. 3\\nDepartment of Molecular Cell Biology, Weizmann Institute of Science Rehovot 76100, Israel. 4\\nDepartment of Systems Biology, Harvard Medical School, Boston,\\nMassachusetts 02115, USA.\\n*These authors contributed equally to this work.\\nS1 S2\\nR1 R2\\nDirect\\nregulation\\nS1 S2\\nR1 R2\\nStochastic\\nswitching\\nR1 R2\\nS1 S2\\nR1 R2\\nSymmetrical\\nanticipatory\\nregulation\\nS1 S2\\nR1 R2\\nAsymmetrical\\nanticipatory\\nregulation\\nFigure 1 | Four possible regulation strategies in response to environmental\\nstimuli. Under direct regulation, each of the stimuli, S1 and S2, activates\\nexclusively the responses R1 and R2, respectively. Under stochastic\\nswitching, cells randomly sample either R1 or R2 in response to either S1 or\\nS2. Under symmetrical anticipatory regulation, each of the stimuli activates\\nboth responses. Under asymmetrical anticipatory regulation, the stimulus\\nthat usually appears first in the ecology activates both responses, whereas the\\nstimulus that appears later induces only the second response.\\nVol 460| 9 July 2009| doi:10.1038/nature08112\\n220\\n©2009 Macmillan Publishers Limited. All rights reserved\\ncost of preparation: pre-induction of genes needed to cope with S2 is\\ncostly and not beneficial during S1. This indicates that early induction\\nis preserved due to a future benefit that is expected to exceed the cost,\\nupon encounter with S2. Third, specificity: the conditioned response is\\nspecific to S1 and not to other unrelated stimuli, suggesting that anticipation evolved in response to the specific conditions of the ecological niche.\\nE. coli carbon source switch\\nDuring its life cycle E. coli alternates between two principal habitats,\\nintestines of mammals and water, sediment and soil10 (Supplementary Information). Focusing on the intestinal ecology reveals a predictable metabolic environment. Specifically, during passage along the\\ndigestive tract, exposure to lactose precedes exposure to another sugar,\\nmaltose11. We thus expect that this environment can select for asymmetric anticipation, so that bacteria that link the presence of lactose\\nwith future exposure to maltose are expected to activate the maltose\\ngenes already upon encounter with lactose. Such pre-induction is\\nexpected to allow cells to better use maltose upon its encounter.\\nWe start by examination of anticipatory behaviour at the molecular level. To test systematically the promoter activity of all relevant\\noperons in the presence of maltose and lactose we used an E. coli\\npromoter-fused green fluorescent protein (GFP) library previously\\ndescribed12. Our experiments revealed the unidirectional regulation\\npattern expected under a conditioned response strategy (Fig. 2a and\\nSupplementary Figs 1–4). Maltose operons, which are induced by\\nmaltose, are also induced, but to a lower level, by lactose. We note\\nthat the basic responsiveness of some of the maltose operons to\\nlactose is not restricted only to the E. coli strain used in our study13.\\nIn contrast, and as expected by our hypothesis, the lactose operon\\nshows no response to maltose (Fig. 2a and Supplementary Figs 1–4).\\nThe crucial question is whether the observed asymmetric response\\nstrategy that implies anticipation is indeed adaptive—that is, whether\\nit could be selected for during evolution. Following the criteria outlined previously, we first validated that the conditioned response\\nindeed provides an asymmetric fitness advantage in an alternating\\nsugar environment. We monitored cell growth on maltose after an\\ninitial growth phase on lactose. Reassuringly, we observed that the\\nwild-type strain displays a fitness advantage if growth on maltose was\\npreceded by growth on lactose (Fig. 3). We ensured that this advantage does not originate from the metabolic contribution of lactose as\\na carbon source (Supplementary Information). In addition, and as\\nrequired by our first principle, we found that pre-exposure to maltose\\nbefore growth on lactose does not improve fitness (Fig. 3).\\nOur second criterion for adaptiveness requires that pre-induction\\nof the maltose genes is maladaptive or neutral during growth on\\nlactose. This criterion is crucial to rule out the possibility that the\\nmaltose genes are induced during exposure to lactose simply because\\nthey are needed during that phase. Built into the classical conditioning paradigm from the cognitive context is the possible extinction of\\nthe association. To put in present case terms, it is expected that\\nrepeated exposure to lactose without consequent arrival of maltose\\nwould select for weakening of the conditioned response due to the\\nfutile cost of preparation. Accordingly, we have examined laboratory-evolved strains of E. coli, which grew for 500 generations on\\nhigh levels of lactose yet without exposure to maltose14. We measured\\nthe promoter activity of relevant operons in three lineages that\\nevolved independently under various lactose concentrations.\\nNotably, we found that, in all three lines, the maltose operons show\\nalmost no activity in response to lactose (Fig. 2b and Supplementary\\nFig. 5). Reassuringly, the strains seem to have preserved the ability to\\nactivate the maltose genes in response to maltose itself (Supplementary Fig. 6), indicating that only the asymmetrical cross talk between\\nthe two pathways was removed during this laboratory evolution\\nperiod. These results imply that the conditioning observed in the\\nwild-type strain is costly and that without a subsequent benefit that\\nshould exceed this cost, purifying selection acts to eliminate this cross\\ntalk. Consistent with the second criterion, the growth advantage LacZ MalE MalK MalP MalS MalZ\\nLacZ MalE MalK MalP MalS MalZ\\n0\\n0.25\\n0.50\\n0.75\\n\"\n },\n {\n \"doc_id\": 449019984534268200,\n \"text\": \" a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character\"\n },\n {\n \"doc_id\": 4510932946447888400,\n \"text\": \" on their traits to the next generation than those with traits that do not confer an advantage. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Consequences of selection include nonrandom mating and genetic hitchhiking.\\nThe central concept of natural selection is the evolutionary fitness of an organism. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, fitness is not the same as the total number of offspring: instead fitness is indicated by\"\n },\n {\n \"doc_id\": 6806517297278293000,\n \"text\": \" to the next generation than those with traits that do not confer an advantage. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Consequences of selection include nonrandom mating and genetic hitchhiking.\\nThe central concept of natural selection is the evolutionary fitness of an organism. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of\"\n },\n {\n \"doc_id\": 5481420576488986000,\n \"text\": \" bacteria, these are much rarer than in eukaryotes.\\nBacteria, as asexual organisms, inherit an identical copy of the parent's genome and are clonal. However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations arise from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. Genetic changes in bacterial genomes emerge from either random mutation during replication or \\\"stress-directed mutation\\\",\"\n },\n {\n \"doc_id\": 8197887054600267000,\n \"text\": \" to composition in diploid organisms such as mammals and (2) bacterial genomes frequently have AT-biased mutation.\\nContemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work showed that the original \\\"pressures\\\" theory assumes that evolution is based on standing variation: when evolution depends on events of mutation that introduce new alleles, mutational and developmental biases in the introduction of variation (arrival biases) can impose biases on evolution without requiring neutral evolution or high mutation rates.\\nSeveral studies report that the mutations implicated in adaptation reflect\"\n },\n {\n \"doc_id\": 241119448779526140,\n \"text\": \", but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard Holaspis guentheri, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Within cells, molecular machines such as the bacterial flagella and protein sorting machinery evolved by the recruitment of several pre-existing proteins that previously had different functions. Another example is the recruitment of\"\n },\n {\n \"doc_id\": 3685274842462513000,\n \"text\": \"Benefits and costs[edit]\\nAntagonistic pleiotropy has several negative consequences. It results in delayed adaptation, an altered path of evolution, and reduced adaptation of other traits. In addition, the overall benefit of alleles is cut down significantly (by about half) by pleiotropy. Still, antagonistic pleiotropy has some evolutionary benefits. In fact, the conservation of genes is directly related to the pleiotropic character of an organism. This implies that genes that control for multiple traits, even if the traits have different implications for the organism's fitness, have more staying power in an evolutionary context.\"\n },\n {\n \"doc_id\": 3731771800428118500,\n \"text\": \" trait works in some cases, most traits are influenced by multiple genes in a quantitative or epistatic manner.\\nEvolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is very similar among all individuals of that species. However, discoveries in the field of evolutionary developmental biology have demonstrated that even relatively small differences in genotype can lead to dramatic differences in phenotype both within\"\n },\n {\n \"doc_id\": 4002705701123597300,\n \"text\": \"Text adapted from OpenStax Biology 2e and used under a [ Creative Commons\\nAttribution License 4.0 ](https://creativecommons.org/licenses/by/4.0/).\"\n },\n {\n \"doc_id\": 6342130594967206000,\n \"text\": \" as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed. These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population,\"\n },\n {\n \"doc_id\": 7335190644482435000,\n \"text\": \" reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles. It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes.\\nCoevolution\\n\\nFurther information: Coevolution\\nThe common garter snake has evolved resistance to the defensive substance tetrodotoxin in its amphibian prey.\\nInteractions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a\"\n },\n {\n \"doc_id\": 221831125176784060,\n \"text\": \"As a constraint on perfection[edit]\\nAntagonistic pleiotropy is one of the several reasons evolutionary biologists give for organisms never being able to reach perfection through natural selection. Antagonistically pleiotropic genes are the explanation for fitness trade-offs. This means that genes that are pleiotropic control for some beneficial traits and some detrimental traits; thus, if they happen to persist through natural selection, this will prevent organisms from reaching perfection because if they possess the benefits of the gene, they must also possess the imperfections or faults. An example of this would be female rodents that live in a nest with other females and may end up feeding young that are not theirs due to their intense parental drive. This strong parental drive will be selected for, but the organisms will still make the mistake of feeding young that are not theirs and misallocating their resources.\"\n },\n {\n \"doc_id\": 2348511018954172400,\n \"text\": \" and between species.\\nAn individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely.\\nMutations are changes in the DNA sequence of a cell's genome and are the ultimate source of genetic variation in all organisms. When mutations occur,\"\n },\n {\n \"doc_id\": 53925992549567576,\n \"text\": \"Go to:\\nAcknowledgments\\nWe thank Ford Doolittle and Susan Rosenberg for organizing this series of papers on “How Microbes ‘Jeopardize’ the Modern Synthesis.”\"\n },\n {\n \"doc_id\": 4519866947457956400,\n \"text\": \" genotype can lead to dramatic differences in phenotype both within and between species.\\nAn individual organism's phenotype results from both its genotype and the influence of the environment it has lived in. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point of fixation—when it either disappears from the population or replaces the ancestral allele entirely.\\nMutation\\n\\nMain article: Mutation\\nDuplication of part of a chromosome\"\n },\n {\n \"doc_id\": 2202892938114897200,\n \"text\": \"ogenic strains of human flu\\nViruses undergo genetic change by several mechanisms. These include a process called antigenic drift where individual bases in the DNA or RNA mutate to other bases. Most of these point mutations are \\\"silent\\\"—they do not change the protein that the gene encodes—but others can confer evolutionary advantages such as resistance to antiviral drugs. Antigenic shift occurs when there is a major change in the genome of the virus. This can be a result of recombination or reassortment. When this happens with influenza viruses, pandemics might result. RNA viruses often\"\n },\n {\n \"doc_id\": 2482187478625555500,\n \"text\": \" and communication among cells can lead to heritable variations that may underlay some of the mechanics in developmental plasticity and canalisation. Heritability may also occur at even larger scales. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment. This generates a legacy of effects that modify and feed back into the selection regime of subsequent generations. Other examples of heritability in evolution that are not under the direct control of genes include the inheritance of cultural traits and symbiogenesis.\"\n },\n {\n \"doc_id\": 7456392705571888000,\n \"text\": \" coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\\nGene flow\\n\\nFurther information: Gene flow\\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the\"\n },\n {\n \"doc_id\": 4659267878746529000,\n \"text\": \"ARTICLES\\nAdaptive prediction of environmental\\nchanges by microorganisms\\nAmir Mitchell1\\n, Gal H. Romano2\\n, Bella Groisman1\\n, Avihu Yona1\\n, Erez Dekel3\\n, Martin Kupiec2\\n, Orna Dahan1\\n*\\n& Yitzhak Pilpel1,4*\\nNatural habitats of some microorganisms may fluctuate erratically, whereas others, which are more predictable, offer the\\nopportunity to prepare in advance for the next environmental change. In analogy to classical Pavlovian conditioning,\\nmicroorganisms may have evolved to anticipate environmental stimuli by adapting to their temporal order of appearance.\\nHere we present evidence for environmental change anticipation in two model microorganisms, Escherichia coli and\\nSaccharomyces cerevisiae. We show that anticipation is an adaptive trait, because pre-exposure to the stimulus that typically\\nappears early in the ecology improves the organism’s fitness when encountered with a second stimulus. Additionally, we\\nobserve loss of the conditioned response in E. coli strains that were repeatedly exposed in a laboratory evolution experiment\\nonly to the first stimulus. Focusing on the molecular level reveals that the natural temporal order of stimuli is embedded in the\\nwiring of the regulatory network—early stimuli pre-induce genes that would be needed for later ones, yet later stimuli only\\ninduce genes needed to cope with them. Our work indicates that environmental anticipation is an adaptive trait that was\\nrepeatedly selected for during evolution and thus may be ubiquitous in biology.\\nMicroorganisms are constantly faced with environmental stimuli and\\nstresses. The cellular response to such challenges has been intensively\\nstudied in several model organisms1–4. The simplest response strategy\\nto a stimulus is to monitor the environment and to respond directly\\nto it using designated mechanisms (Fig. 1). The environmental stress\\nresponse in yeast represents a more complicated strategy in which the\\nresponses to many stresses are partially overlapping1,2. Theoretical\\nwork has shown that when a population of microorganisms evolves\\nunder erratic environmental fluctuations, cells may not effectively\\nmonitor the environment, but rather use stochasticity to randomly\\nalternate between potential states5 (Fig. 1). Stochastic switching\\nmight thus ensure that a portion of the population is prepared in\\nadvance for the unpredicted challenge6,7. However, other, more predictable, environments offer organisms the opportunity to adopt an\\nalternative regulation strategy of anticipating an environmental\\nchange based on a preceding signal. The capacity of some complex\\nmulticellular eukaryotes to capture the statistics that govern the temporal connection between events in their environment, known as\\nclassical Pavlovian conditioning, serves as a central paradigm in the\\nstudy of learning8\\n. Here we ask whether genetic regulatory networks\\nof microorganisms adaptively evolved to capture the temporal connections between subsequent stimuli in their environment. Most\\nrecently, ‘anticipatory regulation’ was discovered9 (Fig. 1)—an association between environmental changes in bacteria. Specifically, this\\nstudy investigated the response of E. coli to temperature increase that\\nis followed by a drop in oxygen availability upon its entry to the\\ndigestive tract. Interestingly, these two signals show a symmetrical\\nassociative regulation pattern—each signal affects the expression of\\ngenes needed to cope with both (Fig. 1). Remarkably, the authors\\nsuccessfully decoupled the two responses during a laboratory evolution experiment in which the two signals were presented out-ofphase from one another. The ability to decouple the two responses\\nis an indication that the coupling seen in the wild type is not a trivial\\ncombined response to the two stresses.\\nHere we show that biological systems that react to a unidirectional\\ntemporal order of environmental changes may manifest a more elaborate predictive capacity. This capacity is reflected in a corresponding\\nasymmetric response strategy between subsequent stimuli, denoted S1\\nand S2 hereafter, and their designated responses R1 and R2, respectively. The first stimulus, S1, activates both responses, R1 and R2, yet\\nbecause the second stimulus, S2, does not predict the appearance of S1,\\nit only activates its own response (Fig. 1). We propose three criteria to\\ndetermine whether the observed cross-regulation pattern forms an\\nadaptive anticipatory response strategy that could be selected for by\\nevolution. First, asymmetric fitness advantage: pre-exposure to S1\\nincreases the fitness under S2, yet pre-exposure to S2 should not\\nenhance fitness upon subsequent growth on S1. This ensures that\\nthe natural order of stimuli was captured during evolution. Second,\\n1\"\n },\n {\n \"doc_id\": 2404171571940034000,\n \"text\": \"Mechanism of action[edit]\\n(a) The survival rate within a population decreases with age, while the reproduction rate remains constant. (b) The reproduction probability peaks early in life, at sexual maturity, and then steadily decreases as an individual ages, with the remaining share of the population decreasing with age as they enter the selection shadow.\\nAmongst almost all populations, the likelihood that an individual will reproduce is related directly to their age. Starting at 0 at birth, the probability increases to its maximum in young adulthood once sexual maturity has been reached, before gradually decreasing with age. This decrease is caused by the increasing likelihood of death due to external pressures such as predation or illness, as well as the internal pressures inherent to organisms that experience senescence. In such cases deleterious mutations which are expressed early on are strongly selected against due to their major impact on the number of offspring produced by that individual. Mutations that present later in life, by contrast, are relatively unaffected by selective pressure, as their carriers have already passed on their genes, assuming they survive long enough for the mutation to be expressed at all. The result, as predicted by Medawar, is that deleterious late-life mutations will accumulate and result in the evolution of aging as it is known colloquially. This concept is portrayed graphically by Medawar through the concept of a \\\"selection shadow\\\". The shaded region represents the 'shadow' of time during which selective pressure has no effect. Mutations that are expressed within this selection shadow will remain as long as reproductive probability within that age range remains low.\"\n },\n {\n \"doc_id\": 6014319786151096000,\n \"text\": \"Applications\\n\\nMain articles: Applications of evolution, Selective breeding, and Evolutionary computation\\nConcepts and models used in evolutionary biology, such as natural selection, have many applications.\\nArtificial selection is the intentional selection of traits in a population of organisms. This has been used for thousands of years in the domestication of plants and animals. More recently, such selection has become a vital part of genetic engineering, with selectable markers such as antibiotic resistance genes being used to manipulate DNA. Proteins with valuable properties have evolved by repeated rounds of mutation and selection (for example modified enzymes\"\n },\n {\n \"doc_id\": 6223078474715175000,\n \"text\": \" enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.\\nAn area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to produce new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. It is also possible for structures that have been lost in evolution to\"\n },\n {\n \"doc_id\": 4385609516791664600,\n \"text\": \"26. Braun, E. & Brenner, N. Transient responses and adaptation to steady state in a\\neukaryotic gene regulation system. Phys. Biol. 1, 67–76 (2004).\\n27. Winzeler, E. A. et al. Functional characterization of the S. cerevisiae genome by\\ngene deletion and parallel analysis. Science 285, 901–906 (1999).\\nSupplementary Information is linked to the online version of the paper at\\nwww.nature.com/nature.\\nAcknowledgements We thank S. Trattner\"\n },\n {\n \"doc_id\": 8650248059159904000,\n \"text\": \" the organism, its position in the food chain and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection.\\nNatural selection can act at different levels of organisation, such as genes, cells, individual organisms, groups of organisms and species. Selection can act at multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection, may allow the evolution of cooperation.\\nGenetic drift\"\n },\n {\n \"doc_id\": 6174758067960444000,\n \"text\": \" the proportion of subsequent generations that carry an organism's genes. For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.\\nIf an allele increases fitness more than the other alleles of that gene, then with each generation this allele has a higher probability of becoming common within the population. These traits are said to be \\\"selected for.\\\" Examples of traits that can increase fitness are enhanced survival and increased fecundity. Conversely, the lower fitness caused by\"\n },\n {\n \"doc_id\": 9021716622346336000,\n \"text\": \" between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.\\nIn the 1920s and 1930s, the modern synthesis connected natural selection and population genetics, based on Mendelian inheritance, into a unified theory that included random genetic drift, mutation, and gene flow. This new version of evolutionary theory focused on changes in allele frequencies in population. It explained patterns observed across species in populations, through fossil transitions in palaeontology.\\nSince then, further syntheses have extended evolution's explanatory power in the light of numerous discoveries, to cover biological phenomena across the whole\"\n },\n {\n \"doc_id\": 9181138567613823000,\n \"text\": \"-adaptive forces in driving evolutionary change is an area of current research.\\nMutation bias is usually conceived as a difference in expected rates for two different kinds of mutation, e.g., transition-transversion bias, GC-AT bias, deletion-insertion bias. This is related to the idea of developmental bias. Haldane and Fisher argued that, because mutation is a weak pressure easily overcome by selection, tendencies of mutation would be ineffectual except under conditions of neutral evolution or extraordinarily high mutation rates. This opposing-pressures argument was long used to dismiss the possibility of internal\"\n },\n {\n \"doc_id\": 17671705224215012,\n \"text\": \"Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome. Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.\\nNew genes can be generated from an ancestral gene when a duplicate\"\n },\n {\n \"doc_id\": 1139347458278782600,\n \"text\": \" a gradual change in the flowering time of the metal-resistant plants, which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.\\nFinally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences\"\n },\n {\n \"doc_id\": 3997471738175323600,\n \"text\": \" increase or decrease in genome size.\\nHowever, mutational hypotheses for the evolution of composition suffered a reduction in scope when it was discovered that (1) GC-biased gene conversion makes an important contribution to composition in diploid organisms such as mammals and (2) bacterial genomes frequently have AT-biased mutation.\\nContemporary thinking about the role of mutation biases reflects a different theory from that of Haldane and Fisher. More recent work showed that the original \\\"pressures\\\" theory assumes that evolution is based on standing variation: when evolution depends on events of mutation that introduce new alleles,\"\n },\n {\n \"doc_id\": 8660851524417328000,\n \"text\": \"Role in disease[edit]\\nThe survival of many serious genetic disorders in our long evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are defined by the existence of deleterious alleles, then natural selection acting over evolutionary time would result in a lower frequency of mutations than are currently observed. In a recent article, Carter and Nguyen identify several genetic disorders, arguing that far from being a rare phenomenon, antagonistic pleiotropy might be a fundamental mechanism for the survival of these non-optimal alleles.\\nIn one of these studies, 99 individuals with Laron syndrome (a rare form of dwarfism) were monitored alongside their non-dwarf kin for a period of ten years. Patients with Laron syndrome possess one of three genotypes for the growth hormone receptor gene (GHR). Most patients have an A->G splice site mutation in position 180 in exon 6. Some others possess a nonsense mutation (R43X), while the rest are heterozygous for the two mutations. Laron syndrome patients experienced a lower incidence of cancer mortality and diabetes compared to their non-dwarf kin. This suggests a role for antagonistic pleiotropy, whereby a deleterious mutation is preserved in a population because it still confers some survival benefit.\\nAnother instance of antagonistic pleiotropy is manifested in Huntington's disease, a rare neurodegenerative disorder characterized by a high number of CAG repeats within the Huntingtin gene. The onset of Huntington's is usually observed post-reproductive age and generally involves involuntary muscle spasms, cognitive difficulties and psychiatric problems. Incidentally, the high number of CAG repeats is associated with increased activity of p53, a tumor suppressing protein that participates in apoptosis. It has been hypothesized that this explains the lower rates of cancer among Huntington's patients. Huntington's disease is also correlated with high fecundity.\\nAdditionally, it was found that individuals with a higher pro-inflammatory ratio TNFα/IL-10 had a significantly higher incidence of death due to cardiovascular disease in old age. Yet, it was hypothesized that this genotype was prevalent because higher ratios of TNFα/IL-10 allow individuals to more effectively combat infection during reproductive years.\\nSickle cell anemia, Beta-thalassemia, and cystic fibrosis are some other examples of the role antagonistic pleiotropy may play in genetic disorders.\"\n },\n {\n \"doc_id\": 832615000926436200,\n \"text\": \"We then tested whether induction of the conditioned gene set is\\nneutral in heat shock, as required by the second criterion. Towards\\nthis aim we collected 29 strains, each of which is deleted for one of the\\ngenes from the cluster described above, and tested their sensitivity to\\nheat shock and oxidative stress (Methods). Analysis of survival ratios\\nreveals that whereas 65% of the strains show increased sensitivity to\\noxidative stress, none shows a significantly increased sensitivity to\\nheat shock relative to the wild-type strain (Fig. 5b and Supplementary\\nFig. 7). This is a direct indication that the expression of many of the\\nheat-induced genes is at least neutral, if not maladaptive, under heat\\nshock. Taken together with the fact that these genes are essential\\nunder oxidative stress, this experiment corroborates the claim that\\nan early preparation to oxidative stress during heat shock is adaptive\\nin this system.\\nFuture outlook\\nAn open question is how prevalent anticipatory regulation is in the\\nworld of microorganisms. One way to address this issue is to develop\\na mathematical framework that estimates the potential advantage of a\\nconditioned response in a given ecology. Intuitively, the added value\\nfrom anticipation depends both on environmental and on biological\\nparameters. Two cellular parameters, gain and cost, should be considered. A conditioned response is beneficial provided that benefit\\ngained from anticipation exceeds the cost of early preparation. These\\nparameters are in turn dependent on the typical time constants of the\\nenvironment, for example, the time gap between the appearances of\\nthe two stimuli. Additionally, predictable environments in which\\nthe stimuli are often sequentially coupled promote anticipation. A\\nsimple equation captures the relationship between these variables\\nand their effect on the net fitness:\\nDF 5 p(B(Dt) 2 C(Dt)) 2 (1 2 p)(C(tS1)),\\nwhere DF is the difference in fitness between an anticipating cell and a\\ncell that adheres to direct regulation (Fig. 1), p is the probability that\\nS2 will occur given that S1 occurred, and B and C are functions\\ndescribing the benefit and cost of early preparation, respectively. Dt\\nis the length of the time interval between the appearance of S1 and S2,\\nand tS1 is the duration of the first stimulus. The dependence of fitness\\non the interplay between the key factors is shown in Supplementary\\nFig. 8. We note that quantitative predictions from the above phenomenological equation can be drawn by incorporating into the\\nmodel biologically meaningful and realistic parameters, such as measurable gain, cost and timescales (A.M. et al. in preparation).\"\n },\n {\n \"doc_id\": 4246999504903770000,\n \"text\": \" be a strategy to circumvent phage resistance.\"\n },\n {\n \"doc_id\": 7326570392395027000,\n \"text\": \" sexual reproduction as a means to enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment. Another hypothesis is that sexual reproduction is primarily an adaptation for promoting accurate recombinational repair of damage in germline DNA, and that increased diversity is a byproduct of this process that may sometimes be adaptively beneficial.\\nGene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene flow can be caused by the movement of individuals between separate populations of organisms, as might\"\n },\n {\n \"doc_id\": 8383252946560158000,\n \"text\": \"The antagonistic pleiotropy hypothesis was first proposed by George C. Williams in 1957 as an evolutionary explanation for senescence. Pleiotropy is the phenomenon where one gene controls more than one phenotypic trait in an organism. A gene is considered to possess antagonistic pleiotropy if it controls more than one trait, where at least one of these traits is beneficial to the organism's fitness early on in life and at least one is detrimental to the organism's fitness later on due to a decline in the force of natural selection. The theme of G. C. William's idea about antagonistic pleiotropy was that if a gene caused both increased reproduction in early life and aging in later life, then senescence would be adaptive in evolution. For example, one study suggests that since follicular depletion in human females causes both more regular cycles in early life and loss of fertility later in life through menopause, it can be selected for by having its early benefits outweigh its late costs.\"\n },\n {\n \"doc_id\": 8435702021988683000,\n \"text\": \"Drug induced[edit]\\nNeural adaptation can occur for other than natural means. Antidepressant drugs, such as those that cause down regulation of β-adrenergic receptors, can cause rapid neural adaptations in the brain. By creating a quick adaptation in the regulation of these receptors, it is possible for drugs to reduce the effects of stress on those taking the medication.\"\n },\n {\n \"doc_id\": 527421555664797440,\n \"text\": \"To conclude, as we observed that anticipation at the cellular level\\nappears to be adaptive both in prokaryotes (E. coli) and eukaryotes\\n(S. cerevisiae), and under remote systems such as sugar metabolism\\nand stress response, we propose that it may be prevalent in a diverse\\nrange of species. Asymmetric anticipation, along with associative\\nlearning9\\n, memory25 and physiological adaptation26, represent a\\npowerful set of tools used by microorganisms to achieve a sophisticated environmental response.\\nMETHODS SUMMARY\\nStrains. E. coli MG1655 was used for most experiments. An E. coli GFP reporter\\nlibrary12 was used to monitor operon expression. The evolved E. coli strains were\\ndescribed previously14\\nS. cerevisiae BY4741 (MATa; his3D1; leu2D0; met15D0; ura3D0) was used in all\\nyeast experiments. All deletion strains were obtained from the Saccharomyces\\nGenome Deletion Project27.\\nS. cerevisiae cross-protection. An overnight culture was diluted into fresh YPD\\nmedium and grown to a concentration of 2 3 106 cells per ml. Cells were diluted\\n1:2 into the first stress (S1). Transfer into the second stress (S2) was done by\\nadding treated medium after centrifugation. Stresses were calibrated to achieve a\\nmild effect (,50% survival) for S1 and a severe effect (,0.5% survival) for S2\\n(Supplementary Table 1). Samples were taken from cultures at the end of each\\ntreatment and plated. Each experiment was carried out in three repeats. The fold\\nprotection was calculated as described in Supplementary Equation (1).\\nE. coli fitness advantage. Overnight cultures were diluted into M9-Gly plus S1\\n(for example, lactose; treated) or M9-Gly (untreated). After 3 h, population size\\nwas determined and used to dilute (,1:100) an equal amount of cells into a new\\ngrowth medium containing low levels of S2 (for example, maltose) as a sole\\ncarbon source. To account for a potential metabolic effect of residual S1, an\\nidentical trace amount of S1 was added to the untreated growth medium. The\\nratio treated/untreated after 2 h of growth was used as an indication of fitness\\nadvantage originating from pre-exposure to S1. To rule out the possibility that\\nthe growth advantage observed due to early exposure to lactose originates from\\nits metabolic value rather than its rule as a conditioning signal, we preformed a\\ncontrol experiment (Supplementary Information).\\nFull Methods and any associated references are available in the online version of\\nthe paper at www.nature.com/nature.\\nReceived 25 March; accepted 7 May 2009.\\nPublished online 17 June 2009.\\n1. Causton, H. C. et al. Remodeling of yeast genome expression in response to\\nenvironmental changes. Mol. Biol. Cell 12, 323–337 (2001).\\n2. Gasch, A. P. et al. Genomic expression programs in the response of yeast cells to\\nenvironmental changes. Mol. Biol. Cell 11, 4241–4257 (2000).\\n3. Cheung, K. J. et al. A microarray-based antibiotic screen identifies a regulatory\\nrole for supercoiling in the osmotic stress response of Escherichia coli. Genome Res.\\n13, 206–215 (2003).\\n4. Tirosh, I., Weinberger, A., Carmi, M. & Barkai, N. A genetic signature of\\ninterspecies variations in gene expression. Nature Genet. 38, 830–834 (2006).\\nNATURE| Vol 460| 9 July 2009 ARTICLES\\n223\\n\"\n },\n {\n \"doc_id\": 1736052534461021000,\n \"text\": \" the European Research Council ‘Ideas Program’ and the Ben May\\nFoundation for grant support. M.K. was supported from grants from the Israel\\nScience Foundation and the Israeli Ministry of Science and Technology.\\nAuthor Contributions A.M. raised the original idea and performed all the\\nexperiments; G.R., B.G. and A.Y. participated in experiments; E.D. evolved the E. coli\\nstrain; A.M., O.D. and Y.P. designed the experiments; A.M., M.K., O\"\n },\n {\n \"doc_id\": 2662230032262301000,\n \"text\": \" the transfer of heritable variations along the germline. To explain how new variants originate, de Vries developed a mutation theory that led to a temporary rift between those who accepted Darwinian evolution and biometricians who allied with de Vries. In the 1930s, pioneers in the field of population genetics, such as Ronald Fisher, Sewall Wright and J. B. S. Haldane set the foundations of evolution onto a robust statistical philosophy. The false contradiction between Darwin's theory, genetic mutations, and Mendelian inheritance was thus reconciled.\\nThe'modern synthesis'\\n\\n\"\n },\n {\n \"doc_id\": 3717223793901211600,\n \"text\": \"Sources of variation\\n\\nMain article: Genetic variation\\n\\nFurther information: Genetic diversity and Population genetics\\nWhite peppered mothBlack morph in peppered moth evolution\\nEvolution can occur if there is genetic variation within a population. Variation comes from mutations in the genome, reshuffling of genes through sexual reproduction and migration between populations (gene flow). Despite the constant introduction of new variation through mutation and gene flow, most of the genome of a species is very similar among all individuals of that species. However, discoveries in the field of evolutionary developmental biology have demonstrated that even relatively small differences in\"\n },\n {\n \"doc_id\": 887670927609664300,\n \"text\": \" effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.\\nSexual selection\\n\\nFurther information: Sexual selection\\nMale moor frogs become blue during the height of mating season. Blue reflectance may be a form of intersexual communication. It is hypothesised that males with brighter blue coloration may signal greater sexual and genetic fitness.\\nA special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the\"\n },\n {\n \"doc_id\": 8389227544347302000,\n \"text\": \" selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilising selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. This would, for example, cause organisms to eventually have a similar height.\\nNatural selection most generally makes nature the measure against which individuals and individual traits, are more or less likely to survive. \\\"Nature\\\" in this sense refers\"\n },\n {\n \"doc_id\": 3428675185126628000,\n \"text\": \". Many human diseases are not static phenomena, but capable of evolution. Viruses, bacteria, fungi and cancers evolve to be resistant to host immune defences, as well as to pharmaceutical drugs. These same problems occur in agriculture with pesticide and herbicide resistance. It is possible that we are facing the end of the effective life of most of available antibiotics and predicting the evolution and evolvability of our pathogens and devising strategies to slow or circumvent it is requiring deeper knowledge of the complex forces driving evolution at the molecular level.\\nIn computer science, simulations of evolution using evolutionary algorithms and artificial life started\"\n },\n {\n \"doc_id\": 3972252304274929000,\n \"text\": \" inherited in a group is called a haplotype. This can be important when one allele in a particular haplotype is strongly beneficial: natural selection can drive a selective sweep that will also cause the other alleles in the haplotype to become more common in the population; this effect is called genetic hitchhiking or genetic draft. Genetic draft caused by the fact that some neutral genes are genetically linked to others that are under selection can be partially captured by an appropriate effective population size.\\nA special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness\"\n },\n {\n \"doc_id\": 5937972175565269000,\n \"text\": \" biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes, enzymes, and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetic and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of Escherichia coli metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineer bacteria\"\n },\n {\n \"doc_id\": 2469823221813689300,\n \"text\": \" that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass Anthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced\"\n },\n {\n \"doc_id\": 8940875874985204000,\n \"text\": \". Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for night vision; all four are descended from a single ancestral gene.\\nNew genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system;\"\n },\n {\n \"doc_id\": 1185125279276353300,\n \"text\": \", introducing foreign DNA in a process known as transduction. Many types of bacteriophage exist; some infect and lyse their host bacteria, while others insert into the bacterial chromosome. Bacteria resist phage infection through restriction modification systems that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. Third, bacteria can transfer genetic material through direct cell contact via conjugation.\\nIn ordinary circumstances, transduction\"\n },\n {\n \"doc_id\": 5136899540305229000,\n \"text\": \" one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA, a phenomenon termed de novo gene birth.\\nThe generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions (exon shuffling). When new genes are assembled from shuffling pre-existing parts, domains act as modules with simple independent functions, which can be mixed together to produce new combinations with new and complex functions\"\n },\n {\n \"doc_id\": 1560288069817212400,\n \"text\": \" phenotype is favoured. · Graph 2 depicts stabilizing selection, where the intermediate phenotype is favoured over the extreme traits. · Graph 3 shows disruptive selection, in which the extreme phenotypes are favoured over the intermediate.\\nNatural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most\"\n },\n {\n \"doc_id\": 8072226226985974000,\n \"text\": \"les of the same gene.\\nMHC allelic diversity has challenged evolutionary biologists for explanation. Most posit balancing selection (see polymorphism (biology)), which is any natural selection process whereby no single allele is absolutely most fit, such as frequency-dependent selection and heterozygote advantage. Pathogenic coevolution, as a type of balancing selection, posits that common alleles are under greatest pathogenic pressure, driving positive selection of uncommon alleles—moving targets, so to say, for pathogens. As pathogenic pressure on the previously common alleles decreases, their frequency in the population stabilizes\"\n },\n {\n \"doc_id\": 6176450517828379000,\n \"text\": \" seem to fit some genetic variation seen in nature. A better-supported version of this model is the nearly neutral theory, according to which a mutation that would be effectively neutral in a small population is not necessarily neutral in a large population. Other theories propose that genetic drift is dwarfed by other stochastic forces in evolution, such as genetic hitchhiking, also known as genetic draft. Another concept is constructive neutral evolution (CNE), which explains that complex systems can emerge and spread into a population through neutral transitions due to the principles of excess capacity, presuppression, and ratcheting, and\"\n },\n {\n \"doc_id\": 7684727185464362000,\n \"text\": \" edits, from single nucleotide base pairs to whole genome or gene networks simultaneously with results in a matter of days.\\nMAGE experiments can be divided into three classes, characterized by varying degrees of scale and complexity: (i) many target sites, single genetic mutations; (ii) single target site, many genetic mutations; and (iii) many target sites, many genetic mutations. An example of class three was reflected in 2009, where Church and colleagues were able to program Escherichia coli to produce five times the normal amount of lycopene, an antioxidant normally found in tomato seeds and\"\n },\n {\n \"doc_id\": 2356343650515559400,\n \"text\": \" copy mutates and acquires a new function. This process is easier once a gene has been duplicated because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function. Other types of mutations can even generate entirely new genes from previously noncoding DNA, a phenomenon termed de novo gene birth.\\nThe generation of new genes can also involve small parts of several genes being duplicated, with these fragments then recombining to form new combinations with new functions (exon shuffling). When new genes are assembled from\"\n }\n]","isConversational":false}}},{"rowIdx":98,"cells":{"query":{"kind":"string","value":"I recently read about a study where researchers found that all *Klebsiella pneumoniae* isolated from car door handles were resistant to Augmentin and Ceftriaxone. Given that these surfaces can act as a reservoir for resistant bacteria, how significant is the risk of infection from touching such surfaces in public settings? Additionally, how effective are common hand hygiene practices, like using hand soap from refillable dispensers, in preventing the spread of Gram-negative bacteria in these environments? Are there any specific guidelines or recommendations for minimizing the risk of infection from resistant bacteria in public spaces?\n\nThanks in advance for any insights!"},"positive_passages":{"kind":"list like","value":[{"doc_id":6669481434202111000,"text":" µg), Streptomycin (30 µg) and Cloxacillin (30 µg). The results showed a significant frequency of occurrence of Staphylococcus aureus at 35% and Klebsiella pneumoniae having least at 5%. From car door handles, S. epidermidis recorded 37% while K. pneumoniae recorded the least with 17.4%. The isolates exhibited resistance to antibiotics including Augmentin and Ceftriaxone (≤22 mm) while they were more susceptible to Ofloxacin (≥16 mm"},{"doc_id":4723040126690596000,"text":"Ekeneokot E. UcheUsman-Wali Maryam\nMultidrug-resistant bacteria have posed a public health concern over the years, especially with the difficulty and cost of treatment of infections they cause. Fomites such as door handles are thus potent means through which pathogens are transmitted from one person to another as contact with them is made. This study thus involves isolating antibiotic-resistant bacteria from car and office door handles in a university environment. Using the simple random sampling method, twenty samples (20) from car door handles and twenty samples (20) from office door"},{"doc_id":6081983879844188000,"text":"\nC. Testimonies Adebayo-OlajideA. Dakoru GoodheadEkeneokot E. UcheUsman-Wali Maryam\nView\nShow abstract\n... Diseases transmitted by direct physical contact are best prevented by effective hand washing (33). However, using contaminated hand soap from refillable dispensers can result in Gram-negative bacteria colonizing the hands in public settings, thus increasing daily risk of infection threshold (not shown) is 1 x 10 −6 (or 1 infection per 1,000,000 individuals)....\nEye infection risks from"},{"doc_id":7705991245379879000,"text":"Go to:\n4. Conclusions\nThe results demonstrate that handwashing with non-antibacterial soap is much more effective in removing bacteria from hands than handwashing with water only. Although handwashing with water alone reduced the presence of bacteria on hands substantially, the study supports the policy of many current hand hygiene campaigns promoting the use of soap [18,19]. The strong association between hand hygiene method and bacterial contamination of hands found in our study suggests that the prevalence of faecal indicator bacteria may also be used to monitor changes in hygiene behaviour in the general population, for example following hygiene promotion"},{"doc_id":7822207916778225000,"text":" were found after no handwashing in 44% of samples. Handwashing with water alone reduced the presence of bacteria to 23% (p < 0.001). Handwashing with plain soap and water reduced the presence of bacteria to 8% (comparison of both handwashing arms: p < 0.001). The effect did not appear to depend on the bacteria species. Handwashing with non-antibacterial soap and water is more effective for the removal of bacteria of potential faecal origin from hands than handwashing with water alone and should therefore be more useful for the prevention of transmission of"},{"doc_id":4093364935287199000,"text":".) were found after no handwashing in 44% of samples. Handwashing with water alone reduced the presence of bacteria to 23% (p < 0.001). Handwashing with plain soap and water reduced the presence of bacteria to 8% (comparison of both handwashing arms: p < 0.001). The effect did not appear to depend on the bacteria species. Handwashing with non-antibacterial soap and water is more effective for the removal of bacteria of potential faecal origin from hands than handwashing with water alone and should therefore be more useful for the prevention of"},{"doc_id":2489356507689180700,"text":"Substances used[edit]\nSoap and detergents[edit]\nRemoval of microorganisms from skin is enhanced by the addition of soaps or detergents to water. Soap and detergents are surfactants that kill microorganisms by disorganizing their membrane lipid bilayer and denaturing their proteins. It also emulsifies oils, enabling them to be carried away by running water.\nSolid soap[edit]\nSolid soap, because of its reusable nature, may hold bacteria acquired from previous uses. A small number of studies which have looked at the bacterial transfer from contaminated solid soap have concluded transfer is unlikely as the bacteria are rinsed off with the foam. The CDC still states \"liquid soap with hands-free controls for dispensing is preferable\".\nAntibacterial soap[edit]\nAntibacterial soaps have been heavily promoted to a health-conscious public. To date, there is no evidence that using recommended antiseptics or disinfectants selects for antibiotic-resistant organisms in nature. However, antibacterial soaps contain common antibacterial agents such as triclosan, which has an extensive list of resistant strains of organisms. So, even if antibiotic resistant strains are not selected for by antibacterial soaps, they might not be as effective as they are marketed to be. Besides the surfactant and skin-protecting agent, the sophisticated formulations may contain acids (acetic acid, ascorbic acid, lactic acid) as pH regulator, antimicrobially active benzoic acid and further skin conditioners (aloe vera, vitamins, menthol, plant extracts).\nA 2007 meta-analysis from the University of Oregon School of Public Health indicated that plain soaps are as effective as consumer-grade anti-bacterial soaps containing triclosan in preventing illness and removing bacteria from the hands. Dissenting, a 2011 meta-analysis in the Journal of Food Protection argued that when properly formulated, triclosan can grant a small but detectable improvement, as can chlorhexidine gluconate, iodophor, or povidone.\nWarm water[edit]\nHot water that is still comfortable for washing hands is not hot enough to kill bacteria. Bacteria grow much faster at body temperature (37 °C). WHO considers warm soapy water to be more effective than cold, soapy water at removing natural oils which hold soils and bacteria. But CDC mentions that warm water causes skin irritations more often and its ecological footprint is more significant. Water temperatures from 4 to 40 °C do not differ significantly regarding removal of microbes. The most important factor is proper scrubbing.\nContrary to popular belief, scientific studies have shown that using warm water has no effect on reducing the microbial load on hands. Using hot water for handwashing can even be regarded as a waste of energy.\nAntiseptics (hand sanitizer)[edit]\nHand disinfection procedure according to the German standard DIN EN 1500\n\nMain article: Hand sanitizer\nIn situations where hand washing with soap is not an option (e.g., when in a public place with no access to wash facilities), a waterless hand sanitizer such as an alcohol hand gel can be used. They can be used in addition to hand washing to minimize risks when caring for \"at-risk\" groups. To be effective, alcohol hand gels should contain not less than 60%v/v alcohol. Enough hand antiseptic or alcohol rub must be used to thoroughly wet or cover both hands. The front and back of both hands and between and the ends of all fingers must be rubbed for approximately 30 seconds until the liquid, foam or gel is dry. Finger tips must be washed well too, rubbing them in both palms.\nA hand sanitizer or hand antiseptic is a non-water-based hand hygiene agent. In the late 1990s and early part of the 21st century, alcohol rub non-water-based hand hygiene agents (also known as alcohol-based hand rubs, antiseptic hand rubs, or hand sanitizers) began to gain popularity. Most are based on isopropyl alcohol or ethanol formulated together with a thickening agent such as Carbomer (polymer of acrylic acid) into a gel, or a humectant such as glycerin into a liquid, or foam for ease of use and to decrease the drying effect of the alcohol. Adding diluted hydrogen peroxide increases further the antimicrobial activity.\nHand sanitizers are most effective against bacteria and less effective against some viruses. Alcohol-based hand sanitizers are almost entirely ineffective against norovirus (or Norwalk) type viruses, the most common cause of contagious gastroenteritis.\nUS Centers for Disease Control and Prevention recommend hand washing with soap over hand sanitizer rubs, particularly when hands are visibly dirty. The increasing use of these agents is based on their ease of use and rapid killing activity against micro-organisms; however, they should not serve as a replacement for proper hand washing unless soap and water are unavailable. Despite their effectiveness, non-water agents do not cleanse the hands of organic material, but simply disinfect them. It is for this reason that hand sanitizers are not as effective as soap and water at preventing the spread of many pathogens, since the pathogens remain on the hands.\nWipes[edit]\nHand washing using hand sanitizing wipes is an alternative during traveling in the absence of soap and water. Alcohol-based hand sanitizer should contain at least 60% alcohol.\nAsh or mud[edit]\nMany people in low-income communities cannot afford soap and use ash or soil instead. The World Health Organization recommended ash or sand as an alternative to soap when soap is not available. Use of ash is common in rural areas of developing countries and has in experiments been shown at least as effective as soap for removing pathogens. However, evidence to support the use of ash to wash hands is of poor quality. It is not clear if washing hands with ash is effective at reducing viral or bacterial spreading compared to washing with mud, not washing, or with washing with water alone. One concern is that if the soil or ash is contaminated with microorganisms it may increase the spread of disease rather than decrease it, however, there is also no clear evidence to determine the level of risk. Like soap, ash is also a disinfecting agent because in contact with water, it forms an alkaline solution."}],"string":"[\n {\n \"doc_id\": 6669481434202111000,\n \"text\": \" µg), Streptomycin (30 µg) and Cloxacillin (30 µg). The results showed a significant frequency of occurrence of Staphylococcus aureus at 35% and Klebsiella pneumoniae having least at 5%. From car door handles, S. epidermidis recorded 37% while K. pneumoniae recorded the least with 17.4%. The isolates exhibited resistance to antibiotics including Augmentin and Ceftriaxone (≤22 mm) while they were more susceptible to Ofloxacin (≥16 mm\"\n },\n {\n \"doc_id\": 4723040126690596000,\n \"text\": \"Ekeneokot E. UcheUsman-Wali Maryam\\nMultidrug-resistant bacteria have posed a public health concern over the years, especially with the difficulty and cost of treatment of infections they cause. Fomites such as door handles are thus potent means through which pathogens are transmitted from one person to another as contact with them is made. This study thus involves isolating antibiotic-resistant bacteria from car and office door handles in a university environment. Using the simple random sampling method, twenty samples (20) from car door handles and twenty samples (20) from office door\"\n },\n {\n \"doc_id\": 6081983879844188000,\n \"text\": \"\\nC. Testimonies Adebayo-OlajideA. Dakoru GoodheadEkeneokot E. UcheUsman-Wali Maryam\\nView\\nShow abstract\\n... Diseases transmitted by direct physical contact are best prevented by effective hand washing (33). However, using contaminated hand soap from refillable dispensers can result in Gram-negative bacteria colonizing the hands in public settings, thus increasing daily risk of infection threshold (not shown) is 1 x 10 −6 (or 1 infection per 1,000,000 individuals)....\\nEye infection risks from\"\n },\n {\n \"doc_id\": 7705991245379879000,\n \"text\": \"Go to:\\n4. Conclusions\\nThe results demonstrate that handwashing with non-antibacterial soap is much more effective in removing bacteria from hands than handwashing with water only. Although handwashing with water alone reduced the presence of bacteria on hands substantially, the study supports the policy of many current hand hygiene campaigns promoting the use of soap [18,19]. The strong association between hand hygiene method and bacterial contamination of hands found in our study suggests that the prevalence of faecal indicator bacteria may also be used to monitor changes in hygiene behaviour in the general population, for example following hygiene promotion\"\n },\n {\n \"doc_id\": 7822207916778225000,\n \"text\": \" were found after no handwashing in 44% of samples. Handwashing with water alone reduced the presence of bacteria to 23% (p < 0.001). Handwashing with plain soap and water reduced the presence of bacteria to 8% (comparison of both handwashing arms: p < 0.001). The effect did not appear to depend on the bacteria species. Handwashing with non-antibacterial soap and water is more effective for the removal of bacteria of potential faecal origin from hands than handwashing with water alone and should therefore be more useful for the prevention of transmission of\"\n },\n {\n \"doc_id\": 4093364935287199000,\n \"text\": \".) were found after no handwashing in 44% of samples. Handwashing with water alone reduced the presence of bacteria to 23% (p < 0.001). Handwashing with plain soap and water reduced the presence of bacteria to 8% (comparison of both handwashing arms: p < 0.001). The effect did not appear to depend on the bacteria species. Handwashing with non-antibacterial soap and water is more effective for the removal of bacteria of potential faecal origin from hands than handwashing with water alone and should therefore be more useful for the prevention of\"\n },\n {\n \"doc_id\": 2489356507689180700,\n \"text\": \"Substances used[edit]\\nSoap and detergents[edit]\\nRemoval of microorganisms from skin is enhanced by the addition of soaps or detergents to water. Soap and detergents are surfactants that kill microorganisms by disorganizing their membrane lipid bilayer and denaturing their proteins. It also emulsifies oils, enabling them to be carried away by running water.\\nSolid soap[edit]\\nSolid soap, because of its reusable nature, may hold bacteria acquired from previous uses. A small number of studies which have looked at the bacterial transfer from contaminated solid soap have concluded transfer is unlikely as the bacteria are rinsed off with the foam. The CDC still states \\\"liquid soap with hands-free controls for dispensing is preferable\\\".\\nAntibacterial soap[edit]\\nAntibacterial soaps have been heavily promoted to a health-conscious public. To date, there is no evidence that using recommended antiseptics or disinfectants selects for antibiotic-resistant organisms in nature. However, antibacterial soaps contain common antibacterial agents such as triclosan, which has an extensive list of resistant strains of organisms. So, even if antibiotic resistant strains are not selected for by antibacterial soaps, they might not be as effective as they are marketed to be. Besides the surfactant and skin-protecting agent, the sophisticated formulations may contain acids (acetic acid, ascorbic acid, lactic acid) as pH regulator, antimicrobially active benzoic acid and further skin conditioners (aloe vera, vitamins, menthol, plant extracts).\\nA 2007 meta-analysis from the University of Oregon School of Public Health indicated that plain soaps are as effective as consumer-grade anti-bacterial soaps containing triclosan in preventing illness and removing bacteria from the hands. Dissenting, a 2011 meta-analysis in the Journal of Food Protection argued that when properly formulated, triclosan can grant a small but detectable improvement, as can chlorhexidine gluconate, iodophor, or povidone.\\nWarm water[edit]\\nHot water that is still comfortable for washing hands is not hot enough to kill bacteria. Bacteria grow much faster at body temperature (37 °C). WHO considers warm soapy water to be more effective than cold, soapy water at removing natural oils which hold soils and bacteria. But CDC mentions that warm water causes skin irritations more often and its ecological footprint is more significant. Water temperatures from 4 to 40 °C do not differ significantly regarding removal of microbes. The most important factor is proper scrubbing.\\nContrary to popular belief, scientific studies have shown that using warm water has no effect on reducing the microbial load on hands. Using hot water for handwashing can even be regarded as a waste of energy.\\nAntiseptics (hand sanitizer)[edit]\\nHand disinfection procedure according to the German standard DIN EN 1500\\n\\nMain article: Hand sanitizer\\nIn situations where hand washing with soap is not an option (e.g., when in a public place with no access to wash facilities), a waterless hand sanitizer such as an alcohol hand gel can be used. They can be used in addition to hand washing to minimize risks when caring for \\\"at-risk\\\" groups. To be effective, alcohol hand gels should contain not less than 60%v/v alcohol. Enough hand antiseptic or alcohol rub must be used to thoroughly wet or cover both hands. The front and back of both hands and between and the ends of all fingers must be rubbed for approximately 30 seconds until the liquid, foam or gel is dry. Finger tips must be washed well too, rubbing them in both palms.\\nA hand sanitizer or hand antiseptic is a non-water-based hand hygiene agent. In the late 1990s and early part of the 21st century, alcohol rub non-water-based hand hygiene agents (also known as alcohol-based hand rubs, antiseptic hand rubs, or hand sanitizers) began to gain popularity. Most are based on isopropyl alcohol or ethanol formulated together with a thickening agent such as Carbomer (polymer of acrylic acid) into a gel, or a humectant such as glycerin into a liquid, or foam for ease of use and to decrease the drying effect of the alcohol. Adding diluted hydrogen peroxide increases further the antimicrobial activity.\\nHand sanitizers are most effective against bacteria and less effective against some viruses. Alcohol-based hand sanitizers are almost entirely ineffective against norovirus (or Norwalk) type viruses, the most common cause of contagious gastroenteritis.\\nUS Centers for Disease Control and Prevention recommend hand washing with soap over hand sanitizer rubs, particularly when hands are visibly dirty. The increasing use of these agents is based on their ease of use and rapid killing activity against micro-organisms; however, they should not serve as a replacement for proper hand washing unless soap and water are unavailable. Despite their effectiveness, non-water agents do not cleanse the hands of organic material, but simply disinfect them. It is for this reason that hand sanitizers are not as effective as soap and water at preventing the spread of many pathogens, since the pathogens remain on the hands.\\nWipes[edit]\\nHand washing using hand sanitizing wipes is an alternative during traveling in the absence of soap and water. Alcohol-based hand sanitizer should contain at least 60% alcohol.\\nAsh or mud[edit]\\nMany people in low-income communities cannot afford soap and use ash or soil instead. The World Health Organization recommended ash or sand as an alternative to soap when soap is not available. Use of ash is common in rural areas of developing countries and has in experiments been shown at least as effective as soap for removing pathogens. However, evidence to support the use of ash to wash hands is of poor quality. It is not clear if washing hands with ash is effective at reducing viral or bacterial spreading compared to washing with mud, not washing, or with washing with water alone. One concern is that if the soil or ash is contaminated with microorganisms it may increase the spread of disease rather than decrease it, however, there is also no clear evidence to determine the level of risk. Like soap, ash is also a disinfecting agent because in contact with water, it forms an alkaline solution.\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":1251800354590232000,"text":"Hafiza YahyaNorlelawati Ariffin\nHanis Nadia Yahya\nView\n... Of all the human organs, the hands are often more exposed to environmental contamination which makes them a means for transmitting microorganisms from one person to another as they are able to harbour transient microbes [4]. Sometimes, the organisms are from the human skin as normal flora while other times, they may have been deposited through air....\nInvestigation of the Multidrug Resistance Pattern of Bacteria Isolated from Car and Office Door Handles in a Tertiary Institution\nArticle\nMar 2024"},{"doc_id":3236496982507107000,"text":"\n... Of all the human organs, the hands are often more exposed to environmental contamination which makes them a means for transmitting microorganisms from one person to another as they are able to harbour transient microbes [4]. Sometimes, the organisms are from the human skin as normal flora while other times, they may have been deposited through air....\nInvestigation of the Multidrug Resistance Pattern of Bacteria Isolated from Car and Office Door Handles in a Tertiary Institution\nArticle\nMar 2024\nC. Testimonies Adebayo-OlajideA. Dakoru Goodhead"},{"doc_id":6402386300369470000,"text":"% of isolated Shigella species and Proteus species were resistance to tetracycline and ceftriaxone. Conclusion The hands of housemaids are important potential sources of pathogenic bacteria that would result in the potential risk of foodborne diseases. Most bacteria isolates were resistant to tetracycline, ceftriaxone, and ceftazidime. Therefore, practicing good hand hygiene helps to prevent and control the spread of antimicrobial-resistant microbes.\nView\n... Apart from causing diseases when transmitted from one person to another, they run the risk"},{"doc_id":581628699642616400,"text":". [PubMed] [Google Scholar]\n11. Luby SP, Agboatwalla M, Feikin DR, Painter J, Billhimer W, Altaf A, Hoekstra RM. Effect of handwashing on child health: A randomised controlled trial. Lancet. 2005;366:225–233. [PubMed] [Google Scholar]\n12. Aiello AE, Larson EL, Levy SB. Consumer antibacterial soaps: Effective or just risky? Clin. Infect. Dis. 2007;45(Suppl 2):S137–S147"},{"doc_id":8816602139670831000,"text":"\n9. Dodrill L, Schmidt WP, Cobb E, Donachie P, Curtis V, de Barra M. Male commuters in North and South England: Risk factors for the presence of faecal bacteria on hands. BMC Public Health. 2010 (in press) [PMC free article] [PubMed] [Google Scholar]\n10. Judah G, Donachie P, Cobb E, Schmidt W, Holland M, Curtis V. Dirty hands: bacteria of faecal origin on commuters’ hands. Epidemiol. Infect. 2010;138:409–414"},{"doc_id":7865256511831836000,"text":"* 2019 AR Threats report. (2021).\n[ https://www.cdc.gov/drugresistance/biggest-threats.html\n](https://www.cdc.gov/drugresistance/biggest-threats.html)"},{"doc_id":8273483064709169000,"text":" soap for both single and multiple exposure events (per day) ranged from 10–1 to 10⁻⁴, with contaminated eye drops having the greater risk. The concentration of P. aeruginosa was identified as the parameter contributing the greatest variance on eye infection risk; therefore, the prevalence and level of bacterial contamination of the product would have the greatest influence on health risk. Using eye drops in a single-use container or with preservatives can mitigate bacterial growth, and using non-refillable soap dispensers is recommended to reduce contamination of hand soap. Given the opportunistic nature of"},{"doc_id":5102934653837881000,"text":"Abstract and Figures\nHandwashing is thought to be effective for the prevention of transmission of diarrhoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handles and railings in public spaces. They were then allocated at random to (1) handwashing with water, (2) handwashing with non-antibacterial soap and (3) no handwashing. Each volunteer underwent this procedure 24 times, yielding 480 samples overall. Bacteria"},{"doc_id":1857752962571922700,"text":"): 97–104.\nPublished online 2011 Jan 6. doi: 10.3390/ijerph8010097\nPMCID: PMC3037063\nPMID: 21318017\nThe Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands\nMaxine Burton, Emma Cobb, Peter Donachie, Gaby Judah, Val Curtis, and Wolf-Peter Schmidt*\nAuthor information Article notes Copyright and License information PMC Disclaimer\nGo to:\nAbstract\nHandwashing is thought to be effective for the prevention of transmission of diarr"},{"doc_id":2081087526715216000,"text":" during hand washing (AOR = 20.844, 95% CI: 2.190, 9.842) were significantly associated with the prevalence of bacterial isolation. Most Staphylococcus aureus isolates were susceptible to chloramphenicol (98.6%). Escherichia coli isolates were susceptible to tetracycline (75%), ceftriaxone (79.2%), chloramphenicol (87.5%), and ceftazidime (77.1%). Eighty percent of isolated Shigella species were susceptible to chloramp"},{"doc_id":4058345546028399000,"text":"Go to:\n2. Experimental Section\nThis study was carried out between July and August 2009. Overall, 20 volunteers were taken to a large, frequently visited British museum, or asked to travel on a bus or the underground. They were asked to deliberately wipe their hands over hand contact surfaces such as handrails, door handles and seats with the aim of contaminating their hands with whatever bacteria were present. Using a pre-determined random sequence, not known to the participants during self-contamination, participants were then asked to wash their hands with soap, to use water only or not to"},{"doc_id":3641988751893605000,"text":" assessment framework of QMRA to evaluate eye infection risks through both consumer products. The study examines the prevalence of this pathogen in eye drops and soap, as well as the critical need to implement measures that will mitigate bacterial exposure (e.g., single-use soap dispensers and eye drops with preservatives). Additionally, limitations and challenges are discussed, including the need to incorporate data regarding consumer practices, which may improve exposure assessments and health risk estimates.\nView\n... In healthcare, it is crucial to uphold proper hand hygiene as it represents a highly effective preventive measure in reducing infections [1]"},{"doc_id":824704417843875100,"text":" handles were collected, the isolation of bacteria was done using standard microbiological procedures and identification of the isolates was done using cultural, microscopic and biochemical characterization. Determination of the antibiotic sensitivity pattern of the isolates was done using the Kirby-Bauer disc diffusion method on Muller Hinton agar. Antibiotics used included Ofloxacin (5 µg), Gentamicin (10 µg), Ceftriaxone (30 µg), Augmentin (30 µg), Ciprofloxacin (5 µg), Erythromycin (5"},{"doc_id":8995723049121223000,"text":" Bacillus spp, known for bearing resistant spores, was also prevalent and has implications for human pathogenesis and food spoilage [15]. The presence of Klebsiella spp, Escherichia coli, Salmonella spp, and Enterococcus faecalis might suggest compromised personal and domestic hygiene, especially concerning hand contamination after restroom visits, thereby potentially predisposing individuals to diseases [15]....\nHand Carriage of Microorganisms by Students of Federal University of Lafia, Nasarawa State, Nigeria\nArticle\nFull-text available\nOct 2023\nPeter U"},{"doc_id":6880289901152002000,"text":". [PubMed] [Google Scholar]\n13. Hoque BA, Mahalanabis D, Pelto B, Alam MJ. Research methodology for developing efficient handwashing options: An example from Bangladesh. J. Trop. Med. Hyg. 1995;98:469–475. [PubMed] [Google Scholar]\n14. Kaltenthaler EC, Drasar BS, Potter CW. The use of microbiology in the study of hygiene behaviour. Microbios. 1996;88:35–43. [PubMed] [Google Scholar]\n15. Pinf"},{"doc_id":3883468894091239400,"text":"Unlike the study by Hoque and colleagues our trial was conducted in an experimental (albeit naturalistic) setting, where volunteers deliberately contaminated their hands. Additional testing showed that this approach increased the prevalence of contamination from around 10% to over 40% of individuals. It also improved control over the conduct of the experiment, but may affect generalisability, as the study primarily aimed at providing a proof of principle. However, we believe that the superior effect of soap on the removal of bacteria compared to water alone as the principal finding of our study is unlikely to depend on the setting."},{"doc_id":7263983639828164000,"text":"\nArticlePDF Available\nThe Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands\nMDPI\nDecember 2011International Journal of Environmental Research and Public Health (IJERPH) 8(1):97-104\nDOI:10.3390/ijerph8010097\nSourcePubMed\nLicenseCC BY 3.0\nAuthors:\nMaxine Burton\nEmma Cobb\nPeter Donachie\nGaby Judah\nImperial College London\nShow all 6 authors"},{"doc_id":202611872303150430,"text":"ijerph-08-000\n97.pdf\nContent uploaded by Gaby Judah\nAuthor content\nContent may be subject to copyright.\nInt. J. Environ. Res. Public Health 2011, 8, 97-104; doi:10.3390/ijerph8010097 International Journal of Environmental Research and Public Health ISSN 1660-4601 www.mdpi.com/journal/ijerph Article The Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands Maxine Burton, Emma Cobb, Peter Donachie,"},{"doc_id":154248976687680930,"text":" 1 infection per 1,000,000 individuals)....\nEye infection risks from Pseudomonas aeruginosa via hand soap and eye drops\nArticle\nFull-text available\nMar 2024APPL ENVIRON MICROB\nAnna GitterKristina D. MenaKarla S. Mendez\nFuqing WuCharles P. Gerba\nEye infections from bacterial contamination of bulk-refillable liquid soap dispensers and artificial tear eye drops continue to occur, resulting in adverse health outcomes that include impaired vision or eye enucleation. Pseudomonas aer"},{"doc_id":4702055053979222000,"text":"It seems reasonable to assume that handwashing with soap is also more effective in reducing bacterial load compared to water alone. Future studies could address the effect of different hand hygiene procedures on removing gastro-intestinal or respiratory viruses such as influenza A. Hands have been implicated especially in the spread of Norovirus [16]. Viral studies are more difficult to conduct as viruses may not be as present in the environment as often as are bacteria of faecal origin, but they may be possible for example if patients with laboratory confirmed infection are recruited as volunteers. Alternatively, healthy volunteers may experimentally contaminate their hands"},{"doc_id":3248572189032461000,"text":"2Fj.gfs.2019.100325&volume=25&publication_year=2020&author=Adesogan%2CAT&author=Havelaar%2CAH&author=McKune%2CSL&author=Eilitt%C3%A4%2CM&author=Dahl%2CGE)"},{"doc_id":2869156314969342500,"text":"ates was 72% (95% CI: 66.2, 77.8). The dominant bacterial isolates were Staphylococcus aureus (31.6%), Escherichia coli (21.3%), Salmonella species (1.3%), Shigella species (6.7%), Klebsiella species (23.1%) and Proteus species (14.7%). Fingernail status (AOR =15.31, 95% CI: 10.372, 22.595) and the removal of a watch, ring, and bracelet"},{"doc_id":8327700382538876000,"text":"The prevalence of bacterial contamination in the three study arms (soap, water, no handwash) was compared using logistic regression. Since the same volunteers repeatedly underwent testing, within-subject correlation was accounted for by the use of generalised estimating equations (GEE) with robust standard errors. If the cell numbers were too low for conducting regression analysis, Fishers exact test was used instead, ignoring clustering (the design effect was found to be low, see results)."},{"doc_id":2019813929377508000,"text":" of potential faecal origin (mostly Enterococcus and Enterobacter spp.) were found after no handwashing in 44% of samples. Handwashing with water alone reduced the presence of bacteria to 23% (p < 0.001). Handwashing with plain soap and water reduced the presence of bacteria to 8% (comparison of both handwashing arms: p < 0.001). The effect did not appear to depend on the bacteria species. Handwashing with non-antibacterial soap and water is more effective for the removal of bacteria of potential faecal origin from hands"},{"doc_id":2684108841186596000,"text":"hoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handles and railings in public spaces. They were then allocated at random to (1) handwashing with water, (2) handwashing with non-antibacterial soap and (3) no handwashing. Each volunteer underwent this procedure 24 times, yielding 480 samples overall. Bacteria of potential faecal origin (mostly Enterococcus and Enterobacter spp"},{"doc_id":8156286088047615000,"text":"\nThe Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands\nArticle\nFull-text available\nDec 2011\nMaxine Burton\nEmma Cobb\nPeter Donachie[...]\nWolf-Peter Schmidt\nHandwashing is thought to be effective for the prevention of transmission of diarrhoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handle...\nCite\nDownload full-text\nCitations"},{"doc_id":2451214201653555000,"text":"But what happens outside the lab is less clear. In the biggest study of its kind, researchers recruited about 240 households in upper Manhattan to participate in a \"real-world\" hand washing study. Half were randomized to use 0.2% triclosan soap; half, to plain soap. After a year, the researchers tested the hands of the primary caregivers in the households for antibiotic-resistant bacteria. The result: no statistically significant difference between antibacterial and plain-soap households. The researchers offered several possible explanations for their findings (resistance may not develop in a year; high antibiotic"},{"doc_id":5409631546574559000,"text":"Not all of the bacteria isolated in our study are known to cause disease in humans. Surprisingly, we found few E. coli on hands which may be due to their short survival time in the environment. Overall, the effect of soap appeared to be independent of the type of bacteria (Figure 1), a view which is supported by the study by Hoque and colleagues who found a similar effect of hand hygiene on unspecified faecal coliform bacteria [6]. However, the power of our study to detect differences between species was low."},{"doc_id":5752874606470013000,"text":" P. aeruginosa and its ability to thrive in unique environments, additional safeguards to mitigate bacterial growth and exposure are warranted. IMPORTANCE Pseudomonas aeruginosa (P. aeruginosa) is a pathogen that can persist in a variety of unusual environments and continues to pose a significant risk for public health. This quantitative microbial risk assessment (QMRA) estimates the potential human health risks, specifically for eye infections, associated with exposure to P. aeruginosa in bacteria-contaminated artificial tear eye drops and hand soap. This study applies the risk"},{"doc_id":6788887078786142000,"text":"uginosa (P. aeruginosa), a common cause of eye infections, can grow in eye drop containers and refillable soap dispensers to high numbers. To assess the risk of eye infection, a quantitative microbial risk assessment for P. aeruginosa was conducted to predict the probability of an eye infection for two potential exposure scenarios: (i) individuals using bacteria-contaminated eye drops and (ii) contact lens wearers washing their hands with bacteria-contaminated liquid soap prior to placing the lens. The median risk of an eye infection using contaminated eye drops and hand"},{"doc_id":343709101918362700,"text":", so hand hygiene might improve over all."},{"doc_id":2423938153737278000,"text":"Go to:\n3. Results and Discussion\nTable 1 shows the different organisms isolated in the three study arms. Enterococcus spp. were the most common bacteria found, followed by Enterobacter spp. Figure 1 shows the effect of handwashing with soap or water only on contamination, compared to no handwashing. Overall, handwashing with water alone reduced the prevalence of bacteria substantially. Handwashing with soap was more effective in reducing the prevalence of contamination and specifically of Enterococcus spp. There was a trend that handwashing with soap was also more effective in reducing the prevalence of"},{"doc_id":1428582819192438800,"text":"Luby and colleagues found that a simple microbiological method with three fingers directly imprinting a MacConkey agar for thermotolerant coliforms was unable to distinguish between households who were given soap during a large randomized handwashing trial and control households [8]. They concluded that the method was unsuitable for the evaluation of handwashing practices. However, the lack of difference in bacterial contamination may have been due to lack of compliance with the intervention. We thought that a proof-of-principle trial was needed where participants would be given specific tasks to contaminate their hands in a"},{"doc_id":1568908759133818000,"text":" slum in Bandung, Indonesia. We observed handwashing and conducted repeated hand swabs before and after handwashing among 137 participants. The mean E. coli concentration on the hands decreased after handwashing, with a higher reduction in E. coli count among students who used soap and had soap contact for more than 10 s during handwashing. Cleaning in-between fingers, using soap, soap contact for more than 10 s, and drying hands with a single-use towel were effective factors for reducing E. coli concentration after handwashing (p < 0.05). More than half of the sw"},{"doc_id":6245401161975709000,"text":" a lack of hand-washing remains at unacceptable levels in most medical environments, with large numbers of doctors and nurses routinely forgetting to wash their hands before touching patients, thus transmitting microorganisms. One study showed that proper hand-washing and other simple procedures can decrease the rate of catheter-related bloodstream infections by 66%.\nThe World Health Organization has published a sheet demonstrating standard hand-washing and hand-rubbing in health-care sectors. The draft guidance of hand hygiene by the organization can also be found at its website for public comment. A relevant review was conducted by Whitby et al. Commercial"},{"doc_id":8609808448557046000,"text":"... Thus, hands serve as the vehicle of infectious disease transmission, especially amongst people living and working in close proximity to one another, such as dormitories, classrooms, camps etc. Close environments, doorknobs and other inanimate objects serving as resting vehicles of transmission all contribute to increased infection rates among these groups [14]. Human hands usually constitute microorganisms both as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms and usually harbour microorganisms both as residents and transients, the"},{"doc_id":6686149660008202000,"text":" touches the dirty surface, they may deposit the virus at the eyes, nose, or mouth where it can enter the body and cause infection. Evidence indicates that contact with infected surfaces is not the main driver of COVID‑19, leading to recommendations for optimised disinfection procedures to avoid issues such as the increase of antimicrobial resistance through the use of inappropriate cleaning products and processes. Deep cleaning and other surface sanitation has been criticised as hygiene theatre, giving a false sense of security against something primarily spread through the air.\nThe amount of time that the virus can survive depends significantly on the type of surface"},{"doc_id":32513998792769484,"text":"lla spp, and Enterococcus faecalis might suggest compromised personal and domestic hygiene, especially concerning hand contamination after restroom visits, thereby potentially predisposing individuals to diseases [15]....\nHand Carriage of Microorganisms by Students of Federal University of Lafia, Nasarawa State, Nigeria\nArticle\nFull-text available\nOct 2023\nPeter Upla\nBashiru SaniOsuyi Gerard Uyi\nGladys ABEL Angbalaga\nView\nShow abstract\n... It has been mentioned transmission of bacteria occur from the handler by direct contact with the"},{"doc_id":5092476230231695000,"text":"A number of studies have compared different hand hygiene methods in hospital settings [5]. In contrast, few studies have been published on the effect of hand hygiene on bacterial contamination of hands in the community. Hoque and colleagues found that a wide variety of hand cleansing means in poor settings (soap, ash, mud) are effective in reducing the contamination with coliform bacteria on hands [6,7]. In a small randomised trial the same author reported that soap may be more effective than water in reducing the presence of coliform bacteria on hands [6]."},{"doc_id":2105600531165297700,"text":" hands with soap (both plain and antibacterial soap) is a proven mechanism to eliminate bacteria and respiratory viruses [31][32][33]. A systematic review of eight studies from a pool of 410 articles found that hand washing lowered the risk of respiratory infection, with risk reductions ranging from 6% to 44%, but noted that a greater number of rigorous studies are urgently needed [34]....\nEstimating spatially disaggregated probability of severe COVID-19 and the impact of handwashing interventions: The case of Zimbabwe\nArticle\nFull-text available\nNov 2023PLOS ONE\nGeorge"},{"doc_id":2852937817197359600,"text":"hoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handles and railings in public spaces. They were then allocated at random to (1) handwashing with water, (2) handwashing with non-antibacterial soap and (3) no handwashing. Each volunteer underwent this procedure 24 times, yielding 480 samples overall. Bacteria of potential faecal origin (mostly Enterococcus and Enterobacter spp.)"},{"doc_id":3426465220136984000,"text":" mascara had the greatest rates....\nIsolation and Identification of Harmful Microorganisms from the Shared Cosmetic Products in Delhi NCR Region\nArticle\nFull-text available\nFeb 2024\nGyan Vandana Yadav\nSandhya Khunger\nSunil KumarMukesh Sharma\nView\nShow abstract\n... In healthcare, it is crucial to uphold proper hand hygiene as it represents a highly effective preventive measure in reducing infections [1] and managing the spread of illnesses [2,3]. Improper hand hygiene among healthcare workers is responsible for approximately 40% of healthcare-associated infections."},{"doc_id":4494967955268539400,"text":"Even if antibiotic resistance weren't an issue, results from this study (and others) make you wonder if the antibacterial soaps available to consumers add much to hand hygiene. In the Manhattan households, a year of washing with an antibacterial soap didn't lower bacterial counts on hands any more than a year of washing with plain soap. Nor did the antibacterial soap households experience fewer cold-like symptoms. That's not surprising: Colds are caused by viruses, not bacteria. Still, the finding is a useful reminder that the antibacterial soaps aren't the all-purpose germ fighters that many"},{"doc_id":795799110941267100,"text":"Faecal Bacteria\tNo Handwashing\tWater only\tSoap and water\nEnterococcus spp.\t46 (29%)\t24 (15%)\t4 (3%)\nEnterobacter amnigenus\t14 (9%)\t4 (3%)\t4 (3%)\nEnterobacter cloacae\t13 (8%)\t5 (3%)\t2 (1%)\nShigella spp.\t2 (1%)\t1 (1%)\t0 (0%)\nKlebsiella spp.\t5 (3%)"},{"doc_id":2677019744351524000,"text":"Sohaib AsifXinyi Xu\nZhao MingYusen Zhu\nView\nShow abstract\n... The other reason might be the quality of the handwashing water. Pieces of evidence revealed that bacterial contamination of hands is significantly affected by handwashing water (54,55)....\nPrevalence and antimicrobial susceptibility profile of bacteria isolated from the hands of housemaids in Jimma City, Ethiopia\nArticle\nFull-text available\nJan 2024\nTadele Shiwito AngoNegalgn Byadgie GelawGirma Mamo ZegeneTes"},{"doc_id":3981141855101259000,"text":"falem Getahun\nView\nShow abstract\nImpact of different hand drying methods on surrounding environment: aerosolization of virus and bacteria and transference to surfaces\nArticle\nMar 2024J HOSP INFECT\nRodolphe HervéCatherine Bryant\nLucy Sutton\nSandra A Wilks\nView\nVertical stratification and seasonality of fecal indicator bacteria in New York City playground sandboxes\nArticle\nFeb 2024ECOTOX ENVIRON SAFE\nAlessandra LeriG Eliana Fassihi\nMatthew LundquistMariette"},{"doc_id":6504884612643356000,"text":"Article\nFull-text available\nJan 2024\nTadele Shiwito AngoNegalgn Byadgie GelawGirma Mamo ZegeneTizita TeshomeTesfalem Getahun\nIntroduction Bacterial pathogens continue to be a major cause of foodborne gastroenteritis in humans and remain a public health problem. Housemaids operating inside a kitchen could be the source of infection and may transmit disease-inflicting pathogens through contaminated hands. Objective This study aimed to assess the prevalence and antimicrobial susceptibility profile of bacteria isolated from the hands of housemaid"},{"doc_id":6624727010274029000,"text":" remains at unacceptable levels in most medical environments, with large numbers of doctors and nurses routinely forgetting to wash their hands before touching patients, thus transmitting microorganisms. One study showed that proper hand-washing and other simple procedures can decrease the rate of catheter-related bloodstream infections by 66%.\nVideo demonstration on hand washing\nThe World Health Organization has published a sheet demonstrating standard hand-washing and hand-rubbing in health-care sectors. The draft guidance of hand hygiene by the organization can also be found at its website for public comment. A relevant review was conducted by Whitby et al. Commercial"},{"doc_id":7346727617076172000,"text":"old JV, Horan NJ. Measuring the effect of a hygiene behaviour intervention by indicators of behaviour and diarrhoeal disease. Trans. Roy. Soc. Trop. Med. Hyg. 1996;90:366–371. [PubMed] [Google Scholar]\n16. Barker J, Vipond IB, Bloomfield SF. Effects of cleaning and disinfection in reducing the spread of Norovirus contamination via environmental surfaces. J. Hosp. Infect. 2004;58:42–49. [PubMed] [Google Scholar]\n17. Grayson ML, Melv"},{"doc_id":3700513633745884700,"text":" [PubMed] [Google Scholar]\n7. Hoque BA, Mahalanabis D, Alam MJ, Islam MS. Post-defecation handwashing in Bangladesh: practice and efficiency perspectives. Public Health. 1995;109:15–24. [PubMed] [Google Scholar]\n8. Luby SP, Agboatwalla M, Billhimer W, Hoekstra RM. Field trial of a low cost method to evaluate hand cleanliness. Trop. Med. Int. Health. 2007;12:765–771. [PubMed] [Google Scholar]"},{"doc_id":1669520942777172500,"text":"\nThe CDC says that in most situations, cleaning surfaces with soap or detergent, not disinfecting, is enough to reduce risk of transmission. The CDC recommends that if a COVID‑19 case is suspected or confirmed at a facility such as an office or day care, all areas such as offices, bathrooms, common areas, shared electronic equipment like tablets, touch screens, keyboards, remote controls, and ATMs used by the ill persons should be disinfected. Surfaces may be decontaminated with 62–71 per cent ethanol, 50–100 per cent isopropanol, 0"},{"doc_id":6490370016564395000,"text":", a total of sixty (60) hand swab samples (thirty (30) from both palm and nails) were collected and tested for bacterial and fungal presence. Bacteria isolated were Staphylococcus epidermidis (80.00 %), Staphylococcus aureus (75.00 %), Enterococcus spp (50.00 %), Micrococcus spp (46.67 %), Escherichia coli (45.00 %), Klebsiella spp (45.00 %), Bacillus spp (30"},{"doc_id":1218148308571330000,"text":" effective than alcohol-based hand rubs for reducing H1N1 influenza A virus and Clostridium difficile spores from hands.\nInterventions to improve hand hygiene in healthcare settings can involve education for staff on hand washing, increasing the availability of alcohol-based hand rub, and written and verbal reminders to staff. There is a need for more research into which of these interventions are most effective in different healthcare settings."},{"doc_id":8877592251602426000,"text":". It also highlights the spatial differences in risk of severe COVID-19, and thus the opportunity for better planning to focus limited resources in high-risk areas in order to potentially reduce the number of severe cases.\nView\n... Thus, hands serve as the vehicle of infectious disease transmission, especially amongst people living and working in close proximity to one another, such as dormitories, classrooms, camps etc. Close environments, doorknobs and other inanimate objects serving as resting vehicles of transmission all contribute to increased infection rates among these groups [14]. Human hands usually constitute microorganisms both"},{"doc_id":8564106436840319000,"text":"s in Jimma City, Ethiopia. Methods A laboratory-based cross-sectional study was employed among 234 housemaids. Hand swab samples from the dominant hand of the study participants were collected under sterile conditions following standard operating procedures. Then, in the laboratory, the swabs were inoculated aseptically using streak-plating methods on the growth media, such as mannitol salt agar [Staphylococcus aureus and coagulase-negative staphylococci], MacConkey agar [Klebsiella species and Proteus"},{"doc_id":2980497090769698300,"text":" as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms and usually harbour microorganisms both as residents and transients, the presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]....\n... Human hands usually constitute microorganisms both as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms"},{"doc_id":5176090711239746,"text":"henicol and gentamicin respectively. In addition, Klebsiella and Proteus species exhibited high susceptibility to chloramphenicol. However, their isolates showed resistance against a number of the tested antimicrobials. Staphylococcus aureus isolates (28.2%) were resistance to tetracycline. Moreover, One-quarter of Escherichia coli isolates were resistance to tetracycline, ceftriaxone, chloramphenicol, and ceftazidime. Whereas 46.7% and 48.5"},{"doc_id":1547159199412704500,"text":"Hygiene behaviour is difficult to measure because people tend to change their behaviour under observation or over-report desired practices [15,20]. We have previously shown that our test kit can be used to study associations between hygiene relevant behaviours and hand contamination [9]. We found that test results positive for bacteria of potential faecal origin were more common in people frequently shaking hands, reporting soil contact or those scoring low on a hygiene score based on self-report [9]. The microbiological method used in this and our earlier studies [9,10] is relatively simple and of low cost (around $3"},{"doc_id":6979437872508612000,"text":"Promotion of improved hand hygiene has been recognised as an important public health measure but it is unclear how much hand hygiene is required to interrupt transmission of diarrhoea pathogens. In particular it has not been conclusively shown whether use of soap is essential to remove pathogens from hands. Recent hygiene promotion campaigns especially in low income settings have not been unanimous in recommending soap use [4]."},{"doc_id":3911170097265289000,"text":".00 %), Salmonella spp (13.33 %) and Streptococcus spp (10.00 %). The fungi isolated were Aspergillus niger (45.00 %), Penicillium spp (23.33 %), Mucor spp (21.67 %), Candida spp (20.00 %) and Saccharomyces spp (15.00 %). Gender, level of study, faculty, and area of hand swab revealed no statistically significant variation in the presence of numerous bacterial and fungal species at"},{"doc_id":8056488143117447000,"text":" be carried away by running water.\nSolid soap, because of its reusable nature, may hold bacteria acquired from previous uses. A small number of studies which have looked at the bacterial transfer from contaminated solid soap have concluded transfer is unlikely as the bacteria are rinsed off with the foam. The CDC still states \"liquid soap with hands-free controls for dispensing is preferable\".\nAntibacterial soaps have been heavily promoted to a health-conscious public. To date, there is no evidence that using recommended antiseptics or disinfectants selects for antibiotic-resistant organisms in nature. However"},{"doc_id":756020715464919600,"text":" presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]....\n... Human hands usually constitute microorganisms both as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms and usually harbour microorganisms both as residents and transients, the presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]. Therefore,"},{"doc_id":7290049185625302000,"text":" p<0.05. These findings revealed that the microbial burden on students' hands was significant and was unaffected by gender, level of study, or faculty. To improve students' overall health, appropriate hygiene, including regular handwashing practice, and public education about the importance of hands in disease transmission should be supported.\nView\n... Moreover, some respondents describe using soap as challenging because it is a new practice, and they believe washing with water is enough to make hands physically clean. Experimental trials have shown that HWWS is more effective in removing bacteria than using water alone (Burton et al"},{"doc_id":6463784370647876000,"text":"The bacteriological methods used in this study provide no quantification of bacterial load, unlike a study by Hoque and colleagues [7]. Quantifying the effect of different hand washing procedures on bacterial load may be particularly helpful for studies in poor settings with poor sanitation facilities, where the environmental contamination with faecal organisms is much higher [13–15]. We also tested a semi-quantitative finger-print method used previously in Thailand [15] not unsimilar to the method used by Luby and colleagues [8] but found that contamination levels were too low to provide consistent results. Therefore we decided"},{"doc_id":8518308529609072000,"text":". No. CD004265. [PubMed] [CrossRef] [Google Scholar]\n5. WHO Guidelines on Hand Hygiene in Health Care. WHO; Geneva, Switzerland: 2009. [(accessed on 16 November 2010).]. Available online: http://whqlibdoc.who.int/publications/2009/9789241597906_eng.pdf [Google Scholar]\n6. Hoque BA, Briend A. A comparison of local handwashing agents in Bangladesh. J. Trop. Med. Hyg. 1991;94:61–64."},{"doc_id":7788926125136410000,"text":" the study aimed to investigate the microorganisms isolated from the hands of students of the Federal University of Lafia, Nasarawa State, Nigeria....\n... Additionally, these microorganisms, being opportunistic human pathogens, pose implications for food safety, particularly in the case of enterotoxin-producing strains of staphylococci linked to food poisoning [27]. Bacillus spp, known for bearing resistant spores, was also prevalent and has implications for human pathogenesis and food spoilage [15]. The presence of Klebsiella spp, Escherichia coli, Salmone"},{"doc_id":8365345683117400000,"text":" (1) current risk of severe disease is heterogeneous across the country, due to differences in individual characteristics and household conditions and (2) that if the quantifiable estimates on the importance of handwashing for transmission are sound, then improvements in handwashing access could lead to reductions in the risk of severe COVID-19 of up to 16% from the estimated current levels across all districts. Conclusions Taken alongside the likely impact on transmission of SARS-CoV-2 itself, as well as countless other pathogens, this result adds further support for the expansion of access to handwashing across the country"},{"doc_id":3138911842569085400,"text":"... HWWS for 20 s has been highlighted as an effective method for removing bacteria from the hands (WHO 2009). Wetting the hands with water and scrubbing with soap creates a lather that traps and eliminates these bacteria (Burton et al. 2011). Although some students performed handwashing for more than 20 s, only 6% of them scrubbed their hands with soap for that entire duration, while others did so briefly....\nEffect of handwashing on the reduction of Escherichia coli on children's hands in urban slum Indonesia\nArticle\nFull-text available\nOct"},{"doc_id":1376553736554764000,"text":" 2023J WATER HEALTH\nMahmud Aditya Rifqi\nUmi Hamidah\nNeni Sintawardani\nHidenori Harada\nTaro Yamauchi\nPoor hand hygiene practice has been linked to an increase in the number of infections among children in urban slums. Hands are considered an intersection for bacterial transmission, but it is unclear whether the handwashing technique affects bacteria elimination. This study investigated the effect of handwashing on the concentration of Escherichia coli (E. coli) and factors related to its reduction among children in an urban"},{"doc_id":1581215079874269400,"text":"teh Upla\nBashiru Eya SaniOsuyi Gerard UyiIgoche Naomi Ibe\nGladys Abel Angbalaga\nMicrobes from the body's regular flora and transient microorganisms from the environment are found on human hands. Hands can also be used to spread disease from one person to another, especially among close persons. This study aimed to isolate and identify microorganisms from students' hands and assess the occurrence of these bacteria based on gender, level of study, faculty, and hand area (palm and nails swab). Using the pour plate method"},{"doc_id":3846559940304106000,"text":"based solution with hand washing with antibacterial soap for a median time of 30 seconds each showed that the alcohol hand-rubbing reduced bacterial contamination 26% more than the antibacterial soap. But soap and water is more effective than alcohol-based hand rubs for reducing H1N1 influenza A virus and Clostridium difficile spores from hands.\nInterventions to improve hand hygiene in healthcare settings can involve education for staff on hand washing, increasing the availability of alcohol-based hand rub, and written and verbal reminders to staff. There is a need for more research into which of these"},{"doc_id":5681005703472282000,"text":" reduce the spread of pathogens, it is better to wash the hands or use a hand antiseptic before and after tending to a sick person.\nFor control of staphylococcal infections in hospitals, it has been found that the greatest benefit from hand-cleansing came from the first 20% of washing, and that very little additional benefit was gained when hand cleansing frequency was increased beyond 35%. Washing with plain soap results in more than triple the rate of bacterial infectious disease transmitted to food as compared to washing with antibacterial soap.\nComparing hand-rubbing with alcohol-"},{"doc_id":7844972954270361000,"text":" areas such as offices, bathrooms, common areas, shared electronic equipment like tablets, touch screens, keyboards, remote controls, and ATMs used by the ill persons should be disinfected. Surfaces may be decontaminated with 62–71 per cent ethanol, 50–100 per cent isopropanol, 0.1 per cent sodium hypochlorite, 0.5 per cent hydrogen peroxide, 0.2–7.5 per cent povidone-iodine, or 50–200 ppm hypochlorous acid. Other solutions, such as benzalkonium"},{"doc_id":6187756745778273000,"text":" and usually harbour microorganisms both as residents and transients, the presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]. Therefore, the study aimed to investigate the microorganisms isolated from the hands of students of the Federal University of Lafia, Nasarawa State, Nigeria....\n... Additionally, these microorganisms, being opportunistic human pathogens, pose implications for food safety, particularly in the case of enterotoxin-producing strains of staphylococci linked to food poisoning [27]."},{"doc_id":5810683772352509000,"text":" which can cause a variety of diseases in humans.\nView\n... The other reason might be the quality of the handwashing water. Pieces of evidence revealed that bacterial contamination of hands is significantly affected by handwashing water (54,55)....\nPrevalence and antimicrobial susceptibility profile of bacteria isolated from the hands of housemaids in Jimma City, Ethiopia"},{"doc_id":4770515370632447000,"text":" Pseudomonas aeruginosa via hand soap and eye drops\nArticle\nFull-text available\nMar 2024APPL ENVIRON MICROB\nAnna GitterKristina D. MenaKarla S. MendezCharles P. Gerba\nView\nShow abstract\n... Seventy to eighty percent of the chemicals used have been found tainted with bacteria [17]. Poor hand hygiene can cause used objects to become contaminated with bacteria like S. aureus and B. cereus [24]. All product categories had fungus contamination; however, the foundation, lipstick, and"},{"doc_id":6477730130678012000,"text":" and moist hands are more easily recontaminated.\nThe most commonly missed areas are the thumb, the wrist, the areas between the fingers, and under fingernails. Artificial nails and chipped nail polish may harbor microorganisms.\nWhen it is recommended[edit]\nThere are five critical times during the day where washing hands with soap is important to reduce fecal-oral transmission of disease: after using the toilet (for urination, defecation, menstrual hygiene), after cleaning a child's bottom (changing diapers), before feeding a child, before eating and before/after preparing"},{"doc_id":8310864475669528000,"text":"hips in a range of subjects and topics for our PhD programmes starting September 2018. Background From global disease outbreaks to the rising burden of chronic conditions to threats...\nView post\nArticle\nMultidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: An international exp...\nMay 2011 · Clinical Microbiology and Infection\nanna-pelagia MagiorakosA Srinivasan\nRoberta B Carey[...]Dominique L Monnet\nMany different definitions for multidrug-resistant (MDR), extensively drug-resistant (XDR) and pandrug"},{"doc_id":322654686885456640,"text":" water for at least 20 seconds or use a hand sanitizer that contains at least 60% alcohol Cover your mouth and nose with a tissue when you cough or sneeze Stay home when sick Get recommended vaccines, such as the flu vaccine. To learn more about antibiotic prescribing and use, visit CDC’s Antibiotic Prescribing and Use website. To learn more about antibiotic resistance, visit CDC’s Antibiotic Resistance website."},{"doc_id":433738502772842800,"text":"); Val.Curtis@lshtm.ac.uk (V.C.) * Author to whom correspondence should be addressed; E-Mail: Wolf-Peter.Schmidt@lshtm.ac.uk; Tel.: +44-20-7927-2461; Fax: +44-20-7636-7843. Received: 24 November 2010; in revised form: 30 December 2010 / Accepted: 31 December 2010 / Published: 6 January 2011 Abstract: Handwashing is thought to be effective for the prevention of transmission of diarr"},{"doc_id":1510222038353803500,"text":"What is new, though, is the range of organisms you might find on even a seemingly clean pair of hands."},{"doc_id":9217255509341745000,"text":".80). Its suitability for large scale use in the evaluation of handwashing campaigns in low income settings where handwashing should be most beneficial remains to be investigated. A sophisticated laboratory infrastructure may not be required to conduct testing. However, modifying the method to allow semi-quantitative or quantitative analysis may be necessary if contamination rates are high [15]."},{"doc_id":8947403650220114000,"text":" no significant spread of micro-organisms. No studies have found a correlation to hand dryers and human health, however, making these findings inconsequential.\nAccessibility[edit]\nA community handwashing facility in Rwanda with sinks for people of different heights. During the COVID-19 pandemic in Rwanda handwashing was part of a system of public health measures encouraged to reduce transmission.\nMaking hand washing facilities accessible (inclusive) to everyone is crucial to maintain hand washing behavior. Considerations for accessibility include age, disability, seasonality (with rains and muddiness), location and"},{"doc_id":3563355511531846700,"text":" immediately. Do your best to stay healthy and keep others healthy: Clean hands by washing with soap and water for at least 20 seconds or use a hand sanitizer that contains at least 60% alcohol Cover your mouth and nose with a tissue when you cough or sneeze Stay home when sick Get recommended vaccines, such as the flu vaccine. To learn more about antibiotic prescribing and use, visit CDC’s Antibiotic Prescribing and Use website. To learn more about antibiotic resistance, visit CDC’s Antibiotic Resistance website."},{"doc_id":8330182738007888000,"text":" devices can measure and validate hand hygiene, if demonstration of regulatory compliance is required.\nThe World Health Organization has \"Five Moments\" for washing hands:\nThe addition of antiseptic chemicals to soap (\"medicated\" or \"antimicrobial\" soaps) confers killing action to a hand-washing agent. Such killing action may be desired before performing surgery or in settings in which antibiotic-resistant organisms are highly prevalent.\nTo'scrub' one's hands for a surgical operation, it is necessary to have a tap that can be turned on and off without touching it with the hands"},{"doc_id":874043767480450600,"text":" JosephSveta MilushevaHugh Sturrock\nTonderai MapakoYi Rong Hoo\nIntroduction The severity of COVID-19 disease varies substantially between individuals, with some infections being asymptomatic while others are fatal. Several risk factors have been identified that affect the progression of SARS-CoV-2 to severe COVID-19. They include age, smoking and presence of underlying comorbidities such as respiratory illness, HIV, anemia and obesity. Given that respiratory illness is one such comorbidity and is affected by hand hygiene, it"},{"doc_id":5744994367201210000,"text":"icrobial resistance through the use of inappropriate cleaning products and processes. Deep cleaning and other surface sanitation has been criticised as hygiene theatre, giving a false sense of security against something primarily spread through the air.\nThe amount of time that the virus can survive depends significantly on the type of surface, the temperature, and the humidity. Coronaviruses die very quickly when exposed to the UV light in sunlight. Like other enveloped viruses, SARS-CoV-2 survives longest when the temperature is at room temperature or lower, and when the relative humidity is low (<50%).\nOn many surfaces"},{"doc_id":7475511348052728000,"text":" others healthy: Clean hands by washing with soap and water for at least 20 seconds or use a hand sanitizer that contains at least 60% alcohol Cover your mouth and nose with a tissue when you cough or sneeze Stay home when sick Get recommended vaccines, such as the flu vaccine. To learn more about antibiotic prescribing and use, visit CDC’s Antibiotic Prescribing and Use website. To learn more about antibiotic resistance, visit CDC’s Antibiotic Resistance website. More Information CDC’s Be Antibiotics Aware Educational Effort U.S. Ant"},{"doc_id":6488001898291672000,"text":"ani S, Druce J, Barr IG, Ballard SA, Johnson PD, Mastorakos T, Birch C. Efficacy of soap and water and alcohol-based hand-rub preparations against live H1N1 influenza virus on the hands of human volunteers. Clin. Infect. Dis. 2009;48:285–291. [PubMed] [Google Scholar]\n18. Curtis V, Sidibe M, Scott BE, lyer P, Sara J. The Handwash Handbook: A Guide for Developing a Hygiene Promotion Program to Increase Handwashing with Soap."},{"doc_id":8246575083859717000,"text":" hygiene is central to preventing the spread of infectious diseases in home and everyday life settings.\nThe World Health Organization (WHO) recommends washing hands for at least 20 seconds before and after certain activities. These include the five critical times during the day where washing hands with soap is important to reduce fecal-oral transmission of disease: after using the toilet (for urination, defecation, menstrual hygiene), after cleaning a child's bottom (changing diapers), before feeding a child, before eating and before/after preparing food or handling raw meat, fish, or poultry.\nWhen neither hand washing nor"},{"doc_id":632135853865087000,"text":"), use of a warm-air hand dryer spread micro-organisms only up to 0.25 metres (0.82 feet) from the dryer, and paper towels showed no significant spread of micro-organisms. No studies have found a correlation to hand dryers and human health, however, making these findings inconsequential.\nMaking hand washing facilities accessible (inclusive) to everyone is crucial to maintain hand washing behavior. Considerations for accessibility include age, disability, seasonality (with rains and muddiness), location and more. Important aspects for good accessibility include: Placement of"},{"doc_id":3239197287654671000,"text":" also be achieved mechanically with higher energy and maintenance costs. The use of large ducts and openings helps to prevent mixing in closed environments. Recirculation and mixing should be avoided because recirculation prevents dilution of harmful particles and redistributes possibly contaminated air, and mixing increases the concentration and range of infectious particles and keeps larger particles in the air.\nThorough hand hygiene after any cough or sneeze is required. The WHO also recommends that individuals wash hands often with soap and water for at least twenty seconds, especially after going to the toilet or when hands are visibly dirty, before eating and"}],"string":"[\n {\n \"doc_id\": 1251800354590232000,\n \"text\": \"Hafiza YahyaNorlelawati Ariffin\\nHanis Nadia Yahya\\nView\\n... Of all the human organs, the hands are often more exposed to environmental contamination which makes them a means for transmitting microorganisms from one person to another as they are able to harbour transient microbes [4]. Sometimes, the organisms are from the human skin as normal flora while other times, they may have been deposited through air....\\nInvestigation of the Multidrug Resistance Pattern of Bacteria Isolated from Car and Office Door Handles in a Tertiary Institution\\nArticle\\nMar 2024\"\n },\n {\n \"doc_id\": 3236496982507107000,\n \"text\": \"\\n... Of all the human organs, the hands are often more exposed to environmental contamination which makes them a means for transmitting microorganisms from one person to another as they are able to harbour transient microbes [4]. Sometimes, the organisms are from the human skin as normal flora while other times, they may have been deposited through air....\\nInvestigation of the Multidrug Resistance Pattern of Bacteria Isolated from Car and Office Door Handles in a Tertiary Institution\\nArticle\\nMar 2024\\nC. Testimonies Adebayo-OlajideA. Dakoru Goodhead\"\n },\n {\n \"doc_id\": 6402386300369470000,\n \"text\": \"% of isolated Shigella species and Proteus species were resistance to tetracycline and ceftriaxone. Conclusion The hands of housemaids are important potential sources of pathogenic bacteria that would result in the potential risk of foodborne diseases. Most bacteria isolates were resistant to tetracycline, ceftriaxone, and ceftazidime. Therefore, practicing good hand hygiene helps to prevent and control the spread of antimicrobial-resistant microbes.\\nView\\n... Apart from causing diseases when transmitted from one person to another, they run the risk\"\n },\n {\n \"doc_id\": 581628699642616400,\n \"text\": \". [PubMed] [Google Scholar]\\n11. Luby SP, Agboatwalla M, Feikin DR, Painter J, Billhimer W, Altaf A, Hoekstra RM. Effect of handwashing on child health: A randomised controlled trial. Lancet. 2005;366:225–233. [PubMed] [Google Scholar]\\n12. Aiello AE, Larson EL, Levy SB. Consumer antibacterial soaps: Effective or just risky? Clin. Infect. Dis. 2007;45(Suppl 2):S137–S147\"\n },\n {\n \"doc_id\": 8816602139670831000,\n \"text\": \"\\n9. Dodrill L, Schmidt WP, Cobb E, Donachie P, Curtis V, de Barra M. Male commuters in North and South England: Risk factors for the presence of faecal bacteria on hands. BMC Public Health. 2010 (in press) [PMC free article] [PubMed] [Google Scholar]\\n10. Judah G, Donachie P, Cobb E, Schmidt W, Holland M, Curtis V. Dirty hands: bacteria of faecal origin on commuters’ hands. Epidemiol. Infect. 2010;138:409–414\"\n },\n {\n \"doc_id\": 7865256511831836000,\n \"text\": \"* 2019 AR Threats report. (2021).\\n[ https://www.cdc.gov/drugresistance/biggest-threats.html\\n](https://www.cdc.gov/drugresistance/biggest-threats.html)\"\n },\n {\n \"doc_id\": 8273483064709169000,\n \"text\": \" soap for both single and multiple exposure events (per day) ranged from 10–1 to 10⁻⁴, with contaminated eye drops having the greater risk. The concentration of P. aeruginosa was identified as the parameter contributing the greatest variance on eye infection risk; therefore, the prevalence and level of bacterial contamination of the product would have the greatest influence on health risk. Using eye drops in a single-use container or with preservatives can mitigate bacterial growth, and using non-refillable soap dispensers is recommended to reduce contamination of hand soap. Given the opportunistic nature of\"\n },\n {\n \"doc_id\": 5102934653837881000,\n \"text\": \"Abstract and Figures\\nHandwashing is thought to be effective for the prevention of transmission of diarrhoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handles and railings in public spaces. They were then allocated at random to (1) handwashing with water, (2) handwashing with non-antibacterial soap and (3) no handwashing. Each volunteer underwent this procedure 24 times, yielding 480 samples overall. Bacteria\"\n },\n {\n \"doc_id\": 1857752962571922700,\n \"text\": \"): 97–104.\\nPublished online 2011 Jan 6. doi: 10.3390/ijerph8010097\\nPMCID: PMC3037063\\nPMID: 21318017\\nThe Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands\\nMaxine Burton, Emma Cobb, Peter Donachie, Gaby Judah, Val Curtis, and Wolf-Peter Schmidt*\\nAuthor information Article notes Copyright and License information PMC Disclaimer\\nGo to:\\nAbstract\\nHandwashing is thought to be effective for the prevention of transmission of diarr\"\n },\n {\n \"doc_id\": 2081087526715216000,\n \"text\": \" during hand washing (AOR = 20.844, 95% CI: 2.190, 9.842) were significantly associated with the prevalence of bacterial isolation. Most Staphylococcus aureus isolates were susceptible to chloramphenicol (98.6%). Escherichia coli isolates were susceptible to tetracycline (75%), ceftriaxone (79.2%), chloramphenicol (87.5%), and ceftazidime (77.1%). Eighty percent of isolated Shigella species were susceptible to chloramp\"\n },\n {\n \"doc_id\": 4058345546028399000,\n \"text\": \"Go to:\\n2. Experimental Section\\nThis study was carried out between July and August 2009. Overall, 20 volunteers were taken to a large, frequently visited British museum, or asked to travel on a bus or the underground. They were asked to deliberately wipe their hands over hand contact surfaces such as handrails, door handles and seats with the aim of contaminating their hands with whatever bacteria were present. Using a pre-determined random sequence, not known to the participants during self-contamination, participants were then asked to wash their hands with soap, to use water only or not to\"\n },\n {\n \"doc_id\": 3641988751893605000,\n \"text\": \" assessment framework of QMRA to evaluate eye infection risks through both consumer products. The study examines the prevalence of this pathogen in eye drops and soap, as well as the critical need to implement measures that will mitigate bacterial exposure (e.g., single-use soap dispensers and eye drops with preservatives). Additionally, limitations and challenges are discussed, including the need to incorporate data regarding consumer practices, which may improve exposure assessments and health risk estimates.\\nView\\n... In healthcare, it is crucial to uphold proper hand hygiene as it represents a highly effective preventive measure in reducing infections [1]\"\n },\n {\n \"doc_id\": 824704417843875100,\n \"text\": \" handles were collected, the isolation of bacteria was done using standard microbiological procedures and identification of the isolates was done using cultural, microscopic and biochemical characterization. Determination of the antibiotic sensitivity pattern of the isolates was done using the Kirby-Bauer disc diffusion method on Muller Hinton agar. Antibiotics used included Ofloxacin (5 µg), Gentamicin (10 µg), Ceftriaxone (30 µg), Augmentin (30 µg), Ciprofloxacin (5 µg), Erythromycin (5\"\n },\n {\n \"doc_id\": 8995723049121223000,\n \"text\": \" Bacillus spp, known for bearing resistant spores, was also prevalent and has implications for human pathogenesis and food spoilage [15]. The presence of Klebsiella spp, Escherichia coli, Salmonella spp, and Enterococcus faecalis might suggest compromised personal and domestic hygiene, especially concerning hand contamination after restroom visits, thereby potentially predisposing individuals to diseases [15]....\\nHand Carriage of Microorganisms by Students of Federal University of Lafia, Nasarawa State, Nigeria\\nArticle\\nFull-text available\\nOct 2023\\nPeter U\"\n },\n {\n \"doc_id\": 6880289901152002000,\n \"text\": \". [PubMed] [Google Scholar]\\n13. Hoque BA, Mahalanabis D, Pelto B, Alam MJ. Research methodology for developing efficient handwashing options: An example from Bangladesh. J. Trop. Med. Hyg. 1995;98:469–475. [PubMed] [Google Scholar]\\n14. Kaltenthaler EC, Drasar BS, Potter CW. The use of microbiology in the study of hygiene behaviour. Microbios. 1996;88:35–43. [PubMed] [Google Scholar]\\n15. Pinf\"\n },\n {\n \"doc_id\": 3883468894091239400,\n \"text\": \"Unlike the study by Hoque and colleagues our trial was conducted in an experimental (albeit naturalistic) setting, where volunteers deliberately contaminated their hands. Additional testing showed that this approach increased the prevalence of contamination from around 10% to over 40% of individuals. It also improved control over the conduct of the experiment, but may affect generalisability, as the study primarily aimed at providing a proof of principle. However, we believe that the superior effect of soap on the removal of bacteria compared to water alone as the principal finding of our study is unlikely to depend on the setting.\"\n },\n {\n \"doc_id\": 7263983639828164000,\n \"text\": \"\\nArticlePDF Available\\nThe Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands\\nMDPI\\nDecember 2011International Journal of Environmental Research and Public Health (IJERPH) 8(1):97-104\\nDOI:10.3390/ijerph8010097\\nSourcePubMed\\nLicenseCC BY 3.0\\nAuthors:\\nMaxine Burton\\nEmma Cobb\\nPeter Donachie\\nGaby Judah\\nImperial College London\\nShow all 6 authors\"\n },\n {\n \"doc_id\": 202611872303150430,\n \"text\": \"ijerph-08-000\\n97.pdf\\nContent uploaded by Gaby Judah\\nAuthor content\\nContent may be subject to copyright.\\nInt. J. Environ. Res. Public Health 2011, 8, 97-104; doi:10.3390/ijerph8010097 International Journal of Environmental Research and Public Health ISSN 1660-4601 www.mdpi.com/journal/ijerph Article The Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands Maxine Burton, Emma Cobb, Peter Donachie,\"\n },\n {\n \"doc_id\": 154248976687680930,\n \"text\": \" 1 infection per 1,000,000 individuals)....\\nEye infection risks from Pseudomonas aeruginosa via hand soap and eye drops\\nArticle\\nFull-text available\\nMar 2024APPL ENVIRON MICROB\\nAnna GitterKristina D. MenaKarla S. Mendez\\nFuqing WuCharles P. Gerba\\nEye infections from bacterial contamination of bulk-refillable liquid soap dispensers and artificial tear eye drops continue to occur, resulting in adverse health outcomes that include impaired vision or eye enucleation. Pseudomonas aer\"\n },\n {\n \"doc_id\": 4702055053979222000,\n \"text\": \"It seems reasonable to assume that handwashing with soap is also more effective in reducing bacterial load compared to water alone. Future studies could address the effect of different hand hygiene procedures on removing gastro-intestinal or respiratory viruses such as influenza A. Hands have been implicated especially in the spread of Norovirus [16]. Viral studies are more difficult to conduct as viruses may not be as present in the environment as often as are bacteria of faecal origin, but they may be possible for example if patients with laboratory confirmed infection are recruited as volunteers. Alternatively, healthy volunteers may experimentally contaminate their hands\"\n },\n {\n \"doc_id\": 3248572189032461000,\n \"text\": \"2Fj.gfs.2019.100325&volume=25&publication_year=2020&author=Adesogan%2CAT&author=Havelaar%2CAH&author=McKune%2CSL&author=Eilitt%C3%A4%2CM&author=Dahl%2CGE)\"\n },\n {\n \"doc_id\": 2869156314969342500,\n \"text\": \"ates was 72% (95% CI: 66.2, 77.8). The dominant bacterial isolates were Staphylococcus aureus (31.6%), Escherichia coli (21.3%), Salmonella species (1.3%), Shigella species (6.7%), Klebsiella species (23.1%) and Proteus species (14.7%). Fingernail status (AOR =15.31, 95% CI: 10.372, 22.595) and the removal of a watch, ring, and bracelet\"\n },\n {\n \"doc_id\": 8327700382538876000,\n \"text\": \"The prevalence of bacterial contamination in the three study arms (soap, water, no handwash) was compared using logistic regression. Since the same volunteers repeatedly underwent testing, within-subject correlation was accounted for by the use of generalised estimating equations (GEE) with robust standard errors. If the cell numbers were too low for conducting regression analysis, Fishers exact test was used instead, ignoring clustering (the design effect was found to be low, see results).\"\n },\n {\n \"doc_id\": 2019813929377508000,\n \"text\": \" of potential faecal origin (mostly Enterococcus and Enterobacter spp.) were found after no handwashing in 44% of samples. Handwashing with water alone reduced the presence of bacteria to 23% (p < 0.001). Handwashing with plain soap and water reduced the presence of bacteria to 8% (comparison of both handwashing arms: p < 0.001). The effect did not appear to depend on the bacteria species. Handwashing with non-antibacterial soap and water is more effective for the removal of bacteria of potential faecal origin from hands\"\n },\n {\n \"doc_id\": 2684108841186596000,\n \"text\": \"hoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handles and railings in public spaces. They were then allocated at random to (1) handwashing with water, (2) handwashing with non-antibacterial soap and (3) no handwashing. Each volunteer underwent this procedure 24 times, yielding 480 samples overall. Bacteria of potential faecal origin (mostly Enterococcus and Enterobacter spp\"\n },\n {\n \"doc_id\": 8156286088047615000,\n \"text\": \"\\nThe Effect of Handwashing with Water or Soap on Bacterial Contamination of Hands\\nArticle\\nFull-text available\\nDec 2011\\nMaxine Burton\\nEmma Cobb\\nPeter Donachie[...]\\nWolf-Peter Schmidt\\nHandwashing is thought to be effective for the prevention of transmission of diarrhoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handle...\\nCite\\nDownload full-text\\nCitations\"\n },\n {\n \"doc_id\": 2451214201653555000,\n \"text\": \"But what happens outside the lab is less clear. In the biggest study of its kind, researchers recruited about 240 households in upper Manhattan to participate in a \\\"real-world\\\" hand washing study. Half were randomized to use 0.2% triclosan soap; half, to plain soap. After a year, the researchers tested the hands of the primary caregivers in the households for antibiotic-resistant bacteria. The result: no statistically significant difference between antibacterial and plain-soap households. The researchers offered several possible explanations for their findings (resistance may not develop in a year; high antibiotic\"\n },\n {\n \"doc_id\": 5409631546574559000,\n \"text\": \"Not all of the bacteria isolated in our study are known to cause disease in humans. Surprisingly, we found few E. coli on hands which may be due to their short survival time in the environment. Overall, the effect of soap appeared to be independent of the type of bacteria (Figure 1), a view which is supported by the study by Hoque and colleagues who found a similar effect of hand hygiene on unspecified faecal coliform bacteria [6]. However, the power of our study to detect differences between species was low.\"\n },\n {\n \"doc_id\": 5752874606470013000,\n \"text\": \" P. aeruginosa and its ability to thrive in unique environments, additional safeguards to mitigate bacterial growth and exposure are warranted. IMPORTANCE Pseudomonas aeruginosa (P. aeruginosa) is a pathogen that can persist in a variety of unusual environments and continues to pose a significant risk for public health. This quantitative microbial risk assessment (QMRA) estimates the potential human health risks, specifically for eye infections, associated with exposure to P. aeruginosa in bacteria-contaminated artificial tear eye drops and hand soap. This study applies the risk\"\n },\n {\n \"doc_id\": 6788887078786142000,\n \"text\": \"uginosa (P. aeruginosa), a common cause of eye infections, can grow in eye drop containers and refillable soap dispensers to high numbers. To assess the risk of eye infection, a quantitative microbial risk assessment for P. aeruginosa was conducted to predict the probability of an eye infection for two potential exposure scenarios: (i) individuals using bacteria-contaminated eye drops and (ii) contact lens wearers washing their hands with bacteria-contaminated liquid soap prior to placing the lens. The median risk of an eye infection using contaminated eye drops and hand\"\n },\n {\n \"doc_id\": 343709101918362700,\n \"text\": \", so hand hygiene might improve over all.\"\n },\n {\n \"doc_id\": 2423938153737278000,\n \"text\": \"Go to:\\n3. Results and Discussion\\nTable 1 shows the different organisms isolated in the three study arms. Enterococcus spp. were the most common bacteria found, followed by Enterobacter spp. Figure 1 shows the effect of handwashing with soap or water only on contamination, compared to no handwashing. Overall, handwashing with water alone reduced the prevalence of bacteria substantially. Handwashing with soap was more effective in reducing the prevalence of contamination and specifically of Enterococcus spp. There was a trend that handwashing with soap was also more effective in reducing the prevalence of\"\n },\n {\n \"doc_id\": 1428582819192438800,\n \"text\": \"Luby and colleagues found that a simple microbiological method with three fingers directly imprinting a MacConkey agar for thermotolerant coliforms was unable to distinguish between households who were given soap during a large randomized handwashing trial and control households [8]. They concluded that the method was unsuitable for the evaluation of handwashing practices. However, the lack of difference in bacterial contamination may have been due to lack of compliance with the intervention. We thought that a proof-of-principle trial was needed where participants would be given specific tasks to contaminate their hands in a\"\n },\n {\n \"doc_id\": 1568908759133818000,\n \"text\": \" slum in Bandung, Indonesia. We observed handwashing and conducted repeated hand swabs before and after handwashing among 137 participants. The mean E. coli concentration on the hands decreased after handwashing, with a higher reduction in E. coli count among students who used soap and had soap contact for more than 10 s during handwashing. Cleaning in-between fingers, using soap, soap contact for more than 10 s, and drying hands with a single-use towel were effective factors for reducing E. coli concentration after handwashing (p < 0.05). More than half of the sw\"\n },\n {\n \"doc_id\": 6245401161975709000,\n \"text\": \" a lack of hand-washing remains at unacceptable levels in most medical environments, with large numbers of doctors and nurses routinely forgetting to wash their hands before touching patients, thus transmitting microorganisms. One study showed that proper hand-washing and other simple procedures can decrease the rate of catheter-related bloodstream infections by 66%.\\nThe World Health Organization has published a sheet demonstrating standard hand-washing and hand-rubbing in health-care sectors. The draft guidance of hand hygiene by the organization can also be found at its website for public comment. A relevant review was conducted by Whitby et al. Commercial\"\n },\n {\n \"doc_id\": 8609808448557046000,\n \"text\": \"... Thus, hands serve as the vehicle of infectious disease transmission, especially amongst people living and working in close proximity to one another, such as dormitories, classrooms, camps etc. Close environments, doorknobs and other inanimate objects serving as resting vehicles of transmission all contribute to increased infection rates among these groups [14]. Human hands usually constitute microorganisms both as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms and usually harbour microorganisms both as residents and transients, the\"\n },\n {\n \"doc_id\": 6686149660008202000,\n \"text\": \" touches the dirty surface, they may deposit the virus at the eyes, nose, or mouth where it can enter the body and cause infection. Evidence indicates that contact with infected surfaces is not the main driver of COVID‑19, leading to recommendations for optimised disinfection procedures to avoid issues such as the increase of antimicrobial resistance through the use of inappropriate cleaning products and processes. Deep cleaning and other surface sanitation has been criticised as hygiene theatre, giving a false sense of security against something primarily spread through the air.\\nThe amount of time that the virus can survive depends significantly on the type of surface\"\n },\n {\n \"doc_id\": 32513998792769484,\n \"text\": \"lla spp, and Enterococcus faecalis might suggest compromised personal and domestic hygiene, especially concerning hand contamination after restroom visits, thereby potentially predisposing individuals to diseases [15]....\\nHand Carriage of Microorganisms by Students of Federal University of Lafia, Nasarawa State, Nigeria\\nArticle\\nFull-text available\\nOct 2023\\nPeter Upla\\nBashiru SaniOsuyi Gerard Uyi\\nGladys ABEL Angbalaga\\nView\\nShow abstract\\n... It has been mentioned transmission of bacteria occur from the handler by direct contact with the\"\n },\n {\n \"doc_id\": 5092476230231695000,\n \"text\": \"A number of studies have compared different hand hygiene methods in hospital settings [5]. In contrast, few studies have been published on the effect of hand hygiene on bacterial contamination of hands in the community. Hoque and colleagues found that a wide variety of hand cleansing means in poor settings (soap, ash, mud) are effective in reducing the contamination with coliform bacteria on hands [6,7]. In a small randomised trial the same author reported that soap may be more effective than water in reducing the presence of coliform bacteria on hands [6].\"\n },\n {\n \"doc_id\": 2105600531165297700,\n \"text\": \" hands with soap (both plain and antibacterial soap) is a proven mechanism to eliminate bacteria and respiratory viruses [31][32][33]. A systematic review of eight studies from a pool of 410 articles found that hand washing lowered the risk of respiratory infection, with risk reductions ranging from 6% to 44%, but noted that a greater number of rigorous studies are urgently needed [34]....\\nEstimating spatially disaggregated probability of severe COVID-19 and the impact of handwashing interventions: The case of Zimbabwe\\nArticle\\nFull-text available\\nNov 2023PLOS ONE\\nGeorge\"\n },\n {\n \"doc_id\": 2852937817197359600,\n \"text\": \"hoea pathogens. However it is not conclusive that handwashing with soap is more effective at reducing contamination with bacteria associated with diarrhoea than using water only. In this study 20 volunteers contaminated their hands deliberately by touching door handles and railings in public spaces. They were then allocated at random to (1) handwashing with water, (2) handwashing with non-antibacterial soap and (3) no handwashing. Each volunteer underwent this procedure 24 times, yielding 480 samples overall. Bacteria of potential faecal origin (mostly Enterococcus and Enterobacter spp.)\"\n },\n {\n \"doc_id\": 3426465220136984000,\n \"text\": \" mascara had the greatest rates....\\nIsolation and Identification of Harmful Microorganisms from the Shared Cosmetic Products in Delhi NCR Region\\nArticle\\nFull-text available\\nFeb 2024\\nGyan Vandana Yadav\\nSandhya Khunger\\nSunil KumarMukesh Sharma\\nView\\nShow abstract\\n... In healthcare, it is crucial to uphold proper hand hygiene as it represents a highly effective preventive measure in reducing infections [1] and managing the spread of illnesses [2,3]. Improper hand hygiene among healthcare workers is responsible for approximately 40% of healthcare-associated infections.\"\n },\n {\n \"doc_id\": 4494967955268539400,\n \"text\": \"Even if antibiotic resistance weren't an issue, results from this study (and others) make you wonder if the antibacterial soaps available to consumers add much to hand hygiene. In the Manhattan households, a year of washing with an antibacterial soap didn't lower bacterial counts on hands any more than a year of washing with plain soap. Nor did the antibacterial soap households experience fewer cold-like symptoms. That's not surprising: Colds are caused by viruses, not bacteria. Still, the finding is a useful reminder that the antibacterial soaps aren't the all-purpose germ fighters that many\"\n },\n {\n \"doc_id\": 795799110941267100,\n \"text\": \"Faecal Bacteria\\tNo Handwashing\\tWater only\\tSoap and water\\nEnterococcus spp.\\t46 (29%)\\t24 (15%)\\t4 (3%)\\nEnterobacter amnigenus\\t14 (9%)\\t4 (3%)\\t4 (3%)\\nEnterobacter cloacae\\t13 (8%)\\t5 (3%)\\t2 (1%)\\nShigella spp.\\t2 (1%)\\t1 (1%)\\t0 (0%)\\nKlebsiella spp.\\t5 (3%)\"\n },\n {\n \"doc_id\": 2677019744351524000,\n \"text\": \"Sohaib AsifXinyi Xu\\nZhao MingYusen Zhu\\nView\\nShow abstract\\n... The other reason might be the quality of the handwashing water. Pieces of evidence revealed that bacterial contamination of hands is significantly affected by handwashing water (54,55)....\\nPrevalence and antimicrobial susceptibility profile of bacteria isolated from the hands of housemaids in Jimma City, Ethiopia\\nArticle\\nFull-text available\\nJan 2024\\nTadele Shiwito AngoNegalgn Byadgie GelawGirma Mamo ZegeneTes\"\n },\n {\n \"doc_id\": 3981141855101259000,\n \"text\": \"falem Getahun\\nView\\nShow abstract\\nImpact of different hand drying methods on surrounding environment: aerosolization of virus and bacteria and transference to surfaces\\nArticle\\nMar 2024J HOSP INFECT\\nRodolphe HervéCatherine Bryant\\nLucy Sutton\\nSandra A Wilks\\nView\\nVertical stratification and seasonality of fecal indicator bacteria in New York City playground sandboxes\\nArticle\\nFeb 2024ECOTOX ENVIRON SAFE\\nAlessandra LeriG Eliana Fassihi\\nMatthew LundquistMariette\"\n },\n {\n \"doc_id\": 6504884612643356000,\n \"text\": \"Article\\nFull-text available\\nJan 2024\\nTadele Shiwito AngoNegalgn Byadgie GelawGirma Mamo ZegeneTizita TeshomeTesfalem Getahun\\nIntroduction Bacterial pathogens continue to be a major cause of foodborne gastroenteritis in humans and remain a public health problem. Housemaids operating inside a kitchen could be the source of infection and may transmit disease-inflicting pathogens through contaminated hands. Objective This study aimed to assess the prevalence and antimicrobial susceptibility profile of bacteria isolated from the hands of housemaid\"\n },\n {\n \"doc_id\": 6624727010274029000,\n \"text\": \" remains at unacceptable levels in most medical environments, with large numbers of doctors and nurses routinely forgetting to wash their hands before touching patients, thus transmitting microorganisms. One study showed that proper hand-washing and other simple procedures can decrease the rate of catheter-related bloodstream infections by 66%.\\nVideo demonstration on hand washing\\nThe World Health Organization has published a sheet demonstrating standard hand-washing and hand-rubbing in health-care sectors. The draft guidance of hand hygiene by the organization can also be found at its website for public comment. A relevant review was conducted by Whitby et al. Commercial\"\n },\n {\n \"doc_id\": 7346727617076172000,\n \"text\": \"old JV, Horan NJ. Measuring the effect of a hygiene behaviour intervention by indicators of behaviour and diarrhoeal disease. Trans. Roy. Soc. Trop. Med. Hyg. 1996;90:366–371. [PubMed] [Google Scholar]\\n16. Barker J, Vipond IB, Bloomfield SF. Effects of cleaning and disinfection in reducing the spread of Norovirus contamination via environmental surfaces. J. Hosp. Infect. 2004;58:42–49. [PubMed] [Google Scholar]\\n17. Grayson ML, Melv\"\n },\n {\n \"doc_id\": 3700513633745884700,\n \"text\": \" [PubMed] [Google Scholar]\\n7. Hoque BA, Mahalanabis D, Alam MJ, Islam MS. Post-defecation handwashing in Bangladesh: practice and efficiency perspectives. Public Health. 1995;109:15–24. [PubMed] [Google Scholar]\\n8. Luby SP, Agboatwalla M, Billhimer W, Hoekstra RM. Field trial of a low cost method to evaluate hand cleanliness. Trop. Med. Int. Health. 2007;12:765–771. [PubMed] [Google Scholar]\"\n },\n {\n \"doc_id\": 1669520942777172500,\n \"text\": \"\\nThe CDC says that in most situations, cleaning surfaces with soap or detergent, not disinfecting, is enough to reduce risk of transmission. The CDC recommends that if a COVID‑19 case is suspected or confirmed at a facility such as an office or day care, all areas such as offices, bathrooms, common areas, shared electronic equipment like tablets, touch screens, keyboards, remote controls, and ATMs used by the ill persons should be disinfected. Surfaces may be decontaminated with 62–71 per cent ethanol, 50–100 per cent isopropanol, 0\"\n },\n {\n \"doc_id\": 6490370016564395000,\n \"text\": \", a total of sixty (60) hand swab samples (thirty (30) from both palm and nails) were collected and tested for bacterial and fungal presence. Bacteria isolated were Staphylococcus epidermidis (80.00 %), Staphylococcus aureus (75.00 %), Enterococcus spp (50.00 %), Micrococcus spp (46.67 %), Escherichia coli (45.00 %), Klebsiella spp (45.00 %), Bacillus spp (30\"\n },\n {\n \"doc_id\": 1218148308571330000,\n \"text\": \" effective than alcohol-based hand rubs for reducing H1N1 influenza A virus and Clostridium difficile spores from hands.\\nInterventions to improve hand hygiene in healthcare settings can involve education for staff on hand washing, increasing the availability of alcohol-based hand rub, and written and verbal reminders to staff. There is a need for more research into which of these interventions are most effective in different healthcare settings.\"\n },\n {\n \"doc_id\": 8877592251602426000,\n \"text\": \". It also highlights the spatial differences in risk of severe COVID-19, and thus the opportunity for better planning to focus limited resources in high-risk areas in order to potentially reduce the number of severe cases.\\nView\\n... Thus, hands serve as the vehicle of infectious disease transmission, especially amongst people living and working in close proximity to one another, such as dormitories, classrooms, camps etc. Close environments, doorknobs and other inanimate objects serving as resting vehicles of transmission all contribute to increased infection rates among these groups [14]. Human hands usually constitute microorganisms both\"\n },\n {\n \"doc_id\": 8564106436840319000,\n \"text\": \"s in Jimma City, Ethiopia. Methods A laboratory-based cross-sectional study was employed among 234 housemaids. Hand swab samples from the dominant hand of the study participants were collected under sterile conditions following standard operating procedures. Then, in the laboratory, the swabs were inoculated aseptically using streak-plating methods on the growth media, such as mannitol salt agar [Staphylococcus aureus and coagulase-negative staphylococci], MacConkey agar [Klebsiella species and Proteus\"\n },\n {\n \"doc_id\": 2980497090769698300,\n \"text\": \" as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms and usually harbour microorganisms both as residents and transients, the presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]....\\n... Human hands usually constitute microorganisms both as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms\"\n },\n {\n \"doc_id\": 5176090711239746,\n \"text\": \"henicol and gentamicin respectively. In addition, Klebsiella and Proteus species exhibited high susceptibility to chloramphenicol. However, their isolates showed resistance against a number of the tested antimicrobials. Staphylococcus aureus isolates (28.2%) were resistance to tetracycline. Moreover, One-quarter of Escherichia coli isolates were resistance to tetracycline, ceftriaxone, chloramphenicol, and ceftazidime. Whereas 46.7% and 48.5\"\n },\n {\n \"doc_id\": 1547159199412704500,\n \"text\": \"Hygiene behaviour is difficult to measure because people tend to change their behaviour under observation or over-report desired practices [15,20]. We have previously shown that our test kit can be used to study associations between hygiene relevant behaviours and hand contamination [9]. We found that test results positive for bacteria of potential faecal origin were more common in people frequently shaking hands, reporting soil contact or those scoring low on a hygiene score based on self-report [9]. The microbiological method used in this and our earlier studies [9,10] is relatively simple and of low cost (around $3\"\n },\n {\n \"doc_id\": 6979437872508612000,\n \"text\": \"Promotion of improved hand hygiene has been recognised as an important public health measure but it is unclear how much hand hygiene is required to interrupt transmission of diarrhoea pathogens. In particular it has not been conclusively shown whether use of soap is essential to remove pathogens from hands. Recent hygiene promotion campaigns especially in low income settings have not been unanimous in recommending soap use [4].\"\n },\n {\n \"doc_id\": 3911170097265289000,\n \"text\": \".00 %), Salmonella spp (13.33 %) and Streptococcus spp (10.00 %). The fungi isolated were Aspergillus niger (45.00 %), Penicillium spp (23.33 %), Mucor spp (21.67 %), Candida spp (20.00 %) and Saccharomyces spp (15.00 %). Gender, level of study, faculty, and area of hand swab revealed no statistically significant variation in the presence of numerous bacterial and fungal species at\"\n },\n {\n \"doc_id\": 8056488143117447000,\n \"text\": \" be carried away by running water.\\nSolid soap, because of its reusable nature, may hold bacteria acquired from previous uses. A small number of studies which have looked at the bacterial transfer from contaminated solid soap have concluded transfer is unlikely as the bacteria are rinsed off with the foam. The CDC still states \\\"liquid soap with hands-free controls for dispensing is preferable\\\".\\nAntibacterial soaps have been heavily promoted to a health-conscious public. To date, there is no evidence that using recommended antiseptics or disinfectants selects for antibiotic-resistant organisms in nature. However\"\n },\n {\n \"doc_id\": 756020715464919600,\n \"text\": \" presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]....\\n... Human hands usually constitute microorganisms both as part of the body's normal flora and transient microorganisms contracted from the environment [15]. Although it is nearly impossible for the hands to be free of microorganisms and usually harbour microorganisms both as residents and transients, the presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]. Therefore,\"\n },\n {\n \"doc_id\": 7290049185625302000,\n \"text\": \" p<0.05. These findings revealed that the microbial burden on students' hands was significant and was unaffected by gender, level of study, or faculty. To improve students' overall health, appropriate hygiene, including regular handwashing practice, and public education about the importance of hands in disease transmission should be supported.\\nView\\n... Moreover, some respondents describe using soap as challenging because it is a new practice, and they believe washing with water is enough to make hands physically clean. Experimental trials have shown that HWWS is more effective in removing bacteria than using water alone (Burton et al\"\n },\n {\n \"doc_id\": 6463784370647876000,\n \"text\": \"The bacteriological methods used in this study provide no quantification of bacterial load, unlike a study by Hoque and colleagues [7]. Quantifying the effect of different hand washing procedures on bacterial load may be particularly helpful for studies in poor settings with poor sanitation facilities, where the environmental contamination with faecal organisms is much higher [13–15]. We also tested a semi-quantitative finger-print method used previously in Thailand [15] not unsimilar to the method used by Luby and colleagues [8] but found that contamination levels were too low to provide consistent results. Therefore we decided\"\n },\n {\n \"doc_id\": 8518308529609072000,\n \"text\": \". No. CD004265. [PubMed] [CrossRef] [Google Scholar]\\n5. WHO Guidelines on Hand Hygiene in Health Care. WHO; Geneva, Switzerland: 2009. [(accessed on 16 November 2010).]. Available online: http://whqlibdoc.who.int/publications/2009/9789241597906_eng.pdf [Google Scholar]\\n6. Hoque BA, Briend A. A comparison of local handwashing agents in Bangladesh. J. Trop. Med. Hyg. 1991;94:61–64.\"\n },\n {\n \"doc_id\": 7788926125136410000,\n \"text\": \" the study aimed to investigate the microorganisms isolated from the hands of students of the Federal University of Lafia, Nasarawa State, Nigeria....\\n... Additionally, these microorganisms, being opportunistic human pathogens, pose implications for food safety, particularly in the case of enterotoxin-producing strains of staphylococci linked to food poisoning [27]. Bacillus spp, known for bearing resistant spores, was also prevalent and has implications for human pathogenesis and food spoilage [15]. The presence of Klebsiella spp, Escherichia coli, Salmone\"\n },\n {\n \"doc_id\": 8365345683117400000,\n \"text\": \" (1) current risk of severe disease is heterogeneous across the country, due to differences in individual characteristics and household conditions and (2) that if the quantifiable estimates on the importance of handwashing for transmission are sound, then improvements in handwashing access could lead to reductions in the risk of severe COVID-19 of up to 16% from the estimated current levels across all districts. Conclusions Taken alongside the likely impact on transmission of SARS-CoV-2 itself, as well as countless other pathogens, this result adds further support for the expansion of access to handwashing across the country\"\n },\n {\n \"doc_id\": 3138911842569085400,\n \"text\": \"... HWWS for 20 s has been highlighted as an effective method for removing bacteria from the hands (WHO 2009). Wetting the hands with water and scrubbing with soap creates a lather that traps and eliminates these bacteria (Burton et al. 2011). Although some students performed handwashing for more than 20 s, only 6% of them scrubbed their hands with soap for that entire duration, while others did so briefly....\\nEffect of handwashing on the reduction of Escherichia coli on children's hands in urban slum Indonesia\\nArticle\\nFull-text available\\nOct\"\n },\n {\n \"doc_id\": 1376553736554764000,\n \"text\": \" 2023J WATER HEALTH\\nMahmud Aditya Rifqi\\nUmi Hamidah\\nNeni Sintawardani\\nHidenori Harada\\nTaro Yamauchi\\nPoor hand hygiene practice has been linked to an increase in the number of infections among children in urban slums. Hands are considered an intersection for bacterial transmission, but it is unclear whether the handwashing technique affects bacteria elimination. This study investigated the effect of handwashing on the concentration of Escherichia coli (E. coli) and factors related to its reduction among children in an urban\"\n },\n {\n \"doc_id\": 1581215079874269400,\n \"text\": \"teh Upla\\nBashiru Eya SaniOsuyi Gerard UyiIgoche Naomi Ibe\\nGladys Abel Angbalaga\\nMicrobes from the body's regular flora and transient microorganisms from the environment are found on human hands. Hands can also be used to spread disease from one person to another, especially among close persons. This study aimed to isolate and identify microorganisms from students' hands and assess the occurrence of these bacteria based on gender, level of study, faculty, and hand area (palm and nails swab). Using the pour plate method\"\n },\n {\n \"doc_id\": 3846559940304106000,\n \"text\": \"based solution with hand washing with antibacterial soap for a median time of 30 seconds each showed that the alcohol hand-rubbing reduced bacterial contamination 26% more than the antibacterial soap. But soap and water is more effective than alcohol-based hand rubs for reducing H1N1 influenza A virus and Clostridium difficile spores from hands.\\nInterventions to improve hand hygiene in healthcare settings can involve education for staff on hand washing, increasing the availability of alcohol-based hand rub, and written and verbal reminders to staff. There is a need for more research into which of these\"\n },\n {\n \"doc_id\": 5681005703472282000,\n \"text\": \" reduce the spread of pathogens, it is better to wash the hands or use a hand antiseptic before and after tending to a sick person.\\nFor control of staphylococcal infections in hospitals, it has been found that the greatest benefit from hand-cleansing came from the first 20% of washing, and that very little additional benefit was gained when hand cleansing frequency was increased beyond 35%. Washing with plain soap results in more than triple the rate of bacterial infectious disease transmitted to food as compared to washing with antibacterial soap.\\nComparing hand-rubbing with alcohol-\"\n },\n {\n \"doc_id\": 7844972954270361000,\n \"text\": \" areas such as offices, bathrooms, common areas, shared electronic equipment like tablets, touch screens, keyboards, remote controls, and ATMs used by the ill persons should be disinfected. Surfaces may be decontaminated with 62–71 per cent ethanol, 50–100 per cent isopropanol, 0.1 per cent sodium hypochlorite, 0.5 per cent hydrogen peroxide, 0.2–7.5 per cent povidone-iodine, or 50–200 ppm hypochlorous acid. Other solutions, such as benzalkonium\"\n },\n {\n \"doc_id\": 6187756745778273000,\n \"text\": \" and usually harbour microorganisms both as residents and transients, the presence and transfer of pathogenic microorganisms could occur between people who access the same areas or surfaces may lead to chronic or acute illnesses [15,16]. Therefore, the study aimed to investigate the microorganisms isolated from the hands of students of the Federal University of Lafia, Nasarawa State, Nigeria....\\n... Additionally, these microorganisms, being opportunistic human pathogens, pose implications for food safety, particularly in the case of enterotoxin-producing strains of staphylococci linked to food poisoning [27].\"\n },\n {\n \"doc_id\": 5810683772352509000,\n \"text\": \" which can cause a variety of diseases in humans.\\nView\\n... The other reason might be the quality of the handwashing water. Pieces of evidence revealed that bacterial contamination of hands is significantly affected by handwashing water (54,55)....\\nPrevalence and antimicrobial susceptibility profile of bacteria isolated from the hands of housemaids in Jimma City, Ethiopia\"\n },\n {\n \"doc_id\": 4770515370632447000,\n \"text\": \" Pseudomonas aeruginosa via hand soap and eye drops\\nArticle\\nFull-text available\\nMar 2024APPL ENVIRON MICROB\\nAnna GitterKristina D. MenaKarla S. MendezCharles P. Gerba\\nView\\nShow abstract\\n... Seventy to eighty percent of the chemicals used have been found tainted with bacteria [17]. Poor hand hygiene can cause used objects to become contaminated with bacteria like S. aureus and B. cereus [24]. All product categories had fungus contamination; however, the foundation, lipstick, and\"\n },\n {\n \"doc_id\": 6477730130678012000,\n \"text\": \" and moist hands are more easily recontaminated.\\nThe most commonly missed areas are the thumb, the wrist, the areas between the fingers, and under fingernails. Artificial nails and chipped nail polish may harbor microorganisms.\\nWhen it is recommended[edit]\\nThere are five critical times during the day where washing hands with soap is important to reduce fecal-oral transmission of disease: after using the toilet (for urination, defecation, menstrual hygiene), after cleaning a child's bottom (changing diapers), before feeding a child, before eating and before/after preparing\"\n },\n {\n \"doc_id\": 8310864475669528000,\n \"text\": \"hips in a range of subjects and topics for our PhD programmes starting September 2018. Background From global disease outbreaks to the rising burden of chronic conditions to threats...\\nView post\\nArticle\\nMultidrug-resistant, extensively drug-resistant and pandrug-resistant bacteria: An international exp...\\nMay 2011 · Clinical Microbiology and Infection\\nanna-pelagia MagiorakosA Srinivasan\\nRoberta B Carey[...]Dominique L Monnet\\nMany different definitions for multidrug-resistant (MDR), extensively drug-resistant (XDR) and pandrug\"\n },\n {\n \"doc_id\": 322654686885456640,\n \"text\": \" water for at least 20 seconds or use a hand sanitizer that contains at least 60% alcohol Cover your mouth and nose with a tissue when you cough or sneeze Stay home when sick Get recommended vaccines, such as the flu vaccine. To learn more about antibiotic prescribing and use, visit CDC’s Antibiotic Prescribing and Use website. To learn more about antibiotic resistance, visit CDC’s Antibiotic Resistance website.\"\n },\n {\n \"doc_id\": 433738502772842800,\n \"text\": \"); Val.Curtis@lshtm.ac.uk (V.C.) * Author to whom correspondence should be addressed; E-Mail: Wolf-Peter.Schmidt@lshtm.ac.uk; Tel.: +44-20-7927-2461; Fax: +44-20-7636-7843. Received: 24 November 2010; in revised form: 30 December 2010 / Accepted: 31 December 2010 / Published: 6 January 2011 Abstract: Handwashing is thought to be effective for the prevention of transmission of diarr\"\n },\n {\n \"doc_id\": 1510222038353803500,\n \"text\": \"What is new, though, is the range of organisms you might find on even a seemingly clean pair of hands.\"\n },\n {\n \"doc_id\": 9217255509341745000,\n \"text\": \".80). Its suitability for large scale use in the evaluation of handwashing campaigns in low income settings where handwashing should be most beneficial remains to be investigated. A sophisticated laboratory infrastructure may not be required to conduct testing. However, modifying the method to allow semi-quantitative or quantitative analysis may be necessary if contamination rates are high [15].\"\n },\n {\n \"doc_id\": 8947403650220114000,\n \"text\": \" no significant spread of micro-organisms. No studies have found a correlation to hand dryers and human health, however, making these findings inconsequential.\\nAccessibility[edit]\\nA community handwashing facility in Rwanda with sinks for people of different heights. During the COVID-19 pandemic in Rwanda handwashing was part of a system of public health measures encouraged to reduce transmission.\\nMaking hand washing facilities accessible (inclusive) to everyone is crucial to maintain hand washing behavior. Considerations for accessibility include age, disability, seasonality (with rains and muddiness), location and\"\n },\n {\n \"doc_id\": 3563355511531846700,\n \"text\": \" immediately. Do your best to stay healthy and keep others healthy: Clean hands by washing with soap and water for at least 20 seconds or use a hand sanitizer that contains at least 60% alcohol Cover your mouth and nose with a tissue when you cough or sneeze Stay home when sick Get recommended vaccines, such as the flu vaccine. To learn more about antibiotic prescribing and use, visit CDC’s Antibiotic Prescribing and Use website. To learn more about antibiotic resistance, visit CDC’s Antibiotic Resistance website.\"\n },\n {\n \"doc_id\": 8330182738007888000,\n \"text\": \" devices can measure and validate hand hygiene, if demonstration of regulatory compliance is required.\\nThe World Health Organization has \\\"Five Moments\\\" for washing hands:\\nThe addition of antiseptic chemicals to soap (\\\"medicated\\\" or \\\"antimicrobial\\\" soaps) confers killing action to a hand-washing agent. Such killing action may be desired before performing surgery or in settings in which antibiotic-resistant organisms are highly prevalent.\\nTo'scrub' one's hands for a surgical operation, it is necessary to have a tap that can be turned on and off without touching it with the hands\"\n },\n {\n \"doc_id\": 874043767480450600,\n \"text\": \" JosephSveta MilushevaHugh Sturrock\\nTonderai MapakoYi Rong Hoo\\nIntroduction The severity of COVID-19 disease varies substantially between individuals, with some infections being asymptomatic while others are fatal. Several risk factors have been identified that affect the progression of SARS-CoV-2 to severe COVID-19. They include age, smoking and presence of underlying comorbidities such as respiratory illness, HIV, anemia and obesity. Given that respiratory illness is one such comorbidity and is affected by hand hygiene, it\"\n },\n {\n \"doc_id\": 5744994367201210000,\n \"text\": \"icrobial resistance through the use of inappropriate cleaning products and processes. Deep cleaning and other surface sanitation has been criticised as hygiene theatre, giving a false sense of security against something primarily spread through the air.\\nThe amount of time that the virus can survive depends significantly on the type of surface, the temperature, and the humidity. Coronaviruses die very quickly when exposed to the UV light in sunlight. Like other enveloped viruses, SARS-CoV-2 survives longest when the temperature is at room temperature or lower, and when the relative humidity is low (<50%).\\nOn many surfaces\"\n },\n {\n \"doc_id\": 7475511348052728000,\n \"text\": \" others healthy: Clean hands by washing with soap and water for at least 20 seconds or use a hand sanitizer that contains at least 60% alcohol Cover your mouth and nose with a tissue when you cough or sneeze Stay home when sick Get recommended vaccines, such as the flu vaccine. To learn more about antibiotic prescribing and use, visit CDC’s Antibiotic Prescribing and Use website. To learn more about antibiotic resistance, visit CDC’s Antibiotic Resistance website. More Information CDC’s Be Antibiotics Aware Educational Effort U.S. Ant\"\n },\n {\n \"doc_id\": 6488001898291672000,\n \"text\": \"ani S, Druce J, Barr IG, Ballard SA, Johnson PD, Mastorakos T, Birch C. Efficacy of soap and water and alcohol-based hand-rub preparations against live H1N1 influenza virus on the hands of human volunteers. Clin. Infect. Dis. 2009;48:285–291. [PubMed] [Google Scholar]\\n18. Curtis V, Sidibe M, Scott BE, lyer P, Sara J. The Handwash Handbook: A Guide for Developing a Hygiene Promotion Program to Increase Handwashing with Soap.\"\n },\n {\n \"doc_id\": 8246575083859717000,\n \"text\": \" hygiene is central to preventing the spread of infectious diseases in home and everyday life settings.\\nThe World Health Organization (WHO) recommends washing hands for at least 20 seconds before and after certain activities. These include the five critical times during the day where washing hands with soap is important to reduce fecal-oral transmission of disease: after using the toilet (for urination, defecation, menstrual hygiene), after cleaning a child's bottom (changing diapers), before feeding a child, before eating and before/after preparing food or handling raw meat, fish, or poultry.\\nWhen neither hand washing nor\"\n },\n {\n \"doc_id\": 632135853865087000,\n \"text\": \"), use of a warm-air hand dryer spread micro-organisms only up to 0.25 metres (0.82 feet) from the dryer, and paper towels showed no significant spread of micro-organisms. No studies have found a correlation to hand dryers and human health, however, making these findings inconsequential.\\nMaking hand washing facilities accessible (inclusive) to everyone is crucial to maintain hand washing behavior. Considerations for accessibility include age, disability, seasonality (with rains and muddiness), location and more. Important aspects for good accessibility include: Placement of\"\n },\n {\n \"doc_id\": 3239197287654671000,\n \"text\": \" also be achieved mechanically with higher energy and maintenance costs. The use of large ducts and openings helps to prevent mixing in closed environments. Recirculation and mixing should be avoided because recirculation prevents dilution of harmful particles and redistributes possibly contaminated air, and mixing increases the concentration and range of infectious particles and keeps larger particles in the air.\\nThorough hand hygiene after any cough or sneeze is required. The WHO also recommends that individuals wash hands often with soap and water for at least twenty seconds, especially after going to the toilet or when hands are visibly dirty, before eating and\"\n }\n]","isConversational":false}}},{"rowIdx":99,"cells":{"query":{"kind":"string","value":"I'm currently studying the role of ATP (Adenosine Triphosphate) in cellular processes and have a specific question. How does the structure of ATP contribute to its function as the primary energy currency in biological systems? What are the key chemical properties of ATP that allow it to efficiently store and release energy during metabolic reactions? Additionally, how do enzymes facilitate the conversion of ATP to ADP (Adenosine Diphosphate) and vice versa, and what is the significance of these conversions in the context of cellular energy management? Any insights or references to relevant studies would be greatly appreciated!"},"positive_passages":{"kind":"list like","value":[{"doc_id":9077804600915274000,"text":"wiki/Dephosphorylation \"Dephosphorylation\") of ATP\nby enzymes known as [ ATPases ](/wiki/ATPase \"ATPase\"). The cleavage of a\nphosphate group from ATP results in the coupling of energy to metabolic\nreactions and a by-product of ADP. [1] ATP is continually reformed from\nlower-energy species ADP and AMP. The biosynthesis of ATP is achieved\nthroughout processes such as [ substrate-level phosphorylation\n](/wiki/Substrate-level"},{"doc_id":7353967940900421000,"text":"Adenosine triphosphate (ATP) is the energy currency for cellular processes.\nATP provides the energy for both energy-consuming endergonic reactions and\nenergy-releasing exergonic reactions, which require a small input of\nactivation energy. When the chemical bonds within ATP are broken, energy is\nreleased and can be harnessed for cellular work. The more bonds in a molecule,\nthe more potential energy it contains. Because the bond in ATP is so easily\nbroken and reformed, ATP is like a rechargeable battery that powers cellular\nprocess ranging from DNA"},{"doc_id":2389699283862194700,"text":"Adenosine Triphosphate\nATP is considered the molecular unit of intracellular energy currency. ATP derives its inherent energy secondary to anhydride bonds connecting adjacent phosphate functional groups. Hydrolysis of ATP energy generates energy for all cellular processes. In addition ATP also serves as a cofactor for signal transduction reactions using a variety of kinases as well as adenyl cyclase. Normally cellular ATP concentration is maintained in the range of 1 to 10 mmol/L, with a normal ratio of ATP/ADP of approximately 1000. Totally quantity of ATP in an adult is approximately"},{"doc_id":8203066485340092000,"text":" broken, release sufficient energy to power\na variety of cellular reactions and processes. The bond between the beta and\ngamma phosphate is considered “high-energy” because when the bond breaks, the\nproducts [adenosine diphosphate (ADP) and one inorganic phosphate group (P i\n)] have a lower free energy than the reactants (ATP and a water molecule). ATP\nbreakdown into ADP and P i is called hydrolysis because it consumes a water\nmolecule (hydro-, meaning “water�"},{"doc_id":8240113904112787000,"text":"ATP is a highly unstable molecule. Unless quickly used to perform work, ATP\nspontaneously dissociates into ADP + P i , and the free energy released\nduring this process is lost as heat. To harness the energy within the bonds of\nATP, cells use a strategy called energy coupling."},{"doc_id":4567960768741740000,"text":"* Adenosine triphosphate is composed of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphate groups.\n* ATP is hydrolyzed to ADP in the reaction ATP+H2O→ADP+Pi+ free energy; the calculated ∆G for the hydrolysis of 1 mole of ATP is -57 kJ/mol.\n* ADP is combined with a phosphate to form ATP in the reaction ADP+Pi+free energy→ATP+H2O.\n* The energy released from the hydro"},{"doc_id":6614567520638174000,"text":"Exactly how much free energy (∆G) is released with the hydrolysis of ATP, and\nhow is that free energy used to do cellular work? The calculated ∆G for the\nhydrolysis of one mole of ATP into ADP and P i is −7.3 kcal/mole (−30.5\nkJ/mol). However, this is only true under standard conditions, and the ∆G for\nthe hydrolysis of one mole of ATP in a living cell is almost double the value\nat standard conditions: 14 kcal/"},{"doc_id":3495922793584627000,"text":"lysis of ATP into ADP is used to perform cellular work, usually by coupling the exergonic reaction of ATP hydrolysis with endergonic reactions.\n* Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane while phosphorylation drives the endergonic reaction."},{"doc_id":7309316382340030000,"text":"Adenosine triphosphate (ATP) is comprised of the molecule adenosine bound to\nthree phosphate groups. Adenosine is a nucleoside consisting of the\nnitrogenous base adenine and the five-carbon sugar ribose. The three phosphate\ngroups, in order of closest to furthest from the ribose sugar, are labeled\nalpha, beta, and gamma. Together, these chemical groups constitute an energy\npowerhouse. The two bonds between the phosphates are equal high-energy bonds\n(phosphoanhydride bonds) that, when"}],"string":"[\n {\n \"doc_id\": 9077804600915274000,\n \"text\": \"wiki/Dephosphorylation \\\"Dephosphorylation\\\") of ATP\\nby enzymes known as [ ATPases ](/wiki/ATPase \\\"ATPase\\\"). The cleavage of a\\nphosphate group from ATP results in the coupling of energy to metabolic\\nreactions and a by-product of ADP. [1] ATP is continually reformed from\\nlower-energy species ADP and AMP. The biosynthesis of ATP is achieved\\nthroughout processes such as [ substrate-level phosphorylation\\n](/wiki/Substrate-level\"\n },\n {\n \"doc_id\": 7353967940900421000,\n \"text\": \"Adenosine triphosphate (ATP) is the energy currency for cellular processes.\\nATP provides the energy for both energy-consuming endergonic reactions and\\nenergy-releasing exergonic reactions, which require a small input of\\nactivation energy. When the chemical bonds within ATP are broken, energy is\\nreleased and can be harnessed for cellular work. The more bonds in a molecule,\\nthe more potential energy it contains. Because the bond in ATP is so easily\\nbroken and reformed, ATP is like a rechargeable battery that powers cellular\\nprocess ranging from DNA\"\n },\n {\n \"doc_id\": 2389699283862194700,\n \"text\": \"Adenosine Triphosphate\\nATP is considered the molecular unit of intracellular energy currency. ATP derives its inherent energy secondary to anhydride bonds connecting adjacent phosphate functional groups. Hydrolysis of ATP energy generates energy for all cellular processes. In addition ATP also serves as a cofactor for signal transduction reactions using a variety of kinases as well as adenyl cyclase. Normally cellular ATP concentration is maintained in the range of 1 to 10 mmol/L, with a normal ratio of ATP/ADP of approximately 1000. Totally quantity of ATP in an adult is approximately\"\n },\n {\n \"doc_id\": 8203066485340092000,\n \"text\": \" broken, release sufficient energy to power\\na variety of cellular reactions and processes. The bond between the beta and\\ngamma phosphate is considered “high-energy” because when the bond breaks, the\\nproducts [adenosine diphosphate (ADP) and one inorganic phosphate group (P i\\n)] have a lower free energy than the reactants (ATP and a water molecule). ATP\\nbreakdown into ADP and P i is called hydrolysis because it consumes a water\\nmolecule (hydro-, meaning “water�\"\n },\n {\n \"doc_id\": 8240113904112787000,\n \"text\": \"ATP is a highly unstable molecule. Unless quickly used to perform work, ATP\\nspontaneously dissociates into ADP + P i , and the free energy released\\nduring this process is lost as heat. To harness the energy within the bonds of\\nATP, cells use a strategy called energy coupling.\"\n },\n {\n \"doc_id\": 4567960768741740000,\n \"text\": \"* Adenosine triphosphate is composed of the nitrogenous base adenine, the five-carbon sugar ribose, and three phosphate groups.\\n* ATP is hydrolyzed to ADP in the reaction ATP+H2O→ADP+Pi+ free energy; the calculated ∆G for the hydrolysis of 1 mole of ATP is -57 kJ/mol.\\n* ADP is combined with a phosphate to form ATP in the reaction ADP+Pi+free energy→ATP+H2O.\\n* The energy released from the hydro\"\n },\n {\n \"doc_id\": 6614567520638174000,\n \"text\": \"Exactly how much free energy (∆G) is released with the hydrolysis of ATP, and\\nhow is that free energy used to do cellular work? The calculated ∆G for the\\nhydrolysis of one mole of ATP into ADP and P i is −7.3 kcal/mole (−30.5\\nkJ/mol). However, this is only true under standard conditions, and the ∆G for\\nthe hydrolysis of one mole of ATP in a living cell is almost double the value\\nat standard conditions: 14 kcal/\"\n },\n {\n \"doc_id\": 3495922793584627000,\n \"text\": \"lysis of ATP into ADP is used to perform cellular work, usually by coupling the exergonic reaction of ATP hydrolysis with endergonic reactions.\\n* Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane while phosphorylation drives the endergonic reaction.\"\n },\n {\n \"doc_id\": 7309316382340030000,\n \"text\": \"Adenosine triphosphate (ATP) is comprised of the molecule adenosine bound to\\nthree phosphate groups. Adenosine is a nucleoside consisting of the\\nnitrogenous base adenine and the five-carbon sugar ribose. The three phosphate\\ngroups, in order of closest to furthest from the ribose sugar, are labeled\\nalpha, beta, and gamma. Together, these chemical groups constitute an energy\\npowerhouse. The two bonds between the phosphates are equal high-energy bonds\\n(phosphoanhydride bonds) that, when\"\n }\n]","isConversational":false},"negative_passages":{"kind":"list like","value":[{"doc_id":4296125947924500000,"text":"## ATP Hydrolysis and Synthesis"},{"doc_id":209756938577391460,"text":"## ATP and Energy Coupling"},{"doc_id":1187127491154726100,"text":"Breaking one of ATP's phosphorus bonds generates approximately 30.5 [\nkilojoules ](/wiki/Kilojoule \"Kilojoule\") per [ mole ](/wiki/Mole_\\(unit\\)\n\"Mole \\(unit\\)\") of ATP (7.3 [ kcal ](/wiki/Calorie \"Calorie\") ). [3] ADP\ncan be converted, or powered back to ATP through the process of releasing the\nchemical energy available in food; in humans, this is constantly performed via\n[ aerobic resp"},{"doc_id":7450226740730646000,"text":"[ ](/wiki/File:ATP-ADP.svg) The cycles of synthesis and\ndegradation of ATP; 2 and 1 represent input and output of energy,\nrespectively."},{"doc_id":5368992354735622000,"text":"# 2.5.6: ATP- Adenosine Triphosphate"},{"doc_id":3035882699362796500,"text":" Beta oxidation\n* 4.1.3.1 Regulation\n* 4.1.4 Ketosis\n* 4.2 Production, anaerobic conditions\n* 4.2.1 ATP replenishment by nucleoside diphosphate kinases\n* 4.3 ATP production during photosynthesis\n* 4.4 ATP recycling\n* 5 Biochemical functions Toggle Biochemical functions subsection\n* 5.1 Intracellular signaling\n* 5.2 DNA and RNA synthesis\n* 5.3 Amino acid activation in"},{"doc_id":6425626719118181000,"text":"modynamics of the hydrolysis of adenosine 5′-triphosphate to adenosine 5′-diphosphate\" ](https://doi.org/10.1016%2FS0021-9258%2818%2967153-4). _J. Biol. Chem_. **261** (27): 12733–12737. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1016/S0021-9258(18)67"},{"doc_id":4926882949318777000,"text":"[\n](/wiki/File:AdenosineTriphosphate.qutemol.svg) Interactive animation of the\nstructure of ATP"},{"doc_id":3849867267559617000,"text":"2.5.6: ATP- Adenosine Triphosphate"},{"doc_id":1603955150925568000,"text":"Like most chemical reactions, the hydrolysis of ATP to ADP is reversible. The\nreverse reaction combines ADP + P i to regenerate ATP from ADP. Since ATP\nhydrolysis releases energy, ATP synthesis must require an input of free\nenergy."},{"doc_id":6837315664912708000,"text":" ATP in the payoff reactions of the [ TCA cycle ](/wiki/TCA_cycle\n\"TCA cycle\") and [ oxidative phosphorylation ](/wiki/Oxidative_phosphorylation\n\"Oxidative phosphorylation\") mechanism. [4] During the payoff phase of\nglycolysis, the enzymes phosphoglycerate kinase and pyruvate kinase facilitate\nthe addition of a phosphate group to ADP by way of [ substrate-level\nphosphorylation ](/wiki/Substrate-level_"},{"doc_id":4985419207898758000,"text":"ATP is involved in [ signal transduction ](/wiki/Signal_transduction \"Signal\ntransduction\") by serving as substrate for kinases, enzymes that transfer\nphosphate groups. Kinases are the most common ATP-binding proteins. They share\na small number of common folds. [31] [ Phosphorylation\n](/wiki/Phosphorylation \"Phosphorylation\") of a protein by a kinase can\nactivate a cascade such as the [ mitogen-activated protein kinase\n](/wiki/Mitogen-"},{"doc_id":7105853885166583000,"text":"ATP is also a substrate of [ adenylate cyclase ](/wiki/Adenylate_cyclase\n\"Adenylate cyclase\"), most commonly in [ G protein-coupled receptor\n](/wiki/G_protein%E2%80%93coupled_receptor \"G protein–coupled receptor\")\nsignal transduction pathways and is transformed to [ second messenger\n](/wiki/Second_messenger \"Second messenger\"), cyclic AMP, which is involved\nin triggering calcium signals by the release"},{"doc_id":1690748729578964200,"text":"[ ](/wiki/File:ATP-ADP.svg) The cycle of synthesis and\ndegradation of ATP; 1 and 2 represent output and input of energy,\nrespectively."},{"doc_id":4649545360479891000,"text":"## References [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=30 \"Edit\nsection: References\") ]"},{"doc_id":4181729790572705300,"text":"(https://pubmed.ncbi.nlm.nih.gov/12161449). [ Archived ](https://web.archive.org/web/20170808062708/http://www.jbc.org/content/277/32/e21) from the original on 2017-08-08 . Retrieved 2017-10-24 .\n48. ** ^ ** Maruyama, K. (March 1991). \"The discovery of adenosine triphosphate and the establishment of its structure\". _J."},{"doc_id":990683542711695000,"text":"umb/8/85/ATP-\nxtal-3D-balls.png/110px-ATP-xtal-3D-balls.png) ](/wiki/File:ATP-\nxtal-3D-balls.png) | [\n ](/wiki"},{"doc_id":3268199830213627000,"text":"During cellular metabolic reactions, or the synthesis and breakdown of\nnutrients, certain molecules must be altered slightly in their conformation to\nbecome substrates for the next step in the reaction series. In the very first\nsteps of cellular respiration, glucose is broken down through the process of\nglycolysis. ATP is required for the phosphorylation of glucose, creating a\nhigh-energy but unstable intermediate. This phosphorylation reaction causes a\nconformational change that allows enzymes to convert the phosphorylated\nglucose molecule to the phosphorylated sugar fructose."},{"doc_id":1915382988049666800,"text":"releases 20.5 kilojoules per mole (4.9 kcal/mol) of [ enthalpy\n](/wiki/Enthalpy \"Enthalpy\"). This may differ under physiological conditions\nif the reactant and products are not exactly in these ionization states. [15]\nThe values of the free energy released by cleaving either a phosphate (P i )\nor a pyrophosphate (PP i ) unit from ATP at [ standard state\n](/wiki/Standard_state \"Standard state\") concentrations of 1 mol"},{"doc_id":5485378309390847000,"text":" ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 31985968 ](https://pubmed.ncbi.nlm.nih.gov/31985968). Retrieved 13 November 2023 .\n3. ** ^ ** Knowles, J. R. (1980). \"Enzyme-catalyzed phosphoryl transfer reactions\". _Annu. Rev. Biochem_. **49** : 877–919. [ doi ](/wiki/Doi_\\("},{"doc_id":4074334652709847000,"text":"ADP can be interconverted to [ adenosine triphosphate\n](/wiki/Adenosine_triphosphate \"Adenosine triphosphate\") (ATP) and [ adenosine\nmonophosphate ](/wiki/Adenosine_monophosphate \"Adenosine monophosphate\")\n(AMP). ATP contains one more phosphate group than does ADP. AMP contains one\nfewer phosphate group. Energy transfer used by all living things is a result\nof [ dephosphorylation ](/"},{"doc_id":3366248625536973000,"text":" 21.4 The ATP cycle ](https://slcc.pressbooks.pub/collegebiology1/chapter/the-atp-cycle/)"},{"doc_id":4364460489719835000,"text":"The 1997 Nobel Prize in Chemistry was divided, one half jointly to [ Paul D.\nBoyer ](/wiki/Paul_D._Boyer \"Paul D. Boyer\") and [ John E. Walker\n](/wiki/John_E._Walker \"John E. Walker\") \"for their elucidation of the\nenzymatic mechanism underlying the synthesis of adenosine triphosphate (ATP)\"\nand the other half to [ Jens C. Skou ](/wiki/Jens_C._Skou \"Jens C. Skou\")"},{"doc_id":3864776642238982700,"text":"## Reactive aspects [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=4 \"Edit\nsection: Reactive aspects\") ]"},{"doc_id":7148843988463092000,"text":"## Chemical properties [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=3 \"Edit\nsection: Chemical properties\") ]"},{"doc_id":1634483825353413400,"text":"iration ](/wiki/Aerobic_respiration \"Aerobic respiration\") in\nthe [ mitochondria ](/wiki/Mitochondrion \"Mitochondrion\"). [2] Plants use [\nphotosynthetic ](/wiki/Photosynthetic \"Photosynthetic\") pathways to convert\nand store energy from sunlight, also conversion of ADP to ATP. [3] Animals\nuse the energy released in the breakdown of glucose and other molecules to\nconvert ADP to ATP, which can then be used to fuel necessary"},{"doc_id":3945391156819531300,"text":"7167-7108-1 ](/wiki/Special:BookSources/978-0-7167-7108-1 \"Special:BookSources/978-0-7167-7108-1\").\n2. ^ _**a** _ _**b** _ _**c** _ Nave, C.R. (2005). [ \"Adenosine Triphosphate\" ](http://hyperphysics.phy-astr.gsu.edu/hbase/biology/atp.html). _Hyper Physics"},{"doc_id":8908385768755888000,"text":"## ATP: Adenosine Triphosphate"},{"doc_id":5278833529683781000,"text":"Adenosine diphosphate [\n](/wiki/File:Adenosindiphosphat_protoniert.svg \"Skeletal formula of ADP\")\n---\n[. Britannica. 11 November 2023 . Retrieved 1 December 2023 .\n20. ^ _**a** _ _**b** _ Rich, P. R. (2003). \"The molecular machinery of Keilin's respiratory chain\". _Biochem. Soc. Trans_. **31** (6): 1095–1105. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(ident"},{"doc_id":1982598695332092000,"text":" Figure:\n**Adenosine Triphosphate (ATP)** : ATP is the primary energy currency of the\ncell. It has an adenosine backbone with three phosphate groups attached."},{"doc_id":936284948103574500,"text":"gov/29502283).\n43. ** ^ ** [ \"Ancient chemistry may explain why living things use ATP as the universal energy currency: An early step in metabolic evolution set the stage for emergence of ATP as the universal energy carrier\" ](https://www.sciencedaily.com/releases/2022/10/221004151228.htm). _ScienceDaily_. Retrieved 2023-08-27 .\n44. ** ^ ** Resetar, A. M.; Chalovich, J. M. (1995). \""},{"doc_id":7668927016311818000,"text":"## Biochemical functions [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=18 \"Edit\nsection: Biochemical functions\") ]"},{"doc_id":178783696209132400,"text":"\\) \"PMID \\(identifier\\)\") [ 11772 ](https://pubmed.ncbi.nlm.nih.gov/11772).\n7. ** ^ ** Wilson, J.; Chin, A. (1991). \"Chelation of divalent cations by ATP, studied by titration calorimetry\". _Anal. Biochem_. **193** (1): 16–19. [ doi ](/wiki/Doi_\\(identifier\\) \"Doi \\(identifier\\)\") : [ 10.1016/0003"},{"doc_id":965184437499335000,"text":"Cells couple the exergonic reaction of ATP hydrolysis with endergonic\nreactions to harness the energy within the bonds of ATP."},{"doc_id":2890202100238816000,"text":"/wiki/Special:SpecialPages \"A list of all special pages \\[q\\]\")\n* [ Permanent link ](/w/index.php?title=Adenosine_triphosphate&oldid=1219758132 \"Permanent link to this revision of this page\")\n* [ Page information ](/w/index.php?title=Adenosine_triphosphate&action=info \"More information about this page\")\n* [ Cite this page ](/w/index.php?title=Special"},{"doc_id":7193772420916502000,"text":"## Structure [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=1 \"Edit\nsection: Structure\") ]"},{"doc_id":1338774658429844500,"text":"* [ Download as PDF ](/w/index.php?title=Special:DownloadAsPdf&page=Adenosine_triphosphate&action=show-download-screen \"Download this page as a PDF file\")\n* [ Printable version ](/w/index.php?title=Adenosine_triphosphate&printable=yes \"Printable version of this page \\[p\\]\")"},{"doc_id":5699269757669299000,"text":"Adenosine-5'-triphosphate [\n\n](/wiki/File:Adenosintriphosphat_protoniert.svg)\n---\n| [![](//upload.wikimedia.org/wikipedia/commons/th"},{"doc_id":1436132316245030400,"text":"* [ Wikimedia Commons ](https://commons.wikimedia.org/wiki/Category:Adenosine_triphosphate)"},{"doc_id":311498269723767200,"text":"* (Top)\n* 1 Structure Toggle Structure subsection\n* 1.1 Metal cation binding\n* 2 Chemical properties\n* 3 Reactive aspects\n* 4 Production from AMP and ADP Toggle Production from AMP and ADP subsection\n* 4.1 Production, aerobic conditions\n* 4.1.1 Glycolysis\n* 4.1.1.1 Regulation\n* 4.1.2 Citric acid cycle\n* 4.1.2.1 Regulation\n* 4.1.3"},{"doc_id":266985005400561470,"text":"https://bio.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fbio.libretexts.org%2FBookshelves%2FMicrobiology%2FMicrobiology_(Boundless)%2F02%253A_Chemistry%2F2.05%253A_Organic_Compounds%2F2.5.06%253A_ATP-\n_Adenosine_Triphosphate"},{"doc_id":7956788937624895000,"text":"* [ ATP bound to proteins ](http://www.ebi.ac.uk/pdbe-srv/PDBeXplore/ligand/?ligand=ATP) in the [ PDB ](/wiki/Protein_Data_Bank \"Protein Data Bank\")\n* [ ScienceAid: Energy ATP and Exercise ](https://web.archive.org/web/20160310134410/http://www.scienceaid.co.uk/biology/biochemistry/atp.html)\n* [ PubChem entry for Ad"},{"doc_id":2779870307507492000,"text":" ](/wiki/PMC_\\(identifier\\) \"PMC \\(identifier\\)\") [ 1172435 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1172435). [ PMID ](/wiki/PMID_\\(identifier\\) \"PMID \\(identifier\\)\") [ 1212224 ](https://pubmed.ncbi.nlm.nih.gov/1212224).\n19. ** ^ ** [ \"Adenosine triphosphate\""},{"doc_id":3030292087953868300,"text":"ATP consists of an [ adenine ](/wiki/Adenine \"Adenine\") attached by the\n9-nitrogen atom to the 1′ [ carbon ](/wiki/Carbon \"Carbon\") [ atom\n](/wiki/Atom \"Atom\") of a sugar ( [ ribose ](/wiki/Ribose \"Ribose\") ), which\nin turn is attached at the 5' carbon atom of the sugar to a triphosphate\ngroup. In its many reactions related to metabolism, the adenine and sugar"},{"doc_id":4385062583235649500,"text":".org/10.1002%2F9780470048672.wecb648). [ ISBN ](/wiki/ISBN_\\(identifier\\) \"ISBN \\(identifier\\)\") [ 9780471754770 ](/wiki/Special:BookSources/9780471754770 \"Special:BookSources/9780471754770\").\n5. ** ^ ** Dunn, Jacob; Grider, Michael H. (2023), [ \"Physiology, Adenosine Triphosphate\" ]("},{"doc_id":8164153329772920000,"text":"From the perspective of [ biochemistry ](/wiki/Biochemistry \"Biochemistry\"),\nATP is classified as a [ nucleoside triphosphate\n](/wiki/Nucleoside_triphosphate \"Nucleoside triphosphate\"), which indicates\nthat it consists of three components: a nitrogenous base ( [ adenine\n](/wiki/Adenine \"Adenine\") ), the sugar [ ribose ](/wiki/Ribose \"Ribose\"),\nand the [ triphosphate ]"},{"doc_id":3232102919466913300,"text":"2. [ Save as PDF ](https://bio.libretexts.org/@api/deki/pages/8804/pdf/2.5.6%253A%2bATP-%2bAdenosine%2bTriphosphate.pdf \"Export page as a PDF\")\n3."},{"doc_id":7989093631317823000,"text":"ATP is stable in aqueous solutions between [ pH ](/wiki/PH \"PH\") 6.8 and 7.4\n(in the absence of catalysts). At more extreme pH levels, it rapidly [\nhydrolyses ](/wiki/Hydrolyses \"Hydrolyses\") to ADP and phosphate. Living cells\nmaintain the ratio of ATP to ADP at a point ten orders of magnitude from\nequilibrium, with ATP concentrations fivefold higher than the concentration of\nADP. [12] [13] In"},{"doc_id":2196154811093627100,"text":"\ngroups remain unchanged, but the triphosphate is converted to di- and\nmonophosphate, giving respectively the derivatives [ ADP\n](/wiki/Adenosine_diphosphate \"Adenosine diphosphate\") and [ AMP\n](/wiki/Adenosine_monophosphate \"Adenosine monophosphate\"). The three\nphosphoryl groups are labeled as alpha (α), beta (β), and, for the terminal\nphosphate, gamma (γ). [5]"},{"doc_id":1146594224582974600,"text":"**Adenosine triphosphate** ( **ATP** ) is a [ nucleotide ](/wiki/Nucleotide\n\"Nucleotide\") [2] that provides [ energy ](/wiki/Energy \"Energy\") to drive\nand support many processes in living [ cells ](/wiki/Cell_\\(biology\\) \"Cell\n\\(biology\\)\"), such as [ muscle contraction ](/wiki/Muscle_contraction\n\"Muscle contraction\"), [ nerve impulse ](/wiki/Nerve_impulse \"Nerve"},{"doc_id":6977782849683060000,"text":"Along with pH, the free energy change of ATP hydrolysis is also associated\nwith Mg 2+ concentration, from ΔG°' = −35.7 kJ/mol at a Mg 2+\nconcentration of zero, to ΔG°' = −31 kJ/mol at [Mg 2+ ] = 5 mM. Higher\nconcentrations of Mg 2+ decrease free energy released in the reaction due to\nbinding of Mg 2+ ions to negatively charged oxygen atoms of ATP at pH 7"},{"doc_id":3118770243673203000,"text":"ATP can also be synthesized through several so-called \"replenishment\"\nreactions catalyzed by the enzyme families of [ nucleoside diphosphate kinases\n](/wiki/Nucleoside_diphosphate_kinase \"Nucleoside diphosphate kinase\") (NDKs),\nwhich use other nucleoside triphosphates as a high-energy phosphate donor, and\nthe [ ATP:guanido-phosphotransferase\n](/wiki/ATP:guanido_phosph"},{"doc_id":3858892696968426000,"text":". Kinetic energy is the energy of an object\nas a result of its motion. The significance of ATP is in its ability to store\npotential energy within the phosphate bonds. The energy stored between these\nbonds can then be transferred to do work. For example, the transfer of energy\nfrom ATP to the protein [ myosin ](/wiki/Myosin \"Myosin\") causes a\nconformational change when connecting to [ actin ](/wiki/Actin \"Actin\") during\n[ muscle contraction ](/wiki/Muscle_"},{"doc_id":8759487471041125000,"text":"## References [ [ edit\n](/w/index.php?title=Adenosine_diphosphate&action=edit§ion=10 \"Edit\nsection: References\") ]"},{"doc_id":7800175872156098000,"text":"### ATP recycling [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=17 \"Edit\nsection: ATP recycling\") ]"},{"doc_id":4961826454351574000,"text":"## Energy Coupling in Metabolism"},{"doc_id":3838722632484139500,"text":"1. Learning Objectives\n2. Key Points\n3. Key Terms\n4. ATP: Adenosine Triphosphate\n5. Molecular Structure\n6. ATP Hydrolysis and Synthesis\n7. ATP and Energy Coupling\n8. Energy Coupling in Sodium-Potassium Pumps\n9. Energy Coupling in Metabolism"},{"doc_id":2827605596764358700,"text":"ADP cycling supplies the [ energy ](/wiki/Energy \"Energy\") needed to do work\nin a biological system, the [ thermodynamic ](/wiki/Thermodynamic\n\"Thermodynamic\") process of transferring energy from one source to another.\nThere are two types of energy: [ potential energy ](/wiki/Potential_energy\n\"Potential energy\") and [ kinetic energy ](/wiki/Kinetic_energy \"Kinetic\nenergy\"). Potential energy can be thought of as stored energy, or usable\nenergy that is available to do work"},{"doc_id":8283334338726298000,"text":"* Explain the role of ATP as the currency of cellular energy"},{"doc_id":5011687759077796000,"text":"P \\(shown here as a 3-\nion\\)](//upload.wikimedia.org/wikipedia/commons/thumb/9/99/Adenosine-\ndiphosphate-3D-balls.png/230px-Adenosine-diphosphate-3D-balls.png)\n](/wiki/File:Adenosine-diphosphate-3D-balls.png \"Ball-and-stick model of ADP\n\\(shown here as a 3- ion\\)\")\nNames\n[ IUPAC"},{"doc_id":3597971617827617000,"text":"* [ Read ](/wiki/Adenosine_triphosphate)\n* [ Edit ](/w/index.php?title=Adenosine_triphosphate&action=edit \"Edit this page \\[e\\]\")\n* [ View history ](/w/index.php?title=Adenosine_triphosphate&action=history \"Past revisions of this page \\[h\\]\")"},{"doc_id":1032511989725205500,"text":"# Adenosine triphosphate"},{"doc_id":613948337767182000,"text":"When consumed in a [ metabolic ](/wiki/Metabolism \"Metabolism\") processes, ATP\nconverts either to [ adenosine diphosphate ](/wiki/Adenosine_diphosphate\n\"Adenosine diphosphate\") (ADP) or to [ adenosine monophosphate\n](/wiki/Adenosine_monophosphate \"Adenosine monophosphate\") (AMP). Other\nprocesses regenerate ATP. It is also a [ precursor\n](/wiki/Precursor_\\"},{"doc_id":1996598610642749400,"text":"&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FAdenosine_triphosphate)\n* [ Wikidata item ](https://www.wikidata.org/wiki/Special:EntityPage/Q80863 \"Structured data on this page hosted by Wikidata \\[g\\]\")"},{"doc_id":978360755576965600,"text":"* [ Article ](/wiki/Adenosine_triphosphate \"View the content page \\[c\\]\")\n* [ Talk ](/wiki/Talk:Adenosine_triphosphate \"Discuss improvements to the content page \\[t\\]\")"},{"doc_id":8514217792710328000,"text":"synthase \"ATP synthase\") enzyme; the\nequation for this can be written as ADP + P i → ATP. [ _[ citation needed\n](/wiki/Wikipedia:Citation_needed \"Wikipedia:Citation needed\") _ ]"},{"doc_id":740495691876270500,"text":"This page titled [ 2.5.6: ATP- Adenosine Triphosphate\n](https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_\\(Boundless\\)/02%3A_Chemistry/2.05%3A_Organic_Compounds/2.5.06%3A_ATP-\n_Adenosine_Triphosphate) is shared under a [ CC BY-SA 4.0\n](https://creativecommons.org/licenses/by"},{"doc_id":6271573277080441000,"text":"\nallosterically inhibited by high concentrations of ATP and activated by high\nconcentrations of AMP. The inhibition of PFK by ATP is unusual since ATP is\nalso a substrate in the reaction catalyzed by PFK; the active form of the\nenzyme is a [ tetramer ](/wiki/Tetramer_protein \"Tetramer protein\") that\nexists in two conformations, only one of which binds the second substrate\nfructose-6-phosphate (F6P). The protein has two [ binding sites\n](/wiki/B"},{"doc_id":3369311530784969000,"text":"_phosphorylation \"Substrate-level phosphorylation\"), [\noxidative phosphorylation ](/wiki/Oxidative_phosphorylation \"Oxidative\nphosphorylation\"), and [ photophosphorylation ](/wiki/Photophosphorylation\n\"Photophosphorylation\"), all of which facilitate the addition of a phosphate\ngroup to ADP."},{"doc_id":6412499927344170000,"text":"## History [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=28 \"Edit\nsection: History\") ]"},{"doc_id":2025235775950557400,"text":"* [ Read ](/wiki/Adenosine_triphosphate)\n* [ Edit ](/w/index.php?title=Adenosine_triphosphate&action=edit \"Edit this page \\[e\\]\")\n* [ View history ](/w/index.php?title=Adenosine_triphosphate&action=history)"},{"doc_id":1907208704393033200,"text":"Cells couple the exergonic reaction of ATP hydrolysis with the endergonic\nreactions of cellular processes. For example, transmembrane ion pumps in nerve\ncells use the energy from ATP to pump ions across the cell membrane and\ngenerate an action potential. The sodium-potassium pump (Na \\+ /K \\+ pump)\ndrives sodium out of the cell and potassium into the cell. When ATP is\nhydrolyzed, it transfers its gamma phosphate to the pump protein in a process\ncalled phosphorylation. The"},{"doc_id":3412699478476082700,"text":"#### ATP replenishment by nucleoside diphosphate kinases [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=15 \"Edit\nsection: ATP replenishment by nucleoside diphosphate kinases\") ]"},{"doc_id":388825824706267900,"text":"ifier\\)\") [ 30713504 ](https://pubmed.ncbi.nlm.nih.gov/30713504).\n25. ^ _**a** _ _**b** _ Dahout-Gonzalez, C.; Nury, H.; Trézéguet, V.; Lauquin, G.; Pebay-Peyroula, E.; Brandolin, G. (2006). \"Molecular, functional, and pathological aspects of the mitochondrial ADP/ATP carrier\". _Physiology_. **21** (4"},{"doc_id":7187161046571647000,"text":"The hydrolysis of ATP into ADP and inorganic phosphate"},{"doc_id":57114420802227810,"text":"-aldrich/docs/Sigma/Product_Information_Sheet/a7699pis.pdf) (PDF) from the original on 2019-03-23 . Retrieved 2019-03-22 .\n2. ** ^ ** Dunn, Jacob; Grider, Michael H. (2023). [ \"Physiology, Adenosine Triphosphate\" ](https://www.ncbi.nlm.nih.gov/books/NBK553175/). _StatPearls_. StatPearls Publishing. [ PMID"},{"doc_id":1990050540664970200,"text":"* [ Article ](/wiki/Adenosine_diphosphate \"View the content page \\[c\\]\")\n* [ Talk ](/wiki/Talk:Adenosine_diphosphate \"Discuss improvements to the content page \\[t\\]\")"},{"doc_id":6493961511181788000,"text":"Adenosine Triphosphate\nATP is a nucleotide consisting of an adenine, a 5-carbon sugar (ribose), and three phosphate groups."},{"doc_id":2464959256312389600,"text":"1. aa + ATP ⟶ aa-AMP + [ PP i ](/wiki/Pyrophosphate \"Pyrophosphate\")\n2. aa-AMP + tRNA ⟶ aa-tRNA + AMP"},{"doc_id":4840394897342918000,"text":"ATP is hydrolyzed into ADP in the following reaction:"},{"doc_id":4028192244389689300,"text":"* [ What links here ](/wiki/Special:WhatLinksHere/Adenosine_triphosphate \"List of all English Wikipedia pages containing links to this page \\[j\\]\")\n* [ Related changes ](/wiki/Special:RecentChangesLinked/Adenosine_triphosphate \"Recent changes in pages linked from this page \\[k\\]\")\n* [ Upload file ](/wiki/Wikipedia:File_Upload_Wizard \"Upload files \\[u\\]\")\n* [ Special pages ]("},{"doc_id":7414217072591005000,"text":"### Intracellular signaling [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=19 \"Edit\nsection: Intracellular signaling\") ]"},{"doc_id":6604415871490042000,"text":"It takes multiple reactions between myosin and actin to effectively produce\none muscle contraction, and, therefore, the availability of large amounts of\nATP is required to produce each muscle contraction. For this reason,\nbiological processes have evolved to produce efficient ways to replenish the\npotential energy of ATP from ADP. [2]"},{"doc_id":8309147740403247000,"text":"A typical intracellular [ concentration ](/wiki/Concentration \"Concentration\")\nof ATP may be 1–10 μmol per gram of tissue in a variety of eukaryotes. [18]\nThe dephosphorylation of ATP and rephosphorylation of ADP and AMP occur\nrepeatedly in the course of aerobic metabolism. [19]"},{"doc_id":6938585289525588000,"text":"At cytoplasmic conditions, where the ADP/ATP ratio is 10 orders of magnitude\nfrom equilibrium, the Δ _G_ is around −57 kJ/mol. [12]"},{"doc_id":8832330378635948000,"text":"Abstract\nAdenosine 5′-triphosphate (ATP) is a neurotransmitter of postganglionic sympathetic nerves and nonadrenergic, noncholinergic nerves of the enteric nervous system. ATP can also modulate autonomic reflexes through actions at sensory afferent nerves. Mechanisms exist within nerves for the vesicular storage and exocytotic release of ATP. Once released, the availability and duration of ATP is regulated by cell surface ectonucleotidases that hydrolyze ATP, producing metabolites including ADP and adenosine in the process"},{"doc_id":2730431974490151000,"text":"Polyanionic and featuring a potentially [ chelating ](/wiki/Chelation\n\"Chelation\") polyphosphate group, ATP binds metal cations with high affinity.\nThe [ binding constant ](/wiki/Binding_constant \"Binding constant\") for [ Mg\n2+\n](/wiki/Magnesium \"Magnesium\") is ( 9 554 ). [7] The binding of a [\ndivalent ](/wiki/Divalent \"Divalent\") [ cation ](/wiki/Cation"},{"doc_id":4136401358466617000,"text":"### Metal cation binding [ [ edit\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=2 \"Edit\nsection: Metal cation binding\") ]"},{"doc_id":1989801421615764700,"text":"ATP + H\n2 O → ADP + P i Δ _G_ °' = −30.5 kJ/mol (−7.3 kcal/mol)"}],"string":"[\n {\n \"doc_id\": 4296125947924500000,\n \"text\": \"## ATP Hydrolysis and Synthesis\"\n },\n {\n \"doc_id\": 209756938577391460,\n \"text\": \"## ATP and Energy Coupling\"\n },\n {\n \"doc_id\": 1187127491154726100,\n \"text\": \"Breaking one of ATP's phosphorus bonds generates approximately 30.5 [\\nkilojoules ](/wiki/Kilojoule \\\"Kilojoule\\\") per [ mole ](/wiki/Mole_\\\\(unit\\\\)\\n\\\"Mole \\\\(unit\\\\)\\\") of ATP (7.3 [ kcal ](/wiki/Calorie \\\"Calorie\\\") ). [3] ADP\\ncan be converted, or powered back to ATP through the process of releasing the\\nchemical energy available in food; in humans, this is constantly performed via\\n[ aerobic resp\"\n },\n {\n \"doc_id\": 7450226740730646000,\n \"text\": \"[ ](/wiki/File:ATP-ADP.svg) The cycles of synthesis and\\ndegradation of ATP; 2 and 1 represent input and output of energy,\\nrespectively.\"\n },\n {\n \"doc_id\": 5368992354735622000,\n \"text\": \"# 2.5.6: ATP- Adenosine Triphosphate\"\n },\n {\n \"doc_id\": 3035882699362796500,\n \"text\": \" Beta oxidation\\n* 4.1.3.1 Regulation\\n* 4.1.4 Ketosis\\n* 4.2 Production, anaerobic conditions\\n* 4.2.1 ATP replenishment by nucleoside diphosphate kinases\\n* 4.3 ATP production during photosynthesis\\n* 4.4 ATP recycling\\n* 5 Biochemical functions Toggle Biochemical functions subsection\\n* 5.1 Intracellular signaling\\n* 5.2 DNA and RNA synthesis\\n* 5.3 Amino acid activation in\"\n },\n {\n \"doc_id\": 6425626719118181000,\n \"text\": \"modynamics of the hydrolysis of adenosine 5′-triphosphate to adenosine 5′-diphosphate\\\" ](https://doi.org/10.1016%2FS0021-9258%2818%2967153-4). _J. Biol. Chem_. **261** (27): 12733–12737. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1016/S0021-9258(18)67\"\n },\n {\n \"doc_id\": 4926882949318777000,\n \"text\": \"[\\n](/wiki/File:AdenosineTriphosphate.qutemol.svg) Interactive animation of the\\nstructure of ATP\"\n },\n {\n \"doc_id\": 3849867267559617000,\n \"text\": \"2.5.6: ATP- Adenosine Triphosphate\"\n },\n {\n \"doc_id\": 1603955150925568000,\n \"text\": \"Like most chemical reactions, the hydrolysis of ATP to ADP is reversible. The\\nreverse reaction combines ADP + P i to regenerate ATP from ADP. Since ATP\\nhydrolysis releases energy, ATP synthesis must require an input of free\\nenergy.\"\n },\n {\n \"doc_id\": 6837315664912708000,\n \"text\": \" ATP in the payoff reactions of the [ TCA cycle ](/wiki/TCA_cycle\\n\\\"TCA cycle\\\") and [ oxidative phosphorylation ](/wiki/Oxidative_phosphorylation\\n\\\"Oxidative phosphorylation\\\") mechanism. [4] During the payoff phase of\\nglycolysis, the enzymes phosphoglycerate kinase and pyruvate kinase facilitate\\nthe addition of a phosphate group to ADP by way of [ substrate-level\\nphosphorylation ](/wiki/Substrate-level_\"\n },\n {\n \"doc_id\": 4985419207898758000,\n \"text\": \"ATP is involved in [ signal transduction ](/wiki/Signal_transduction \\\"Signal\\ntransduction\\\") by serving as substrate for kinases, enzymes that transfer\\nphosphate groups. Kinases are the most common ATP-binding proteins. They share\\na small number of common folds. [31] [ Phosphorylation\\n](/wiki/Phosphorylation \\\"Phosphorylation\\\") of a protein by a kinase can\\nactivate a cascade such as the [ mitogen-activated protein kinase\\n](/wiki/Mitogen-\"\n },\n {\n \"doc_id\": 7105853885166583000,\n \"text\": \"ATP is also a substrate of [ adenylate cyclase ](/wiki/Adenylate_cyclase\\n\\\"Adenylate cyclase\\\"), most commonly in [ G protein-coupled receptor\\n](/wiki/G_protein%E2%80%93coupled_receptor \\\"G protein–coupled receptor\\\")\\nsignal transduction pathways and is transformed to [ second messenger\\n](/wiki/Second_messenger \\\"Second messenger\\\"), cyclic AMP, which is involved\\nin triggering calcium signals by the release\"\n },\n {\n \"doc_id\": 1690748729578964200,\n \"text\": \"[ ](/wiki/File:ATP-ADP.svg) The cycle of synthesis and\\ndegradation of ATP; 1 and 2 represent output and input of energy,\\nrespectively.\"\n },\n {\n \"doc_id\": 4649545360479891000,\n \"text\": \"## References [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=30 \\\"Edit\\nsection: References\\\") ]\"\n },\n {\n \"doc_id\": 4181729790572705300,\n \"text\": \"(https://pubmed.ncbi.nlm.nih.gov/12161449). [ Archived ](https://web.archive.org/web/20170808062708/http://www.jbc.org/content/277/32/e21) from the original on 2017-08-08 . Retrieved 2017-10-24 .\\n48. ** ^ ** Maruyama, K. (March 1991). \\\"The discovery of adenosine triphosphate and the establishment of its structure\\\". _J.\"\n },\n {\n \"doc_id\": 990683542711695000,\n \"text\": \"umb/8/85/ATP-\\nxtal-3D-balls.png/110px-ATP-xtal-3D-balls.png) ](/wiki/File:ATP-\\nxtal-3D-balls.png) | [\\n ](/wiki\"\n },\n {\n \"doc_id\": 3268199830213627000,\n \"text\": \"During cellular metabolic reactions, or the synthesis and breakdown of\\nnutrients, certain molecules must be altered slightly in their conformation to\\nbecome substrates for the next step in the reaction series. In the very first\\nsteps of cellular respiration, glucose is broken down through the process of\\nglycolysis. ATP is required for the phosphorylation of glucose, creating a\\nhigh-energy but unstable intermediate. This phosphorylation reaction causes a\\nconformational change that allows enzymes to convert the phosphorylated\\nglucose molecule to the phosphorylated sugar fructose.\"\n },\n {\n \"doc_id\": 1915382988049666800,\n \"text\": \"releases 20.5 kilojoules per mole (4.9 kcal/mol) of [ enthalpy\\n](/wiki/Enthalpy \\\"Enthalpy\\\"). This may differ under physiological conditions\\nif the reactant and products are not exactly in these ionization states. [15]\\nThe values of the free energy released by cleaving either a phosphate (P i )\\nor a pyrophosphate (PP i ) unit from ATP at [ standard state\\n](/wiki/Standard_state \\\"Standard state\\\") concentrations of 1 mol\"\n },\n {\n \"doc_id\": 5485378309390847000,\n \"text\": \" ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 31985968 ](https://pubmed.ncbi.nlm.nih.gov/31985968). Retrieved 13 November 2023 .\\n3. ** ^ ** Knowles, J. R. (1980). \\\"Enzyme-catalyzed phosphoryl transfer reactions\\\". _Annu. Rev. Biochem_. **49** : 877–919. [ doi ](/wiki/Doi_\\\\(\"\n },\n {\n \"doc_id\": 4074334652709847000,\n \"text\": \"ADP can be interconverted to [ adenosine triphosphate\\n](/wiki/Adenosine_triphosphate \\\"Adenosine triphosphate\\\") (ATP) and [ adenosine\\nmonophosphate ](/wiki/Adenosine_monophosphate \\\"Adenosine monophosphate\\\")\\n(AMP). ATP contains one more phosphate group than does ADP. AMP contains one\\nfewer phosphate group. Energy transfer used by all living things is a result\\nof [ dephosphorylation ](/\"\n },\n {\n \"doc_id\": 3366248625536973000,\n \"text\": \" 21.4 The ATP cycle ](https://slcc.pressbooks.pub/collegebiology1/chapter/the-atp-cycle/)\"\n },\n {\n \"doc_id\": 4364460489719835000,\n \"text\": \"The 1997 Nobel Prize in Chemistry was divided, one half jointly to [ Paul D.\\nBoyer ](/wiki/Paul_D._Boyer \\\"Paul D. Boyer\\\") and [ John E. Walker\\n](/wiki/John_E._Walker \\\"John E. Walker\\\") \\\"for their elucidation of the\\nenzymatic mechanism underlying the synthesis of adenosine triphosphate (ATP)\\\"\\nand the other half to [ Jens C. Skou ](/wiki/Jens_C._Skou \\\"Jens C. Skou\\\")\"\n },\n {\n \"doc_id\": 3864776642238982700,\n \"text\": \"## Reactive aspects [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=4 \\\"Edit\\nsection: Reactive aspects\\\") ]\"\n },\n {\n \"doc_id\": 7148843988463092000,\n \"text\": \"## Chemical properties [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=3 \\\"Edit\\nsection: Chemical properties\\\") ]\"\n },\n {\n \"doc_id\": 1634483825353413400,\n \"text\": \"iration ](/wiki/Aerobic_respiration \\\"Aerobic respiration\\\") in\\nthe [ mitochondria ](/wiki/Mitochondrion \\\"Mitochondrion\\\"). [2] Plants use [\\nphotosynthetic ](/wiki/Photosynthetic \\\"Photosynthetic\\\") pathways to convert\\nand store energy from sunlight, also conversion of ADP to ATP. [3] Animals\\nuse the energy released in the breakdown of glucose and other molecules to\\nconvert ADP to ATP, which can then be used to fuel necessary\"\n },\n {\n \"doc_id\": 3945391156819531300,\n \"text\": \"7167-7108-1 ](/wiki/Special:BookSources/978-0-7167-7108-1 \\\"Special:BookSources/978-0-7167-7108-1\\\").\\n2. ^ _**a** _ _**b** _ _**c** _ Nave, C.R. (2005). [ \\\"Adenosine Triphosphate\\\" ](http://hyperphysics.phy-astr.gsu.edu/hbase/biology/atp.html). _Hyper Physics\"\n },\n {\n \"doc_id\": 8908385768755888000,\n \"text\": \"## ATP: Adenosine Triphosphate\"\n },\n {\n \"doc_id\": 5278833529683781000,\n \"text\": \"Adenosine diphosphate [\\n](/wiki/File:Adenosindiphosphat_protoniert.svg \\\"Skeletal formula of ADP\\\")\\n---\\n[. Britannica. 11 November 2023 . Retrieved 1 December 2023 .\\n20. ^ _**a** _ _**b** _ Rich, P. R. (2003). \\\"The molecular machinery of Keilin's respiratory chain\\\". _Biochem. Soc. Trans_. **31** (6): 1095–1105. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(ident\"\n },\n {\n \"doc_id\": 1982598695332092000,\n \"text\": \" Figure:\\n**Adenosine Triphosphate (ATP)** : ATP is the primary energy currency of the\\ncell. It has an adenosine backbone with three phosphate groups attached.\"\n },\n {\n \"doc_id\": 936284948103574500,\n \"text\": \"gov/29502283).\\n43. ** ^ ** [ \\\"Ancient chemistry may explain why living things use ATP as the universal energy currency: An early step in metabolic evolution set the stage for emergence of ATP as the universal energy carrier\\\" ](https://www.sciencedaily.com/releases/2022/10/221004151228.htm). _ScienceDaily_. Retrieved 2023-08-27 .\\n44. ** ^ ** Resetar, A. M.; Chalovich, J. M. (1995). \\\"\"\n },\n {\n \"doc_id\": 7668927016311818000,\n \"text\": \"## Biochemical functions [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=18 \\\"Edit\\nsection: Biochemical functions\\\") ]\"\n },\n {\n \"doc_id\": 178783696209132400,\n \"text\": \"\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 11772 ](https://pubmed.ncbi.nlm.nih.gov/11772).\\n7. ** ^ ** Wilson, J.; Chin, A. (1991). \\\"Chelation of divalent cations by ATP, studied by titration calorimetry\\\". _Anal. Biochem_. **193** (1): 16–19. [ doi ](/wiki/Doi_\\\\(identifier\\\\) \\\"Doi \\\\(identifier\\\\)\\\") : [ 10.1016/0003\"\n },\n {\n \"doc_id\": 965184437499335000,\n \"text\": \"Cells couple the exergonic reaction of ATP hydrolysis with endergonic\\nreactions to harness the energy within the bonds of ATP.\"\n },\n {\n \"doc_id\": 2890202100238816000,\n \"text\": \"/wiki/Special:SpecialPages \\\"A list of all special pages \\\\[q\\\\]\\\")\\n* [ Permanent link ](/w/index.php?title=Adenosine_triphosphate&oldid=1219758132 \\\"Permanent link to this revision of this page\\\")\\n* [ Page information ](/w/index.php?title=Adenosine_triphosphate&action=info \\\"More information about this page\\\")\\n* [ Cite this page ](/w/index.php?title=Special\"\n },\n {\n \"doc_id\": 7193772420916502000,\n \"text\": \"## Structure [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=1 \\\"Edit\\nsection: Structure\\\") ]\"\n },\n {\n \"doc_id\": 1338774658429844500,\n \"text\": \"* [ Download as PDF ](/w/index.php?title=Special:DownloadAsPdf&page=Adenosine_triphosphate&action=show-download-screen \\\"Download this page as a PDF file\\\")\\n* [ Printable version ](/w/index.php?title=Adenosine_triphosphate&printable=yes \\\"Printable version of this page \\\\[p\\\\]\\\")\"\n },\n {\n \"doc_id\": 5699269757669299000,\n \"text\": \"Adenosine-5'-triphosphate [\\n\\n](/wiki/File:Adenosintriphosphat_protoniert.svg)\\n---\\n| [![](//upload.wikimedia.org/wikipedia/commons/th\"\n },\n {\n \"doc_id\": 1436132316245030400,\n \"text\": \"* [ Wikimedia Commons ](https://commons.wikimedia.org/wiki/Category:Adenosine_triphosphate)\"\n },\n {\n \"doc_id\": 311498269723767200,\n \"text\": \"* (Top)\\n* 1 Structure Toggle Structure subsection\\n* 1.1 Metal cation binding\\n* 2 Chemical properties\\n* 3 Reactive aspects\\n* 4 Production from AMP and ADP Toggle Production from AMP and ADP subsection\\n* 4.1 Production, aerobic conditions\\n* 4.1.1 Glycolysis\\n* 4.1.1.1 Regulation\\n* 4.1.2 Citric acid cycle\\n* 4.1.2.1 Regulation\\n* 4.1.3\"\n },\n {\n \"doc_id\": 266985005400561470,\n \"text\": \"https://bio.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fbio.libretexts.org%2FBookshelves%2FMicrobiology%2FMicrobiology_(Boundless)%2F02%253A_Chemistry%2F2.05%253A_Organic_Compounds%2F2.5.06%253A_ATP-\\n_Adenosine_Triphosphate\"\n },\n {\n \"doc_id\": 7956788937624895000,\n \"text\": \"* [ ATP bound to proteins ](http://www.ebi.ac.uk/pdbe-srv/PDBeXplore/ligand/?ligand=ATP) in the [ PDB ](/wiki/Protein_Data_Bank \\\"Protein Data Bank\\\")\\n* [ ScienceAid: Energy ATP and Exercise ](https://web.archive.org/web/20160310134410/http://www.scienceaid.co.uk/biology/biochemistry/atp.html)\\n* [ PubChem entry for Ad\"\n },\n {\n \"doc_id\": 2779870307507492000,\n \"text\": \" ](/wiki/PMC_\\\\(identifier\\\\) \\\"PMC \\\\(identifier\\\\)\\\") [ 1172435 ](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1172435). [ PMID ](/wiki/PMID_\\\\(identifier\\\\) \\\"PMID \\\\(identifier\\\\)\\\") [ 1212224 ](https://pubmed.ncbi.nlm.nih.gov/1212224).\\n19. ** ^ ** [ \\\"Adenosine triphosphate\\\"\"\n },\n {\n \"doc_id\": 3030292087953868300,\n \"text\": \"ATP consists of an [ adenine ](/wiki/Adenine \\\"Adenine\\\") attached by the\\n9-nitrogen atom to the 1′ [ carbon ](/wiki/Carbon \\\"Carbon\\\") [ atom\\n](/wiki/Atom \\\"Atom\\\") of a sugar ( [ ribose ](/wiki/Ribose \\\"Ribose\\\") ), which\\nin turn is attached at the 5' carbon atom of the sugar to a triphosphate\\ngroup. In its many reactions related to metabolism, the adenine and sugar\"\n },\n {\n \"doc_id\": 4385062583235649500,\n \"text\": \".org/10.1002%2F9780470048672.wecb648). [ ISBN ](/wiki/ISBN_\\\\(identifier\\\\) \\\"ISBN \\\\(identifier\\\\)\\\") [ 9780471754770 ](/wiki/Special:BookSources/9780471754770 \\\"Special:BookSources/9780471754770\\\").\\n5. ** ^ ** Dunn, Jacob; Grider, Michael H. (2023), [ \\\"Physiology, Adenosine Triphosphate\\\" ](\"\n },\n {\n \"doc_id\": 8164153329772920000,\n \"text\": \"From the perspective of [ biochemistry ](/wiki/Biochemistry \\\"Biochemistry\\\"),\\nATP is classified as a [ nucleoside triphosphate\\n](/wiki/Nucleoside_triphosphate \\\"Nucleoside triphosphate\\\"), which indicates\\nthat it consists of three components: a nitrogenous base ( [ adenine\\n](/wiki/Adenine \\\"Adenine\\\") ), the sugar [ ribose ](/wiki/Ribose \\\"Ribose\\\"),\\nand the [ triphosphate ]\"\n },\n {\n \"doc_id\": 3232102919466913300,\n \"text\": \"2. [ Save as PDF ](https://bio.libretexts.org/@api/deki/pages/8804/pdf/2.5.6%253A%2bATP-%2bAdenosine%2bTriphosphate.pdf \\\"Export page as a PDF\\\")\\n3.\"\n },\n {\n \"doc_id\": 7989093631317823000,\n \"text\": \"ATP is stable in aqueous solutions between [ pH ](/wiki/PH \\\"PH\\\") 6.8 and 7.4\\n(in the absence of catalysts). At more extreme pH levels, it rapidly [\\nhydrolyses ](/wiki/Hydrolyses \\\"Hydrolyses\\\") to ADP and phosphate. Living cells\\nmaintain the ratio of ATP to ADP at a point ten orders of magnitude from\\nequilibrium, with ATP concentrations fivefold higher than the concentration of\\nADP. [12] [13] In\"\n },\n {\n \"doc_id\": 2196154811093627100,\n \"text\": \"\\ngroups remain unchanged, but the triphosphate is converted to di- and\\nmonophosphate, giving respectively the derivatives [ ADP\\n](/wiki/Adenosine_diphosphate \\\"Adenosine diphosphate\\\") and [ AMP\\n](/wiki/Adenosine_monophosphate \\\"Adenosine monophosphate\\\"). The three\\nphosphoryl groups are labeled as alpha (α), beta (β), and, for the terminal\\nphosphate, gamma (γ). [5]\"\n },\n {\n \"doc_id\": 1146594224582974600,\n \"text\": \"**Adenosine triphosphate** ( **ATP** ) is a [ nucleotide ](/wiki/Nucleotide\\n\\\"Nucleotide\\\") [2] that provides [ energy ](/wiki/Energy \\\"Energy\\\") to drive\\nand support many processes in living [ cells ](/wiki/Cell_\\\\(biology\\\\) \\\"Cell\\n\\\\(biology\\\\)\\\"), such as [ muscle contraction ](/wiki/Muscle_contraction\\n\\\"Muscle contraction\\\"), [ nerve impulse ](/wiki/Nerve_impulse \\\"Nerve\"\n },\n {\n \"doc_id\": 6977782849683060000,\n \"text\": \"Along with pH, the free energy change of ATP hydrolysis is also associated\\nwith Mg 2+ concentration, from ΔG°' = −35.7 kJ/mol at a Mg 2+\\nconcentration of zero, to ΔG°' = −31 kJ/mol at [Mg 2+ ] = 5 mM. Higher\\nconcentrations of Mg 2+ decrease free energy released in the reaction due to\\nbinding of Mg 2+ ions to negatively charged oxygen atoms of ATP at pH 7\"\n },\n {\n \"doc_id\": 3118770243673203000,\n \"text\": \"ATP can also be synthesized through several so-called \\\"replenishment\\\"\\nreactions catalyzed by the enzyme families of [ nucleoside diphosphate kinases\\n](/wiki/Nucleoside_diphosphate_kinase \\\"Nucleoside diphosphate kinase\\\") (NDKs),\\nwhich use other nucleoside triphosphates as a high-energy phosphate donor, and\\nthe [ ATP:guanido-phosphotransferase\\n](/wiki/ATP:guanido_phosph\"\n },\n {\n \"doc_id\": 3858892696968426000,\n \"text\": \". Kinetic energy is the energy of an object\\nas a result of its motion. The significance of ATP is in its ability to store\\npotential energy within the phosphate bonds. The energy stored between these\\nbonds can then be transferred to do work. For example, the transfer of energy\\nfrom ATP to the protein [ myosin ](/wiki/Myosin \\\"Myosin\\\") causes a\\nconformational change when connecting to [ actin ](/wiki/Actin \\\"Actin\\\") during\\n[ muscle contraction ](/wiki/Muscle_\"\n },\n {\n \"doc_id\": 8759487471041125000,\n \"text\": \"## References [ [ edit\\n](/w/index.php?title=Adenosine_diphosphate&action=edit§ion=10 \\\"Edit\\nsection: References\\\") ]\"\n },\n {\n \"doc_id\": 7800175872156098000,\n \"text\": \"### ATP recycling [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=17 \\\"Edit\\nsection: ATP recycling\\\") ]\"\n },\n {\n \"doc_id\": 4961826454351574000,\n \"text\": \"## Energy Coupling in Metabolism\"\n },\n {\n \"doc_id\": 3838722632484139500,\n \"text\": \"1. Learning Objectives\\n2. Key Points\\n3. Key Terms\\n4. ATP: Adenosine Triphosphate\\n5. Molecular Structure\\n6. ATP Hydrolysis and Synthesis\\n7. ATP and Energy Coupling\\n8. Energy Coupling in Sodium-Potassium Pumps\\n9. Energy Coupling in Metabolism\"\n },\n {\n \"doc_id\": 2827605596764358700,\n \"text\": \"ADP cycling supplies the [ energy ](/wiki/Energy \\\"Energy\\\") needed to do work\\nin a biological system, the [ thermodynamic ](/wiki/Thermodynamic\\n\\\"Thermodynamic\\\") process of transferring energy from one source to another.\\nThere are two types of energy: [ potential energy ](/wiki/Potential_energy\\n\\\"Potential energy\\\") and [ kinetic energy ](/wiki/Kinetic_energy \\\"Kinetic\\nenergy\\\"). Potential energy can be thought of as stored energy, or usable\\nenergy that is available to do work\"\n },\n {\n \"doc_id\": 8283334338726298000,\n \"text\": \"* Explain the role of ATP as the currency of cellular energy\"\n },\n {\n \"doc_id\": 5011687759077796000,\n \"text\": \"P \\\\(shown here as a 3-\\nion\\\\)](//upload.wikimedia.org/wikipedia/commons/thumb/9/99/Adenosine-\\ndiphosphate-3D-balls.png/230px-Adenosine-diphosphate-3D-balls.png)\\n](/wiki/File:Adenosine-diphosphate-3D-balls.png \\\"Ball-and-stick model of ADP\\n\\\\(shown here as a 3- ion\\\\)\\\")\\nNames\\n[ IUPAC\"\n },\n {\n \"doc_id\": 3597971617827617000,\n \"text\": \"* [ Read ](/wiki/Adenosine_triphosphate)\\n* [ Edit ](/w/index.php?title=Adenosine_triphosphate&action=edit \\\"Edit this page \\\\[e\\\\]\\\")\\n* [ View history ](/w/index.php?title=Adenosine_triphosphate&action=history \\\"Past revisions of this page \\\\[h\\\\]\\\")\"\n },\n {\n \"doc_id\": 1032511989725205500,\n \"text\": \"# Adenosine triphosphate\"\n },\n {\n \"doc_id\": 613948337767182000,\n \"text\": \"When consumed in a [ metabolic ](/wiki/Metabolism \\\"Metabolism\\\") processes, ATP\\nconverts either to [ adenosine diphosphate ](/wiki/Adenosine_diphosphate\\n\\\"Adenosine diphosphate\\\") (ADP) or to [ adenosine monophosphate\\n](/wiki/Adenosine_monophosphate \\\"Adenosine monophosphate\\\") (AMP). Other\\nprocesses regenerate ATP. It is also a [ precursor\\n](/wiki/Precursor_\\\\\"\n },\n {\n \"doc_id\": 1996598610642749400,\n \"text\": \"&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FAdenosine_triphosphate)\\n* [ Wikidata item ](https://www.wikidata.org/wiki/Special:EntityPage/Q80863 \\\"Structured data on this page hosted by Wikidata \\\\[g\\\\]\\\")\"\n },\n {\n \"doc_id\": 978360755576965600,\n \"text\": \"* [ Article ](/wiki/Adenosine_triphosphate \\\"View the content page \\\\[c\\\\]\\\")\\n* [ Talk ](/wiki/Talk:Adenosine_triphosphate \\\"Discuss improvements to the content page \\\\[t\\\\]\\\")\"\n },\n {\n \"doc_id\": 8514217792710328000,\n \"text\": \"synthase \\\"ATP synthase\\\") enzyme; the\\nequation for this can be written as ADP + P i → ATP. [ _[ citation needed\\n](/wiki/Wikipedia:Citation_needed \\\"Wikipedia:Citation needed\\\") _ ]\"\n },\n {\n \"doc_id\": 740495691876270500,\n \"text\": \"This page titled [ 2.5.6: ATP- Adenosine Triphosphate\\n](https://bio.libretexts.org/Bookshelves/Microbiology/Microbiology_\\\\(Boundless\\\\)/02%3A_Chemistry/2.05%3A_Organic_Compounds/2.5.06%3A_ATP-\\n_Adenosine_Triphosphate) is shared under a [ CC BY-SA 4.0\\n](https://creativecommons.org/licenses/by\"\n },\n {\n \"doc_id\": 6271573277080441000,\n \"text\": \"\\nallosterically inhibited by high concentrations of ATP and activated by high\\nconcentrations of AMP. The inhibition of PFK by ATP is unusual since ATP is\\nalso a substrate in the reaction catalyzed by PFK; the active form of the\\nenzyme is a [ tetramer ](/wiki/Tetramer_protein \\\"Tetramer protein\\\") that\\nexists in two conformations, only one of which binds the second substrate\\nfructose-6-phosphate (F6P). The protein has two [ binding sites\\n](/wiki/B\"\n },\n {\n \"doc_id\": 3369311530784969000,\n \"text\": \"_phosphorylation \\\"Substrate-level phosphorylation\\\"), [\\noxidative phosphorylation ](/wiki/Oxidative_phosphorylation \\\"Oxidative\\nphosphorylation\\\"), and [ photophosphorylation ](/wiki/Photophosphorylation\\n\\\"Photophosphorylation\\\"), all of which facilitate the addition of a phosphate\\ngroup to ADP.\"\n },\n {\n \"doc_id\": 6412499927344170000,\n \"text\": \"## History [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=28 \\\"Edit\\nsection: History\\\") ]\"\n },\n {\n \"doc_id\": 2025235775950557400,\n \"text\": \"* [ Read ](/wiki/Adenosine_triphosphate)\\n* [ Edit ](/w/index.php?title=Adenosine_triphosphate&action=edit \\\"Edit this page \\\\[e\\\\]\\\")\\n* [ View history ](/w/index.php?title=Adenosine_triphosphate&action=history)\"\n },\n {\n \"doc_id\": 1907208704393033200,\n \"text\": \"Cells couple the exergonic reaction of ATP hydrolysis with the endergonic\\nreactions of cellular processes. For example, transmembrane ion pumps in nerve\\ncells use the energy from ATP to pump ions across the cell membrane and\\ngenerate an action potential. The sodium-potassium pump (Na \\\\+ /K \\\\+ pump)\\ndrives sodium out of the cell and potassium into the cell. When ATP is\\nhydrolyzed, it transfers its gamma phosphate to the pump protein in a process\\ncalled phosphorylation. The\"\n },\n {\n \"doc_id\": 3412699478476082700,\n \"text\": \"#### ATP replenishment by nucleoside diphosphate kinases [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=15 \\\"Edit\\nsection: ATP replenishment by nucleoside diphosphate kinases\\\") ]\"\n },\n {\n \"doc_id\": 388825824706267900,\n \"text\": \"ifier\\\\)\\\") [ 30713504 ](https://pubmed.ncbi.nlm.nih.gov/30713504).\\n25. ^ _**a** _ _**b** _ Dahout-Gonzalez, C.; Nury, H.; Trézéguet, V.; Lauquin, G.; Pebay-Peyroula, E.; Brandolin, G. (2006). \\\"Molecular, functional, and pathological aspects of the mitochondrial ADP/ATP carrier\\\". _Physiology_. **21** (4\"\n },\n {\n \"doc_id\": 7187161046571647000,\n \"text\": \"The hydrolysis of ATP into ADP and inorganic phosphate\"\n },\n {\n \"doc_id\": 57114420802227810,\n \"text\": \"-aldrich/docs/Sigma/Product_Information_Sheet/a7699pis.pdf) (PDF) from the original on 2019-03-23 . Retrieved 2019-03-22 .\\n2. ** ^ ** Dunn, Jacob; Grider, Michael H. (2023). [ \\\"Physiology, Adenosine Triphosphate\\\" ](https://www.ncbi.nlm.nih.gov/books/NBK553175/). _StatPearls_. StatPearls Publishing. [ PMID\"\n },\n {\n \"doc_id\": 1990050540664970200,\n \"text\": \"* [ Article ](/wiki/Adenosine_diphosphate \\\"View the content page \\\\[c\\\\]\\\")\\n* [ Talk ](/wiki/Talk:Adenosine_diphosphate \\\"Discuss improvements to the content page \\\\[t\\\\]\\\")\"\n },\n {\n \"doc_id\": 6493961511181788000,\n \"text\": \"Adenosine Triphosphate\\nATP is a nucleotide consisting of an adenine, a 5-carbon sugar (ribose), and three phosphate groups.\"\n },\n {\n \"doc_id\": 2464959256312389600,\n \"text\": \"1. aa + ATP ⟶ aa-AMP + [ PP i ](/wiki/Pyrophosphate \\\"Pyrophosphate\\\")\\n2. aa-AMP + tRNA ⟶ aa-tRNA + AMP\"\n },\n {\n \"doc_id\": 4840394897342918000,\n \"text\": \"ATP is hydrolyzed into ADP in the following reaction:\"\n },\n {\n \"doc_id\": 4028192244389689300,\n \"text\": \"* [ What links here ](/wiki/Special:WhatLinksHere/Adenosine_triphosphate \\\"List of all English Wikipedia pages containing links to this page \\\\[j\\\\]\\\")\\n* [ Related changes ](/wiki/Special:RecentChangesLinked/Adenosine_triphosphate \\\"Recent changes in pages linked from this page \\\\[k\\\\]\\\")\\n* [ Upload file ](/wiki/Wikipedia:File_Upload_Wizard \\\"Upload files \\\\[u\\\\]\\\")\\n* [ Special pages ](\"\n },\n {\n \"doc_id\": 7414217072591005000,\n \"text\": \"### Intracellular signaling [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=19 \\\"Edit\\nsection: Intracellular signaling\\\") ]\"\n },\n {\n \"doc_id\": 6604415871490042000,\n \"text\": \"It takes multiple reactions between myosin and actin to effectively produce\\none muscle contraction, and, therefore, the availability of large amounts of\\nATP is required to produce each muscle contraction. For this reason,\\nbiological processes have evolved to produce efficient ways to replenish the\\npotential energy of ATP from ADP. [2]\"\n },\n {\n \"doc_id\": 8309147740403247000,\n \"text\": \"A typical intracellular [ concentration ](/wiki/Concentration \\\"Concentration\\\")\\nof ATP may be 1–10 μmol per gram of tissue in a variety of eukaryotes. [18]\\nThe dephosphorylation of ATP and rephosphorylation of ADP and AMP occur\\nrepeatedly in the course of aerobic metabolism. [19]\"\n },\n {\n \"doc_id\": 6938585289525588000,\n \"text\": \"At cytoplasmic conditions, where the ADP/ATP ratio is 10 orders of magnitude\\nfrom equilibrium, the Δ _G_ is around −57 kJ/mol. [12]\"\n },\n {\n \"doc_id\": 8832330378635948000,\n \"text\": \"Abstract\\nAdenosine 5′-triphosphate (ATP) is a neurotransmitter of postganglionic sympathetic nerves and nonadrenergic, noncholinergic nerves of the enteric nervous system. ATP can also modulate autonomic reflexes through actions at sensory afferent nerves. Mechanisms exist within nerves for the vesicular storage and exocytotic release of ATP. Once released, the availability and duration of ATP is regulated by cell surface ectonucleotidases that hydrolyze ATP, producing metabolites including ADP and adenosine in the process\"\n },\n {\n \"doc_id\": 2730431974490151000,\n \"text\": \"Polyanionic and featuring a potentially [ chelating ](/wiki/Chelation\\n\\\"Chelation\\\") polyphosphate group, ATP binds metal cations with high affinity.\\nThe [ binding constant ](/wiki/Binding_constant \\\"Binding constant\\\") for [ Mg\\n2+\\n](/wiki/Magnesium \\\"Magnesium\\\") is ( 9 554 ). [7] The binding of a [\\ndivalent ](/wiki/Divalent \\\"Divalent\\\") [ cation ](/wiki/Cation\"\n },\n {\n \"doc_id\": 4136401358466617000,\n \"text\": \"### Metal cation binding [ [ edit\\n](/w/index.php?title=Adenosine_triphosphate&action=edit§ion=2 \\\"Edit\\nsection: Metal cation binding\\\") ]\"\n },\n {\n \"doc_id\": 1989801421615764700,\n \"text\": \"ATP + H\\n2 O → ADP + P i Δ _G_ °' = −30.5 kJ/mol (−7.3 kcal/mol)\"\n }\n]","isConversational":false}}}],"truncated":false,"partial":false},"paginationData":{"pageIndex":0,"numItemsPerPage":100,"numTotalItems":81659,"offset":0,"length":100}},"jwt":"eyJhbGciOiJFZERTQSIsImtpZCI6IjVHZDBvd0g5MTM2eDZjc1FvbE1zcktNWUZoRVFoUm5PVVVybEpjOUhaUEEifQ.eyJyZWFkIjp0cnVlLCJwZXJtaXNzaW9ucyI6eyJyZXBvLmNvbnRlbnQucmVhZCI6dHJ1ZX0sImlhdCI6MTc4ODE3OTc4NCwianRpIjoiNDg0MGIzMjMtNzcyYy00YWU4LTlmZWItMWZhZmY0YmNlMzE3Iiwic3ViIjoiL2RhdGFzZXRzL2gzaW5rci9yZWFzb25fZW1iZWRfZGF0YV9yIiwiZXhwIjoxNzg4MTgzMzg0LCJpc3MiOiJodHRwczovL2h1Z2dpbmdmYWNlLmNvIn0.-i9sJ5kv_IQrPj4dBmeR6pGjPKEKZV9kX1TAbS7CHKM5TEd38VGSx1gl4W1WGfQC_Vwc8mU2DIOZwTrfBRWnBA","displayUrls":true,"splitSizeSummaries":[{"config":"default","split":"train","numRows":81659,"numBytesParquet":2013074877}]},"discussionsStats":{"closed":0,"open":1,"total":1},"fullWidth":true,"hasGatedAccess":true,"hasFullAccess":true,"isEmbedded":false,"savedQueries":{"community":[],"user":[]}}">
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I'm reading a paper on the survival and learning capabilities of newly generated neurons and how timing plays a crucial role in these processes. The study highlights the importance of the timing of neuronal integration into existing neural circuits for the survival of these neurons and their contribution to learning an... | [
{
"doc_id": 8438656648216676000,
"text": "The difference in survival mechanisms between embryonically and adult-born\nneurons illustrates the importance of time period of neuronal survival with\nregard to brain maturation, since embryonically born _immature_ neurons must\nsurvive in _immature_ brain, wherea... | [
{
"doc_id": 3457460377553067000,
"text": " A, Gheusi G, Gabellec MM, de Chaumont\nF, Olivo-Marin JC, Lledo PM. Learning and survival of newly generated\nneurons: when time matters. J Neurosci 2008; 28: 11511–11516.\"), [ 9\n](/articles/cddis201764#ref-CR9 \"Khodosevich K, Lazarini F, von Engelhardt J,\nKane... |
I'm studying the evolution of sexual reproduction and came across some interesting points about the advantages and mechanisms that have led to the prevalence of sexual reproduction over asexual reproduction in many species. From what I understand, sexual reproduction, despite the costs involved, offers significant bene... | [
{
"doc_id": 5786531833455687000,
"text": "Biologists studying [ evolution ](/wiki/Evolution \"Evolution\") propose several\nexplanations for the development of sexual reproduction and its maintenance.\nThese reasons include reducing the likelihood of the [ accumulation\n](/wiki/Mullers_ratchet \"Mullers rat... | [
{
"doc_id": 571901081079250000,
"text": "(identifier\\) \"S2CID \\(identifier\\)\") [ 7714974 ](https://api.semanticscholar.org/CorpusID:7714974).\n24. ** ^ ** Colegrave, N. (2002). \"Sex releases the speed limit on evolution\". _Nature_. **420** (6916): 664–6. [ Bibcode ](/wiki/Bibcode_\\(identifier\\) \"... |
I've been reading about cancer vaccines and how these can help the body fight disease. From what I understand, a cancer vaccine exposes the immune system to a foreign protein, known as an antigen, which then triggers the immune system to recognize and destroy that antigen or related substances. There are two main types... | [
{
"doc_id": 5560400009856783000,
"text": "One example of a cancer prevention vaccine is Gardasil, the vaccine to protect\nagainst the [ human papillomavirus (HPV) ](/node/24561), a virus that can\ncause specific types of cancer. An example of a treatment vaccine includes [\nspuleucel-T (Provenge) ](https://... | [
{
"doc_id": 7541977959245189000,
"text": "### What are cancer vaccines?"
},
{
"doc_id": 3040989394500767000,
"text": "* **Treatment vaccines**, which work against cancer by boosting your immune system’s response to cancer cells. Treatment vaccines are different from the ones that help prevent d... |
I'm curious about the methods used to enhance forest resilience against environmental threats. Specifically, how effective is the strategy of identifying and cultivating seedlings from trees that show resistance to certain stressors, such as diseases or climate change, and then replanting these seedlings in natural set... | [
{
"doc_id": 3610466548274692600,
"text": "Efforts are under way to collect seeds from apparently disease-resistant\nindividuals and outplant seedlings. It’s also important to protect the places\non the landscape where the tree is managing to survive, and critical habitat\nprotection could have helped with t... | [
{
"doc_id": 5381096558001503000,
"text": " created in Montana’s\nCuster Gallatin National Forest in <a\nhref=\"https://www.fs.usda.gov/detail/custergallatin/news-\nevents/?cid=STELPRDB5426775\">2013</a>, are meant to speed up and simplify the\nseed sourcing process. While it takes the orchards <a\nhref=\"ht... |
I'm studying the role of tandem repeats in genetics and came across the fact that tandem repeats make up a significant portion of the human and fruit fly genomes. I understand that tandem repeats can have functional roles, like the TTAGGG sequence in mammalian telomeres which helps protect chromosome ends. However, I'm... | [
{
"doc_id": 4953413197420232000,
"text": " results in Huntington's disease, a neurodegenerative disease. Twenty human disorders are known to result from similar tandem repeat expansions in various genes. The mechanism by which proteins with expanded polygulatamine tracts cause death of neurons is not fully ... | [
{
"doc_id": 6812175503030930000,
"text": " the sequence (TTAGGG)n.\nTandem repeats of longer sequences (arrays of repeated sequences 10–60 nucleotides long) are termed minisatellites.\nTransposable genetic elements, DNA sequences that can replicate and insert copies of themselves at other locations within a... |
I'm curious about the differences between sperm and ovum cells in human reproduction, particularly focusing on the structures that surround each cell. How do the plasma membrane of a sperm cell and the egg envelopes of an ovum cell play a role in the process of fertilization? What specific functions do these structures... | [
{
"doc_id": 668939432810031900,
"text": "ental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane. The fusion of these two membranes creates an opening throug... | [
{
"doc_id": 2039517890329344300,
"text": " transferred into the ovum. Fusion between the oocyte plasma membrane and sperm follows and allows the sperm nucleus, centriole, and flagellum, but not the mitochondria, to enter the oocyte. The nuclear membranes of the egg and sperm break down and the two haploid g... |
I recently came across some fascinating information about the coconut crab, a species of terrestrial hermit crab. The body structure of coconut crabs is quite unique, with a cephalothorax and an abdomen, similar to other decapods. The front-most pair of legs features large chelae (claws), with the left claw being large... | [
{
"doc_id": 4381194709270548500,
"text": " with tweezer-like chelae at the end allowing young coconut crabs to grip the inside of the shell or coconut husks that juveniles habitually carry for protection. Adults use this pair for walking and climbing. The last pair of legs is very small and is used by femal... | [
{
"doc_id": 2110735589865615600,
"text": "ers on one of them until it breaks. Afterwards, it turns around and uses the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier con... |
I'm curious about the morphology of uniramous appendages in arthropods. Could someone provide a detailed explanation of the structure and function of these appendages, particularly how they differ from biramous appendages? Additionally, any insights into the evolutionary advantages of uniramous appendages would be grea... | [
{
"doc_id": 4803317430044290000,
"text": "The legs of [ insects ](/wiki/Insect \"Insect\") and [ myriapods\n](/wiki/Myriapoda \"Myriapoda\") are uniramous. In crustaceans, the first\nantennae are uniramous, but the second antennae are biramous, as are the legs\nin most species."
},
{
"doc_id": 41428... | [
{
"doc_id": 7485487499135020000,
"text": "The appendages of arthropods may be either **biramous** or **uniramous**. A\nuniramous limb comprises a single series of segments attached end-to-end. A\nbiramous limb, however, branches into two, and each branch consists of a\nseries of segments attached end-to-end... |
I'm interested in learning more about the applications of phage therapy beyond human medicine. Specifically, how effective has phage therapy been in treating bacterial infections in animals and plants? Are there any notable studies or applications in livestock, companion animals, honeybees, or aquaculture that demonstr... | [
{
"doc_id": 1169967128443746000,
"text": " raw food and Listeria in fresh food or to reduce food spoilage bacteria. In agricultural practice, phages have been used to fight pathogens like Campylobacter, Escherichia, and Salmonella in farm animals, Lactococcus and Vibrio pathogens in fish aquaculture, and Er... | [
{
"doc_id": 6761493782054093000,
"text": "Applications[edit]\nCollection[edit]\nPhages for therapeutic use can be collected from environmental sources that likely contain high quantities of bacteria and bacteriophages, such as effluent outlets, sewage, or even soil. The samples are taken and applied to bact... |
In the context of synaptic transmission, what are the potential drawbacks of not efficiently recycling neurotransmitters? Specifically, how might these issues affect the timing and intensity of neural responses, and what complications could arise if a new stimulus occurs before neurotransmitter levels are replenished? | [
{
"doc_id": 693613700246829000,
"text": "Reuptake by the presynaptic neuron, enzymatic degradation, and diffusion away\nfrom the synapse reduce neurotransmitter levels, terminating the signal."
},
{
"doc_id": 1732826137367918000,
"text": " recycled, which will prevent the neuron from having to r... | [
{
"doc_id": 3067998502938363000,
"text": " can be recycled, which will prevent the neuron from having to re-synthesize the neurotransmitter every cycle of release."
},
{
"doc_id": 4281873121114820000,
"text": "What about temporal summation? A key point is that postsynaptic potentials\naren’t ins... |
I'm studying the impact of soluble fiber on canine digestion and came across a research paper discussing the in vitro fermentation of various soluble fiber sources using dog fecal inoculum. Could someone elaborate on how different types of soluble fibers affect the microbial activity and gas production in the canine gu... | [
{
"doc_id": 8289483096762136000,
"text": " ]. Thus, if the fiber source is\nconcentrated in soluble and fermentable fibers rather than insoluble and non-\nfermentable fibers, more SCFA will be produced [ 75 , 76 , 77 ].\nMiscanthus grass has been evaluated in an in vitro fermentation model using\ncanine... | [
{
"doc_id": 7907510552424612000,
"text": "S337. DOI: 10.1007/s11259-008-9142-y\n74. 74\\. Guevara MA, Bauer LL, Abbas CA, Berry KE, Holzgaefe DP, Cecava MJ, Fahey GC. Chemical composition, in vitro fermentation characteristics, and in vivo digestibility responses, by dogs to selected corn fibers. Journal o... |
I recently came across the fascinating structure of the macronuclear genome in ciliates, particularly the fact that some species have around 16,000 tiny chromosomes. How does such a complex genome structure evolve, and what are the potential advantages or disadvantages of having so many small chromosomes in the macronu... | [
{
"doc_id": 2127123058911355000,
"text": "Like other ciliates, Oxytricha possesses two distinct versions of its genome, a germline version and a somatic version. Oxytricha’s germline genome is an archive of approximately 1 Gb of DNA sequence containing approximately one-quarter million embedded gene segment... | [
{
"doc_id": 4687173420313829000,
"text": "Google Scholar]\n10. Goldman AD, Landweber LF. Oxytricha as a modern analog of ancient genome evolution. Trends Genet. 2012;28:382–388. 10.1016/j.tig.2012.03.010 [PMC free article] [PubMed] [CrossRef] [Google Scholar]\n11. Bracht JR, Fang W, Goldman AD, Dolzhenko E,... |
I'm exploring the topic of lipids and their roles in biological systems. Could someone explain how the structure of lipids, particularly phospholipids, influences the fluidity and permeability of cell membranes? Additionally, I'm curious about the mechanisms by which cells regulate the lipid composition of their membra... | [
{
"doc_id": 3244567816454370000,
"text": " the hydrophilic heads\nof the glycerophospholipids in a cell's plasma membrane face both the water-\nbased cytoplasm and the exterior of the cell.\n\nAltogether, lipids account for about half the mass of cell membranes.\nCholesterol molecules, although less abundan... | [
{
"doc_id": 7479183341722698000,
"text": "1. [ Lipids ](https://slcc.pressbooks.pub/collegebiology1/chapter/lipids/#chapter-1081-section-1)\n2. [ 9.2 The Plasma Membrane ](https://slcc.pressbooks.pub/collegebiology1/chapter/the-plasma-membrane/)"
},
{
"doc_id": 4299990639444634600,
"text": "The ... |
I've been reading about the differences in sweat glands between humans and chimpanzees, and it seems humans have a much more extensive distribution of sweat glands over the body. How does this difference in sweat gland distribution contribute to the thermoregulation capabilities of humans compared to chimpanzees? Addit... | [
{
"doc_id": 1060148758333610100,
"text": "edal respiration. Apart from bipedalism and brain size, humans differ from chimpanzees mostly in smelling, hearing and digesting proteins. While humans have a density of hair follicles comparable to other apes, it is predominantly vellus hair, most of which is so sh... | [
{
"doc_id": 2235341452207540700,
"text": "ispy as to be practically invisible. Humans have about 2 million sweat glands spread over their entire bodies, many more than chimpanzees, whose sweat glands are scarce and are mainly located on the palm of the hand and on the soles of the feet. It is estimated that... |
I'm curious about the susceptibility of different animals to the SARS-CoV-2 virus, which causes COVID-19. Specifically, how do the risks of transmission to and from pets like cats and dogs compare? Are there any particular behaviors that pet owners should avoid to minimize the risk of spreading the virus to their anima... | [
{
"doc_id": 377344527947257700,
"text": " transmission of SARS-CoV-2 has not been proven. Compared to cats, dogs are less susceptible to this infection. Behaviours which increase the risk of transmission include kissing, licking, and petting the animal.\nThe virus does not appear to be able to infect pigs, ... | [
{
"doc_id": 4797163782711433000,
"text": " SARS-CoV-2, Manis javanica, smuggled from Southeast Asia\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nPangolin SARSr-CoV-GD, 90.1% to SARS-CoV-2, Manis javanica, smuggled from Southeast Asia\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\nBat RshSTT182, 92.6% to SARS-CoV-2, Rhinoloph... |
I'm curious about the evolutionary trade-offs between nocturnal and diurnal vision in mammals. How does the presence or absence of a tapetum lucidum affect the visual capabilities of nocturnal animals compared to humans and other diurnal species? Specifically, how does the blurring of initial images due to the tapetum ... | [
{
"doc_id": 71821428347335730,
"text": "Classification[edit]\nA classification of anatomical variants of tapeta lucida defines four types:\nRetinal tapetum, as seen in teleosts (with a variety of reflecting materials from lipids to phenols), crocodiles (with guanine), marsupials (with lipid spheres), and fr... | [
{
"doc_id": 2513742522659926500,
"text": "Gallery[edit]\nTraditionally it has been difficult to take retinal images of animals with a tapetum lucidum because ophthalmoscopy devices designed for humans rely on a high level of on-axis illumination. This kind of illumination causes a great deal of reflex, or b... |
I'm curious about the mechanisms of seed dispersal in different plant species. How do the oil droplets in alder tree seeds and the buoyant propagules of mangroves contribute to the dispersal and germination of these plants in aquatic environments? Additionally, how do the physical characteristics of seeds, like the tho... | [
{
"doc_id": 8707872328246562000,
"text": "Description\nCoconut palm leaves\nCocos nucifera is a large palm, growing up to 30 metres (100 feet) tall, with pinnate leaves 4–6 m (13–20 ft) long, and pinnae 60–90 centimetres (2–3 ft) long; old leaves break away cleanly, leaving the trunk smooth. On fertile soil... | [
{
"doc_id": 2574380222144259000,
"text": " If seeds only fell straight to the ground, competition among the concentrated saplings and the shade of the parent would likely prevent it from flourishing. Many seeds such as birch are small and have papery wings to aid dispersal by the wind. Ash trees and maples ... |
I'm curious about the vitamin C content in various fruits and vegetables. I've seen a list that includes items like green bell pepper, Brussels sprouts, and strawberries, all with high levels of vitamin C. However, I'm wondering how these levels compare to the recommended daily intake and if there are any other less co... | [
{
"doc_id": 6222603341707957000,
"text": "Sources[edit]\nAlthough also present in other plant-derived foods, the richest natural sources of vitamin C are fruits and vegetables. Vitamin C is the most widely taken dietary supplement.\nPlant sources[edit]\n\nFor vitamin C content in ten common staple foods suc... | [
{
"doc_id": 1597000135986091500,
"text": "Diet[edit]\nRecommended consumption[edit]\nRecommendations for vitamin C intake by adults have been set by various national agencies:\n40 mg/day: India National Institute of Nutrition, Hyderabad\n45 mg/day or 300 mg/week: the World Health Organization\n80 mg/day: th... |
I'm reading up on the history of immunology and came across the work of Jean Dausset, George Snell, and Baruj Benacerraf, who were awarded the 1980 Nobel Prize in Physiology or Medicine. Dausset demonstrated the existence of MHC genes in humans and described the first human leucocyte antigen, HLA-A2. Benacerraf, on the... | [
{
"doc_id": 6078984594418064000,
"text": "In transplant rejection[edit]\nIn a transplant procedure, as of an organ or stem cells, MHC molecules themselves act as antigens and can provoke immune response in the recipient, thus causing transplant rejection. MHC molecules were identified and named after their ... | [
{
"doc_id": 2743968896703315000,
"text": "Discovery[edit]\nThe first descriptions of the MHC were made by British immunologist Peter Gorer in 1936. MHC genes were first identified in inbred mice strains. Clarence Little transplanted tumors across different strains and found rejection of transplanted tumor... |
I'm researching the evolutionary adaptations of the human pelvis and the impact on childbirth. Could someone provide insights into how the human pelvis has evolved over time and what specific changes have occurred that affect the process of childbirth? Additionally, any information on the prevalence of malnutrition amo... | [
{
"doc_id": 7257696604395589000,
"text": "Pavličev M, Romero R, Mitteroecker P (January 2020). \"Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma\". American Journal of Obstetrics and Gynecology. 222 (1): 3–16. doi : 10.1016/j.ajog.2019.06.043. PMC 9069416. ... | [
{
"doc_id": 8256634771846564000,
"text": "Khor GL (December 2003). \"Update on the prevalence of malnutrition among children in Asia\". Nepal Medical College Journal. 5 (2): 113–122. PMID 15024783.\n\nRosenberg KR (1992). \"The evolution of modern human childbirth\". American Journal of Physical Anthropolog... |
I'm curious about the historical development of understanding in bioelectromagnetics, particularly how early discoveries influenced modern neuroscience. How did Luigi Galvani's work in 1791, which demonstrated that electricity is the medium by which neurons pass signals to muscles, impact the field? Additionally, how d... | [
{
"doc_id": 8327330670949300000,
"text": " the investigations of Luigi Galvani (1737–1798), who discovered that a shock of static electricity applied to an exposed nerve of a dead frog could cause its leg to contract. Since that time, each major advance in understanding has followed more or less directly fr... | [
{
"doc_id": 1315331049365631200,
"text": " decomposed by the current from a voltaic pile was discovered by Nicholson and Carlisle in 1800, a process now known as electrolysis. Their work was greatly expanded upon by Michael Faraday in 1833. Current through a resistance causes localised heating, an effect Ja... |
I've been reading about the house centipede, *Scutigera coleoptrata*, and noticed that the legs of this species are seven-segmented. Could someone explain the evolutionary advantage of having seven segments in the legs of *Scutigera coleoptrata*? How does this specific number of segments contribute to the agility and s... | [
{
"doc_id": 4142826719316004400,
"text": "[ Myriapods ](/wiki/Myriapod \"Myriapod\") ( [ millipedes ](/wiki/Millipede\n\"Millipede\"), [ centipedes ](/wiki/Centipede \"Centipede\") and their\nrelatives) have seven-segmented walking legs, comprising coxa, trochanter,\nprefemur, femur, tibia, tarsus, and a ta... | [
{
"doc_id": 3472123691328049700,
"text": "[ Homologies ](/wiki/Homology_\\(biology\\) \"Homology \\(biology\\)\") of leg\nsegments between groups are difficult to prove and are the source of much\nargument. Some authors posit up to eleven segments per leg for the [ most\nrecent common ancestor ](/wiki/Most_... |
I've been reading about the potential neurotoxic effects of early-life exposure to fluoride, particularly how these effects might differ between males and females. Could someone provide a detailed explanation of the current understanding of sex-specific impacts of fluoride on neurodevelopment, based on both epidemiolog... | [
{
"doc_id": 6395756373247603000,
"text": " ]. An\nincrease of 0.5 mg/L of F concentration in the water (approximately equal to\nthe difference between fluoridated and non-fluoridated regions) was associated\nwith a 7.9-point lower IQ score in formula-fed infants and 6.3-point lower IQ\nscore in breastfed c... | [
{
"doc_id": 576392534316992600,
"text": "Sex-\nspecific+neurotoxic+effects+of+early-\nlife+exposure+to+fluoride%3A+a+review+of+the+epidemiologic+and+animal+literature&journal=Curr+Epidemiol+Rep&volume=7&pages=263-73)"
},
{
"doc_id": 4211093138690642000,
"text": "[ PubMed Abstract ](https://pubme... |
I'm curious about the genetic divergence between tall and dwarf coconut varieties. How did the alteration in genes related to gibberellin metabolism contribute to the development of dwarf coconuts? Additionally, how does the niu leka of Polynesia, often referred to as "Compact Dwarfs," fit into the genetic lineage of d... | [
{
"doc_id": 7362595851458222000,
"text": "ars are fully domesticated, in contrast to tall cultivars which display greater diversity in terms of domestication (and lack thereof). The fact that all dwarf coconuts share three genetic markers out of thirteen (which are only present at low frequencies in tall cu... | [
{
"doc_id": 3584090569727402500,
"text": " practiced by the Austronesian Lapita people.\nCoconuts can also be broadly divided into two general types based on habit: the \"Tall\" (var. typica) and \"Dwarf\" (var. nana) varieties. The two groups are genetically distinct, with the dwarf variety showing a great... |
I'm curious about the mechanisms cells employ to utilize the energy stored in ATP efficiently. Since ATP is known to be a highly unstable molecule, how do cells ensure that the energy within ATP bonds is harnessed before spontaneous dissociation into ADP and inorganic phosphate occurs, leading to the loss of free energ... | [
{
"doc_id": 965184437499335000,
"text": "Cells couple the exergonic reaction of ATP hydrolysis with endergonic\nreactions to harness the energy within the bonds of ATP."
},
{
"doc_id": 9077804600915274000,
"text": "wiki/Dephosphorylation \"Dephosphorylation\") of ATP\nby enzymes known as [ ATPas... | [
{
"doc_id": 209756938577391460,
"text": "## ATP and Energy Coupling"
},
{
"doc_id": 7450226740730646000,
"text": "[ ](/wiki/File:ATP-ADP.svg) The cycles of synthesis and\ndegradation of ATP; 2 and 1 repr... |
I'm curious about the process of egg development in women and how premature menopause affects fertility. Could someone explain how eggs develop from the beginning of a menstrual cycle and what factors contribute to premature ovarian failure, leading to a situation where women might need egg donation even if they are ot... | [
{
"doc_id": 7407081584516896000,
"text": " that happen during this time. About 1% of women will experience premature menopause (or premature ovarian failure), meaning that they will run out of eggs well before the normal age of menopause, sometimes when they are still teenagers. This is one of the reasons w... | [
{
"doc_id": 2868170182457191000,
"text": " How does an egg develop? At the beginning of each menstrual cycle, a group of 10-20 primary follicles begin to develop under the influence of Follicle Stimulating Hormone (FSH). By around Day 9 of the cycle, only one healthy follicle normally remains, with the rest... |
I'm working on a project about bacterial classification and came across the Gram-staining technique. Could someone explain how Gram staining is used to classify bacteria into Gram-positive and Gram-negative groups? What are the key differences between these two groups in terms of cell wall structure and how does this s... | [
{
"doc_id": 6328065218086852000,
"text": " in bacteria, that classify bacteria into Gram-positive bacteria and Gram-negative bacteria. The names originate from the reaction of cells to the Gram stain, a long-standing test for the classification of bacterial species.\nGram-positive bacteria possess a thick c... | [
{
"doc_id": 7787398884946974000,
"text": " reliably used to assess familial relationships of bacteria. Nevertheless, staining often gives reliable information about the composition of the cell membrane, distinguishing between the presence or absence of an outer lipid membrane.\nOf these two structurally dis... |
I'm studying the role of programmed cell death in the development of the nervous system and came across a scholarly article that discusses the mechanisms and significance of apoptosis in neural development. The article highlights how programmed cell death is a critical process for sculpting the nervous system, eliminat... | [
{
"doc_id": 2137971112251909400,
"text": "ddis201764#ref-CR98 \"Heck N, Golbs A, Riedemann T, Sun JJ,\nLessmann V, Luhmann HJ. Activity-dependent regulation of neuronal apoptosis\nin neonatal mouse cerebral cortex. Cereb Cortex 2008; 18: 1335–1349.\"), [ 99\n](/articles/cddis201764#ref-CR99 \"Wagner-Golbs A... | [
{
"doc_id": 869375188465495300,
"text": " Scholar\n](http://scholar.google.com/scholar_lookup?&title=Adaptive%20roles%20of%20programmed%20cell%20death%20during%20nervous%20system%20development&journal=Annu%20Rev%20Neurosci&doi=10.1146%2Fannurev.neuro.29.051605.112800&volume=29&pages=1-35&publication_year=20... |
I'm exploring the role of Bax in neuronal death and came across a study that discusses the widespread elimination of naturally occurring neuronal death in Bax-deficient mice. How does the absence of Bax affect neuronal survival and what are the implications of these findings for understanding neurodegenerative diseases... | [
{
"doc_id": 1577163544796728600,
"text": ", Roth KA, Sawa H,\nNakayama K, Nakayama K et al. Massive cell death of immature hematopoietic\ncells and neurons in Bcl-x-deficient mice. Science 1995; 267: 1506–1510.\"), [\n87 ](/articles/cddis201764#ref-CR87 \"Shindler KS, Latham CB, Roth KA. Bax\ndeficiency pre... | [
{
"doc_id": 4726265538697730000,
"text": " ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6573662) [\nGoogle Scholar\n](http://scholar.google.com/scholar_lookup?&title=Bax%20deficiency%20prevents%20the%20increased%20cell%20death%20of%20immature%20neurons%20in%20bcl-\nx-\ndeficient%20mice&journal=J%20Neurosci... |
I'm curious about the role of fiber in weight management and overall health for both humans and pets. How does the type of fiber used in food or supplements influence factors like food acceptance, digestibility, fecal consistency, and defecation frequency? Additionally, how does fiber impact colonic fermentation and wh... | [
{
"doc_id": 8259303601759555000,
"text": "In general, fiber rich ingredients have been gaining more attention. In part\nbecause obesity in the pet and human population is a substantial issue [ 23\n, 24 ] and fiber is one possible solution to decrease the energy density of\nfood. It may also increase the ... | [
{
"doc_id": 7966778160608792000,
"text": ".1439-0396.2009.00931.x\n78. 78\\. Kienzle E, Opitz B, Earle KE, Smith PM, Maskell IE. The influence of dietary fiber components on the apparent digestibility of organic matter in prepared dog and cat foods. Journal of Animal Physiology and Animal Nutrition. 1998:7... |
I'm studying the mechanisms of bacteriophages in bacterial infection control and came across the concept of temperate (or lysogenic) bacteriophages. From what I understand, these phages can integrate their DNA into the bacterial genome, leading to the potential for horizontal gene transfer among bacteria. This seems to... | [
{
"doc_id": 6636832515479921000,
"text": " confers resistance to complement killing in the blood. For this reason, temperate phages are generally to be avoided as candidates for phage therapy, although in some cases, the lack of lytic phage candidates and emergency conditions may make such considerations mo... | [
{
"doc_id": 2215246223427518700,
"text": " years, mycobacteria such as Mycobacterium tuberculosis have specific bacteriophages. No lytic phage has yet been discovered for Clostridium difficile, which is responsible for many nosocomial diseases, but some temperate phages (integrated in the genome, also calle... |
I'm curious about the historical classification of bacteria and how that has changed over time. How did scientists originally classify bacteria as plants, and what specific characteristics led to the classification of bacteria as prokaryotes instead? What are the key differences between prokaryotic and eukaryotic cells... | [
{
"doc_id": 8522548530290737000,
"text": "acteria.\nHistorically, bacteria were considered a part of the Plantae, the Plant kingdom, and were called \"Schizomycetes\" (fission-fungi). For this reason, collective bacteria and other microorganisms in a host are often called \"flora\".\nThe term \"bacteria\" w... | [
{
"doc_id": 6967618051290112000,
"text": " treated)\n\nProtista\n\nProkaryota\n\nMonera\n\nMonera\n\nBacteria\n\nBacteria\n\n\nArchaea\n\nArchaea (2015)\n\n\nEukaryota\n\nProtoctista\n\nProtista\n\nEucarya\n\n\"Protozoa\"\n\n\n\"Chromista\"\n\n\nVegetabilia\n\nPlantae\n\nPlantae\n\nPlantae\n\nPlantae\n\n\nF... |
In the context of evolutionary biology, how do environmental stability and change influence the balance between learned and innate behaviors in animals? Specifically, how do the costs and benefits associated with acquiring knowledge through learning versus having innate knowledge impact an animal's survival and evoluti... | [
{
"doc_id": 7833690765638908000,
"text": " disadvantageous to have a specific innate behavior. In these situations, learning evolves in the species.\n\nIn a changing environment, an animal must constantly gain new information to survive. However, in a stable environment, this same individual needs to gather... | [
{
"doc_id": 5503964453586466000,
"text": " is more likely to evolve. Learning is beneficial in these scenarios because an animal can adapt to the new situation, but can still apply the knowledge that it learns for a somewhat extended period of time. Therefore, learning increases the chances of success as op... |
I'm exploring the topic of monozygotic twins and the factors that can lead to differences in genotypes and phenotypes between them. I came across a research paper from 1996 published in the American Journal of Medical Genetics that delves into this subject. The study discusses various causes of discordance in monozygot... | [
{
"doc_id": 2271463723300432400,
"text": ".\nMonozygotic twins always have the same genotype. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes... | [
{
"doc_id": 7238615163234048000,
"text": "\". Journal of Medical Genetics. 3 (2): 117–123. doi : 10.1136/jmg.3.2.117. PMC 1012913. PMID 6007033. ^ Machin GA (January 1996). \"Some causes of genotypic and phenotypic discordance in monozygotic twin pairs\". American Journal of Medical Genetics. 61 (3): 216–22... |
I'm reading about neurotransmitter recycling and the role of glial cells in the process, particularly with glutamate. How does the mechanism of glutamate re-uptake and conversion into glutamine by glial cells contribute to the protection of neurons from glutamate toxicity, and what are the specific steps involved in th... | [
{
"doc_id": 7604020983200802000,
"text": "* In some cases, such as with glutamate, a glial cell will be involved in the re-uptake. Glutamate is toxic to the cell, so it is stored inside the neuron as glutamine. When glutamate is released into the synapse, it will be taken up by the glial cell using a specif... | [
{
"doc_id": 328126944299836600,
"text": "* Re-uptake: Re-uptake can either be pre-synaptic or by glial cells. One important point to remember involving reuptake is that only small molecule chemical neurotransmitters can be taken back up, neuropeptides cannot participate in re-uptake; they must be eliminated... |
I've been reading about the evolution of estimates for the number of human genes over the years. Initially, scientists thought there might be between 50,000 to 140,000 genes, but as the quality of genome sequencing and gene identification methods improved, the number of recognized protein-coding genes was revised to ar... | [
{
"doc_id": 5990119419189581000,
"text": "Size of the human genome[edit]\nIn 2003, scientists reported the sequencing of 85% of the entire human genome, but as of 2020 at least 8% was still missing. In 2021, scientists reported sequencing the complete female genome (i.e., without the Y chromosome). This seq... | [
{
"doc_id": 3289506149260711000,
"text": " Nature 518, 317–330 \\(2015\\).\"),\nand the recent completion of ENCODE phase 3 has further highlighted the\nimportance of functional elements on evolutionary biology, human medicine, and\ngenotype-to-phenotype prediction [ 9 ](/articles/s41467-021-22100-8#ref-CR... |
In the context of evolutionary biology, how does the concept of evolutionary fitness differ from the mere number of offspring an organism produces? Can there be scenarios where an organism with fewer offspring is considered more fit than another with a higher number of offspring? How do nonrandom mating and genetic hit... | [
{
"doc_id": 6806517297278293000,
"text": " to the next generation than those with traits that do not confer an advantage. This teleonomy is the quality whereby the process of natural selection creates and preserves traits that are seemingly fitted for the functional roles they perform. Consequences of selec... | [
{
"doc_id": 7588234283367312000,
"text": " are additive has been contested, in its application to the evolution of sexually selected signals. It can be reasoned that since fitness depends on the production of offspring, this is a multiplicative rather than additive function of reproductive success.\nFurther... |
I'm curious about the evolutionary trade-offs in sensory development among different animal groups. For instance, bears have an incredibly acute sense of smell, which helps them locate food from great distances and even drive away other predators from new kills. On the other hand, catarrhine primates and cetaceans have... | [
{
"doc_id": 8910631799905085000,
"text": " may have decreased primate reliance on olfaction, which explains the relaxation of selective pressure that accounts for the accumulation of olfactory receptor pseudogenes in primates. However, recent evidence has rendered the vision priority hypothesis obsolete, b... | [
{
"doc_id": 9121594639098404000,
"text": "I think this paper will satisfy you, everyzhing int he one place:\n[ https://jeb.biologists.org/content/218/4/506\n](https://jeb.biologists.org/content/218/4/506)"
},
{
"doc_id": 694789828925001700,
"text": "Physiological basis in vertebrates[edit]\nMain... |
I'm researching genetic disorders that are more prevalent in specific populations and came across a few that seem to have a higher incidence rate among individuals of Eastern European Jewish ancestry. For instance, there is a disorder with a frequency of 1:3600 births in Ashkenazi Jews, linked to a gene on chromosome 1... | [
{
"doc_id": 416581987007618750,
"text": " small portion of the genome, around 1–2%. The results of this sequencing can be used for clinical diagnosis of a genetic condition, including Usher syndrome, retinal disease, hearing impairments, diabetes, epilepsy, Leigh disease, hereditary cancers, neuromuscular d... | [
{
"doc_id": 8263828021801513000,
"text": ":280 in Native Americans and Yupik Eskimos\n1:15000 in American Caucasians\n\n\nCYP21A2\n\n\nGlycogen storage disease type I\n\n1:100000 births in America\n\nG6PC\n\n\nMaple syrup urine disease\n\n1:180000 in the U.S.\n1:176 in Mennonite/Amish communities\n1:250000 ... |
I recently read a study that highlighted the prevalence of retinal tears in patients experiencing a new onset of floaters. The study, part of the Rational Clinical Examination Series in the Journal of the American Medical Association, reported a surprisingly high rate of 14% (95% confidence interval, 12–16%). Given the... | [
{
"doc_id": 6332380944406823000,
"text": "Causes[edit]\nThere are various causes for the appearance of floaters, of which the most common are described here.\nFloaters can occur when eyes age; in rare cases, floaters may be a sign of retinal detachment or a retinal tear.\nVitreous syneresis[edit]\nVitreous ... | [
{
"doc_id": 1894619407401531400,
"text": "oproteinase-9 is associated with diabetic vitreous hemorrhage. Exp Eye Res. 2006;83:401–407. [PubMed] [Google Scholar]\n101. Drance SM, Fairclough M, Butler DM, Kottler MS. The importance of disc hemorrhage in the prognosis of chronic open angle glaucoma. Arch Ophth... |
I'm studying the survival mechanisms of microorganisms and came across the concept of biofilms and microbial mats. How do these structures contribute to the survival and proliferation of bacteria in various environments? What are the key differences between biofilms and microbial mats, and how do the secondary structur... | [
{
"doc_id": 4713999916369586000,
"text": " a cellular division of labour, accessing resources that cannot effectively be used by single cells, collectively defending against antagonists, and optimising population survival by differentiating into distinct cell types. For example, bacteria in biofilms can hav... | [
{
"doc_id": 6718401852687065000,
"text": ", through which there are networks of channels to enable better diffusion of nutrients. In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms. Biofilms are also important in medicine, as these str... |
I'm studying the evolutionary advantages of symmetry in organisms and came across the concept of radial symmetry in cnidarians and some flowers. How does radial symmetry benefit sessile animals like sea anemones and floating animals like jellyfish compared to bilateral symmetry? Additionally, why is radial symmetry par... | [
{
"doc_id": 7094785909081461000,
"text": "Evolution of symmetry[edit]\nLike all the traits of organisms, symmetry (or indeed asymmetry) evolves due to an advantage to the organism – a process of natural selection. This involves changes in the frequency of symmetry-related genes throughout time.\nEvolution o... | [
{
"doc_id": 2632758265103453700,
"text": "Radial symmetry[edit]\n\n\"Radial symmetry\" redirects here. For radial symmetry in mathematics, see rotational symmetry.\nOrganisms with radial symmetry show a repeating pattern around a central axis such that they can be separated into several identical pieces whe... |
I'm curious about the balance between the benefits and risks of fluoride intake in children. How does the amount of fluoride consumed affect the development of dental caries and dental fluorosis? Are there specific intake levels that are considered safe and effective for preventing tooth decay without causing fluorosis... | [
{
"doc_id": 8950067325623963000,
"text": "Toumba, K.J. et al.: [ Guidelines on the use of fuoride for caries prevention\nin children: an updated EAPD policy document\n](https://www.eapd.eu/uploads/files/EAPD_Fluoride_Guidelines_2019.pdf), in:\nEuropean Archives of Paediatric Dentistry. 2019."
},
{
"... | [
{
"doc_id": 7912906134241381000,
"text": "/assets/Oral-Health-Caries-Community-Water-Fluoridation_3.pdf) [ ](/About/exit_disclaimer.aspx \"External Website\") 2013\\.\n29. Slade GD, Grider WB, Maas WR, Sanders AE. Water Fluoridation and Dental Cari... |
I'm studying the relationship between protein size and domain composition and came across some interesting data. An early study analyzed 170,000 proteins and found that about two-thirds of these proteins were assigned at least one domain. The study also noted that larger proteins, particularly those exceeding 600 amino... | [
{
"doc_id": 3240459778741617700,
"text": " synthesized protein can be measured by the number of amino acids it contains and by its total molecular mass, which is normally reported in units of daltons (synonymous with atomic mass units), or the derivative unit kilodalton (kDa). The average size of a protein ... | [
{
"doc_id": 7656469436757046000,
"text": "20domains&journal=BMC%20Genom.&doi=10.1186%2Fs12864-016-2516-6&volume=17&publication_year=2016&author=Oti%2CM&author=Falck%2CJ&author=Huynen%2CMA&author=Zhou%2CH)"
},
{
"doc_id": 4627827396162630000,
"text": " superfamily\" ](https://www.ncbi.nlm.nih.gov... |
I've been reading about the potential origins of SARS-CoV-2 and came across the hypothesis that pangolins might have played a role in the spillover event. However, I noticed that recent studies suggest otherwise. Could someone provide more detailed insights into why pangolin virus samples are considered too distant fro... | [
{
"doc_id": 328311051701325400,
"text": " closely related being bat coronaviruses, including BANAL-52 and RaTG13. The virus is of zoonotic origin; its close genetic similarity to bat coronaviruses suggests it emerged from a bat-borne virus. Research is ongoing as to whether SARS‑CoV‑2 came directly from bat... | [
{
"doc_id": 8486258661291537000,
"text": "\n\n\n\n\n\n\n\n\n\n(Bat) RpYN06, 94.4% to SARS-CoV-2, Rhinolophus pusillus, Xishuangbanna, Yunnan\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n(Bat) RaTG13, 96.1% to SARS-CoV-2, Rhinolophus affinis, Mojiang, Yunnan\n\n\n\n\n\n\n\n\n\n(Bat) BANAL-52, 96.8% to SARS-CoV-2, Rhinolop... |
I've been reading up on bacterial and viral infections and how they are treated. It's clear that antibiotics are the go-to treatment for bacterial infections, but what are the key considerations in choosing the right antibiotic? How does antibiotic resistance play a role in the decision-making process of healthcare pro... | [
{
"doc_id": 3942722163036119000,
"text": "pox, COVID-19 and HIV/AIDS. What treatment will I receive for bacterial and viral infections? Treating a bacterial infection Doctors usually treat bacterial infections with antibiotics. It's important to match the antibiotic with the specific type of bacterial infec... | [
{
"doc_id": 139363084535758700,
"text": " search results Results for medical professionals Top results Infections – bacterial and viral - Better Health Channel Many bacterial infections can be treated with antibiotics, but they are useless against viral infections. Read more on Better Health Channel website... |
I'm exploring the role of transcription factors in tissue-specific gene regulation and came across a study that discusses how transcription factors binding to accessible chromatin within regulatory elements (REs) exhibit distinct and conserved activity between mouse and human. Could someone elaborate on how chromatin a... | [
{
"doc_id": 4614223827884031000,
"text": "These results, taken together, revealed patterns of tissue-specificity and\nenrichment of histone modifications following known characteristics of\npromoters and enhancers. TSS proximal REs were promoter-like, as expected,\nwhile genic and intergenic REs exhibited c... | [
{
"doc_id": 9172209259006890000,
"text": "72. Li, Z. et al. Identification of transcription factor binding sites using ATAC-seq. _Genome Biol._ **20**, 45 (2019)."
},
{
"doc_id": 891533840275468700,
"text": "To identify transcription factor footprints, the HINT tool from the Regulatory\nGenomics... |
I recently came across a study that discusses the development of immature excitatory neurons during adolescence. The research highlights how these neurons play a crucial role in brain development and how their maturation might be linked to the onset of certain neurological disorders. Could someone elaborate on how the ... | [
{
"doc_id": 1654455207960992800,
"text": ".\") The process of neuronal\noverproduction and elimination is necessary to optimize brain connectivity.\nDisturbances in regulating developmental neuronal death not only change cell\ncomposition and connectivity within local neuronal networks, but also alter\nglob... | [
{
"doc_id": 8454476950696215000,
"text": "Sorrells, S. F., Paredes, M. F., Velmeshev, D., Herranz-Pérez, V., Sandoval,\nK., Mayer, S., et al. (2019). Immature excitatory neurons develop during\nadolescence in the human amygdala. _Nat. Commun._ 10, 2748. doi:\n10.1038/s41467-019-10765-1"
},
{
"doc_id... |
I'm studying the process of protein digestion and absorption in the gastro-intestinal tract and have a few questions. Proteins are broken down into single amino acids or small peptides during digestion, primarily in the stomach and small intestine. Pepsinogen is converted to pepsin in the stomach due to the action of h... | [
{
"doc_id": 1192813942070109400,
"text": "Digestion[edit]\nMost proteins are decomposed to single amino acids by digestion in the gastro-intestinal tract.\nDigestion typically begins in the stomach when pepsinogen is converted to pepsin by the action of hydrochloric acid, and continued by trypsin and chymot... | [
{
"doc_id": 8830466065560970000,
"text": " not the end. There, most of the peptides are broken into single amino acids.\nAbsorption of the amino acids and their derivatives into which dietary protein is degraded is done by the gastrointestinal tract. The absorption rates of individual amino acids are highly... |
I'm studying the hemodynamic changes in patients with Postural Orthostatic Tachycardia Syndrome (POTS), specifically the POTS-Faint subtype. In the literature, there's a mention of splanchnic pooling occurring despite sustained splanchnic vasoconstriction. Could someone explain how splanchnic pooling can happen if vaso... | [
{
"doc_id": 6347706772008196000,
"text": "Therefore, intrahepatic resistance is likely increased in POTS‐Faint, resulting in pooling despite sustained splanchnic arterial vasoconstriction. In addition, both intrahepatic resistance and splanchnic arterial vasoconstriction are likely reduced in Control‐Faint,... | [
{
"doc_id": 7108691501048846000,
"text": "‐Faint. We propose that intrahepatic resistance is increased in POTS‐Faint resulting in pooling and that both intrahepatic resistance and splanchnic arterial vasoconstriction are reduced in Control‐Faint resulting in increased splanchnic blood flow and reduced splan... |
I'm curious about the role of muscle fiber types in achieving muscle growth. Specifically, how does targeting fast twitch (type II) muscle fibers contribute more significantly to muscle hypertrophy compared to slow twitch (type I) fibers? What are the physiological mechanisms behind this difference, and are there speci... | [
{
"doc_id": 3095219866847003000,
"text": "While it used to be thought that only heavy loads—weights you can only lift\nabout three to five times—could activate the fast-twitch fibers, we now know\nthat’s not the case, Dr. Schoenfeld says. “Provided that you train with a lot\nof effort where the last reps ar... | [
{
"doc_id": 7484735891377214000,
"text": "\"Different muscle fiber characteristics, Type I and II, slow and fast twitch\nrespectively can both increase in size, but the latter can have more growth\npotential,\" explains Turner."
},
{
"doc_id": 4132241093155054600,
"text": "Research has found tha... |
In the context of pharmacology and neurobiology, how do medications that target enzymes affecting neurotransmitter degradation work to alleviate symptoms of neurological disorders? Specifically, what are the mechanisms by which inhibiting these enzymes prolongs the presence of neurotransmitters in the synaptic cleft, a... | [
{
"doc_id": 8352680171115676000,
"text": " can be destroyed directly either in the cleft or in the pre-synaptic terminal using certain enzymes. Two major enzymes are involved in the destruction of the neurotransmitter:\nMonoamine Oxidases (MAO): These enzymes are responsible for oxidizing, and therefore ina... | [
{
"doc_id": 7583930385946143000,
"text": "Anything that interferes with the processes that terminate the synaptic signal\ncan have significant physiological effects. For instance, some insecticides\nkill insects by inhibiting an enzyme that breaks down the neurotransmitter\nacetylcholine. On a more positive... |
I'm curious about the reproductive biology of humans, specifically focusing on the differences between sperm and egg cells. Could someone provide a detailed comparison of these two types of gametes, including where they are produced and any unique characteristics each has? Additionally, how does the timing of ovum rele... | [
{
"doc_id": 8167485988086013000,
"text": " their whole reproduction cycle. One Ovum is released by both ovaries on an alternate basis in the mid-day of the menstrual cycle. After that, the Ovum waits in the fallopian tube for the Sperm to reach there and get fertilised. Want to read offline? download full P... | [
{
"doc_id": 8371452073971669000,
"text": " ova are released on a cycled basis throughout their whole reproduction cycle. One Ovum is released by both ovaries on an alternate basis in the mid-day of the menstrual cycle. After that, the Ovum waits in the fallopian tube for the Sperm to reach there and get fer... |
I'm studying the role of synapses in neuronal communication and came across some information about diseases that affect synapses. Could someone provide detailed examples of diseases that specifically target synapses and explain how these diseases impact neuronal function and clinical outcomes? I'm particularly interest... | [
{
"doc_id": 2350845706478820400,
"text": "Lambert-Eaton syndrome is also an auto-immune condition producing dysfunction\nat the neuromuscular junction; however, it involves the pre-synaptic neuron.\nInstead of antibodies directed against the ACh receptors as in myasthenia\ngravis, the antibodies here are di... | [
{
"doc_id": 7292431269231704000,
"text": "In both of these disease processes, the causative agent is a toxin produced by\na bacteria that acts as a protease that cleaves the SNARE proteins. This\nprevents the release of neurotransmitters at the junction by inhibiting\nvesicular fusion."
},
{
"doc_id... |
I'm currently delving into the mechanisms of anesthesia-induced unconsciousness and have come across a variety of anesthetic agents, including propofol, halogenated vapors, ketamine, dexmedetomidine, benzodiazepines, and xenon. Each of these substances seems to have unique effects on the sleep-wake cycle and biochemica... | [
{
"doc_id": 8338445386005574000,
"text": ", it is still uneasy to draw a comprehensive picture of the precise cascades during general anesthesia. Several questions remain unsolved, including the exact identification of the neural substrate of consciousness and its components, the detection of specific consc... | [
{
"doc_id": 7932422769889129000,
"text": "49. Bao W.-W., Jiang S., Qu W.-M., Li W.-X., Miao C.-H., Huang Z.-L. Understanding the neural mechanisms of general anesthesia from interaction with sleep-wake state: a decade of discovery. Pharmacol Rev. 2023;75:532–553. [PubMed] [Google Scholar]\n50. Franks N.P. G... |
I've been reading about the correlation between skin pigmentation and ultraviolet radiation exposure in different geographic areas. How does the evolution of darker skin in equatorial regions serve as a protective mechanism against ultraviolet radiation, and what is the role of light skin pigmentation in maintaining vi... | [
{
"doc_id": 1156119316732601600,
"text": " correlates with the level of ultraviolet radiation in a particular geographic area, with darker skin mostly around the equator. Skin darkening may have evolved as protection against ultraviolet solar radiation. Light skin pigmentation protects against depletion of ... | [
{
"doc_id": 3409312761917171000,
"text": "Nina J (2004). \"The evolution of human skin and skin color\". Annual Review of Anthropology. 33 : 585–623. doi : 10.1146/annurev.anthro.33.070203.143955.\n\nJablonski NG, Chaplin G (May 2010). \"Colloquium paper: human skin pigmentation as an adaptation to UV radia... |
I'm curious about the fundamental differences between bacteria and viruses, especially in terms of how these microorganisms reproduce and cause diseases. Could someone provide a detailed comparison, including examples of specific diseases caused by each? Additionally, how does the treatment differ for illnesses caused ... | [
{
"doc_id": 8332165319357622000,
"text": " • Do not self-medicate antibiotics Use antibiotics only when they are likely to be beneficial Frequently asked questions 1. What is the difference between bacteria and viruses? o Bacteria are a class of microorganisms which cannot be seen with naked eye. They can r... | [
{
"doc_id": 4424040280233481700,
"text": " by viruses which do not need antibiotics. The following table shows some examples of bacteria and viruses as well as the diseases they cause: Germs Disease example(s) Bacteria Escherichia coli (E.coli) Urinary tract infection, diarrhoeal diseases Streptococcus pneu... |
I'm studying the effects of oxidative stress on the developing mammalian brain and came across a statement that oxidative stress can lead to severe neurodevelopmental deficits due to chronic exposure to either reduced (hypoxia–ischemia) or elevated (hyperoxia) levels of oxygen. Could someone provide more detailed infor... | [
{
"doc_id": 1919026019128062200,
"text": "google.com/scholar_lookup?&title=Neuronal%20death%20and%20oxidative%20stress%20in%20the%20developing%20brain&journal=Antioxid%20Redox%20Signal&doi=10.1089%2Fars.2010.3581&volume=14&pages=1535-1550&publication_year=2011&author=Ikonomidou%2CC&author=Kaindl%2CAM)"
},... | [
{
"doc_id": 1166877009329983000,
"text": " M, Zabel C, Nebrich G,\nWacker MA, Felderhoff-Mueser U et al. Acute and long-term proteome changes\ninduced by oxidative stress in the developing brain. Cell Death Differ 2006;\n13: 1097–1109.\") Importantly, the effect is limited to immature neurons, as\nhyperoxia... |
I'm curious about the different geographic modes of speciation and how these modes influence the definitions of species. In biology, the concept of a "species" can vary depending on the reproductive methods and ecological contexts of the organisms in question. How do the geographic modes of speciation (allopatric, symp... | [
{
"doc_id": 5482007351581509000,
"text": " genealogical divergence. There are four primary geographic modes of speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these ... | [
{
"doc_id": 6981789283127673000,
"text": " the rest of their species, separation may eventually produce organisms that cannot interbreed.\nThe second mode of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatr... |
I'm curious about the role of palms in the study of monocot evolution. Palms are known to have separated from the monocots earlier than other families and have developed a significant amount of intrafamilial specialization and diversity. How does the early adaptive radiation of palms and the presence of fossil Nypa pol... | [
{
"doc_id": 4906740703608605000,
"text": "Taxonomy[edit]\nTwo Roystonea regia specimens. The characteristic crownshaft and apex shoot, or 'spear', are visible.\nPalms are a monophyletic group of plants, meaning the group consists of a common ancestor and all its descendants. Extensive taxonomic research on ... | [
{
"doc_id": 6538340417566965000,
"text": "Morphology[edit]\nWhether as shrubs, tree-like, or vines, palms have two methods of growth: solitary or clustered. The common representation is that of a solitary shoot ending in a crown of leaves. This monopodial character may be exhibited by prostrate, trunkless, ... |
I'm studying the effects of trauma on the brain and came across a research paper that discusses how apoptotic neurodegeneration is significantly more pronounced in immature brains compared to mature ones. The paper, "Apoptotic neurodegeneration following trauma is markedly enhanced in the immature brain" by Bittigau et... | [
{
"doc_id": 648207360896988300,
"text": " the P3–P7 brains being most vulnerable. [ 130\n](/articles/cddis201764#ref-CR130 \"Bittigau P, Sifringer M, Pohl D, Stadthaus\nD, Ishimaru M, Shimizu H et al. Apoptotic neurodegeneration following trauma\nis markedly enhanced in the immature brain. Ann Neurol 1999;... | [
{
"doc_id": 8874234972044492000,
"text": "entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=10360764)\n[ Google Scholar\n](http://scholar.google.com/scholar_lookup?&title=Apoptotic%20neurodegeneration%20following%20trauma%20is%20markedly%20enhanced%20in%20the%20immature%20brain&journal=Ann%20... |
In the field of biology, the importance of invertebrates cannot be overstated, as these organisms represent the majority of known species. However, invertebrates often receive less attention compared to vertebrates in both research and public interest. Could someone elaborate on why invertebrates might be overshadowed ... | [
{
"doc_id": 7105012453218095000,
"text": " not considered, and no mention was made about concurrent gains in species diversity. About 90% of the terrestrial world's species are insects. A recent estimate by an entomologist (Dunn, 2005) indicates a species diversity of about 3.4 million. While many insect sp... | [
{
"doc_id": 83154112646699820,
"text": "Several leading analysts applauded the estimation technique used by Regnier.\nBut others have been more cautious about reading across taxa. They say it is\ndangerous to assume that other invertebrates are suffering extinctions at a\nsimilar rate to land snails. Mark C... |
I'm curious about the process of egg production in women. From what I understand, eggs develop in the ovaries through a process called oogenesis and are released monthly during ovulation. Typically, each ovary alternates in releasing eggs. However, I've also read that a woman is born with a finite number of potential e... | [
{
"doc_id": 1536504770287136300,
"text": "A woman is born with approximately 500,000 potential eggs, or follicles, in each ovary. From birth onwards she will not produce any more; in fact the number of eggs will steadily decline over her lifetime and be absorbed back into the body in a process known as atre... | [
{
"doc_id": 1607593371945853000,
"text": "ary takes turns releasing eggs every month; however, if one ovary is absent or dysfunctional then the other ovary continues to provide eggs to be released."
},
{
"doc_id": 2205090386878406000,
"text": " small and measures approximately 0.12 mm in diamete... |
I'm reading about the historical classification of organisms and came across the five-kingdom system proposed by Ernst Haeckel. According to the text, Haeckel united certain microorganisms under the kingdom Protista, which were previously classified as either animals or plants. Later, the prokaryotes were separated int... | [
{
"doc_id": 7910025794867290000,
"text": " divided into two separate groups, the Bacteria and the Archaea. This led to the six-kingdom system and eventually to the current three-domain system, which is based on evolutionary relationships. However, the classification of eukaryotes, especially of protists, is... | [
{
"doc_id": 6967618051290112000,
"text": " treated)\n\nProtista\n\nProkaryota\n\nMonera\n\nMonera\n\nBacteria\n\nBacteria\n\n\nArchaea\n\nArchaea (2015)\n\n\nEukaryota\n\nProtoctista\n\nProtista\n\nEucarya\n\n\"Protozoa\"\n\n\n\"Chromista\"\n\n\nVegetabilia\n\nPlantae\n\nPlantae\n\nPlantae\n\nPlantae\n\n\nF... |
In the study of mammalian brain evolution, how does the development of the neocortex, particularly involving structures like the hippocampus and amygdala, influence the size and function of other brain regions such as the superior colliculus and the neocerebellum? What specific roles do these regions play in the overal... | [
{
"doc_id": 2587331062120260600,
"text": " six-layered structure called neocortex or isocortex. Several areas at the edge of the neocortex, including the hippocampus and amygdala, are also much more extensively developed in mammals than in other vertebrates.\nThe elaboration of the cerebral cortex carries w... | [
{
"doc_id": 7867027993540241000,
"text": " largely within the cerebellum. 10% of the brain's total volume consists of the cerebellum and 50% of all neurons are held within its structure.\nThe optic tectum allows actions to be directed toward points in space, most commonly in response to visual input. In mam... |
I'm curious about the ecological role of Prototaxites during the time when vascular plants were just beginning to rise to prominence. Given that Prototaxites was significantly taller than any other living organism of its time, reaching heights far above the 6 centimeter (2.4 in) Cooksonia and the even shorter "moss for... | [
{
"doc_id": 7515045353919362000,
"text": ".\nA 2022 paper suggested that Prototaxites was a fungal rhizomorph that grew on its side and likely at least partially underground, as opposed to the traditional view that it grew upright."
},
{
"doc_id": 8497197202292679000,
"text": " height would have... | [
{
"doc_id": 8608169437708964000,
"text": "\nPrototaxites /ˌproʊtoʊˈtæksɪtiːz/ is an extinct genus of terrestrial fossil fungi dating from the Late Silurian until the Late Devonian periods. Prototaxites formed large trunk-like structures up to 1 metre (3 ft) wide, reaching 8 metres (26 ft) in length, made up... |
I'm curious about the evolution of eyes in different organisms. How do the two main types of photoreceptor cells—ciliated and rhabdomeric—differ in structure and function, and what evolutionary significance do these differences hold? Additionally, how does the presence of both types of photoreceptors in some gastropods... | [
{
"doc_id": 4966117999943788000,
"text": " vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells. The eyes of vertebrates usually contain ciliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells i... | [
{
"doc_id": 4941223347637707000,
"text": "Function[edit]\nPhotoreception[edit]\nAnatomy of a Rod Cell\nIn vertebrates, activation of a photoreceptor cell is a hyperpolarization (inhibition) of the cell. When they are not being stimulated, such as in the dark, rod cells and cone cells depolarize and release ... |
I'm curious about the mechanisms of focusing in different vertebrates. How do the focusing mechanisms in amphibians and cartilaginous fish compare to those in higher vertebrates, and what specific muscles are involved in these processes? Additionally, how do lampreys and hagfish manage to focus without the lens being a... | [
{
"doc_id": 1743566479811413800,
"text": "Focusing mechanisms[edit]\nThe path of light through the eye calculated using four refractive indexes, cornea and lens curvatures approximating components of real eyes. Note objects in some size ranges and distances do not require the light path to bend noticeably t... | [
{
"doc_id": 4260415276799774000,
"text": "Lenses of birds, reptiles, amphibians, fish and others[edit]\nDiving bird (Cormorant) lens focusing can be up to 80 dioptres for clearer underwater vision.\nBony fish eye. Note the spherical lens and muscle to pull the lens backward\nIn reptiles and birds, the cilia... |
I'm curious about the process of meiosis and how gametes are formed in humans. Specifically, how does meiosis ensure that each sperm and ovum ends up with exactly 23 chromosomes, and what mechanisms are in place to maintain this precise number? Additionally, how does the reduction in chromosome number during meiosis af... | [
{
"doc_id": 27742980187808492,
"text": " the number of Chromosomes after Fertilisation to be 46 each gamete should have 23 Chromosomes. In Sexual Reproduction there exists two types of gamete one male and one female. The male gamete is known as Sperm and the female gamete is known as Ovum. These gametes are... | [
{
"doc_id": 5751336519220241000,
"text": "[\n ](/wiki/File:Sexual_cycle_N-2N.svg) In the first\nstage of sexual reproduction, [ meiosis ](/wiki/Meiosis \"Meiosis\"), the\nnumber of chromosomes is ... |
I'm curious about the evolution of the visual spectrum sensitivity in land animals compared to aquatic creatures. How does the peak sensitivity of rhodopsin at 500 nm relate to the evolutionary adaptations of eyes in different environments? Also, how do small genetic changes affect the sensitivity of visual pigments, a... | [
{
"doc_id": 6483048601942811000,
"text": " photoreceptors, each being maximally sensitive to a specific wavelength of light depending on the structure of their opsin photopigment. The various cone cells are maximally sensitive to either short wavelengths (blue light), medium wavelengths (green light), or lo... | [
{
"doc_id": 9147177034961203000,
"text": " considerably thicker, almost spherical resulting in increased light refraction. This difference helps compensate for the smaller angle of refraction between the eye's cornea and the watery environment, as they have more similar refractive indices than cornea and ai... |
How does temperature affect the shelf life of baked goods such as bread? What are the optimal storage conditions to prevent mold growth and maintain freshness? Are there specific temperature ranges and storage practices that are more effective than others? | [
{
"doc_id": 2045538538117355300,
"text": "Although they can survive in less, molds are happiest in warm, humid,\nnutrient-rich environments, according to Elisabetta Lambertini, a senior\nresearch scientist at the Global Alliance for Improved Nutrition. Think of\nthat steaming sourdough loaf or half-eaten ap... | [
{
"doc_id": 1467586632880187000,
"text": "Leavened bread, containing raising agents such as yeast, contains air pockets\nthat are formed following the fermentation process.\n\nDue to the porous nature of bread, mould can easily spread and contaminate\nbelow its surface."
},
{
"doc_id": 6604070330034... |
I recently read about a clinical study investigating the use of recombinant human tropoelastin for enhancing elastin fiber production in skin wounds and scarring. According to the research, subcutaneous injections of recombinant human tropoelastin did not lead to any noticeable improvement in scarring or skin flexibili... | [
{
"doc_id": 2572777622571808000,
"text": "g. AAAKAAKAA) whereas in KP domains the lysine residues are separated mainly by proline residues (e.g. KPLKP).\nAggregation[edit]\nTropoelastin aggregates at physiological temperature due to interactions between hydrophobic domains in a process called coacervation. ... | [
{
"doc_id": 2469368809738647000,
"text": " regulated under at least eight different transcription start sites.\nTissue specific variants of elastin are produced by alternative splicing of the tropoelastin gene. There are at least 11 known human tropoelastin isoforms. these isoforms are under developmental r... |
I'm researching the effects of respiratory infections on pregnant women and came across some interesting points regarding the changes in lung capacity and coagulation during pregnancy. It seems that pregnant women might experience a total reduction in lung capacity, which could affect their ability to clear respiratory... | [
{
"doc_id": 1455924866011442000,
"text": "emic. Given that they are prone to have complications and severe disease infection with other types of coronaviruses, they have been identified as a vulnerable group and advised to take supplementary preventive measures.\nPhysiological responses to pregnancy can inc... | [
{
"doc_id": 7842438099122838000,
"text": "[ Article ](https://doi.org/10.1164%2Frccm.201911-2203LE) [ PubMed\n](http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=32083949)\n[ PubMed Central ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7301748) [\nGoogle Scholar\n]... |
I'm currently studying different types of natural selection in my biology class, and I'm having trouble understanding how stabilizing selection, directional selection, and diversifying selection can be distinguished in real-world scenarios. Could someone provide examples where each type of selection is clearly observab... | [
{
"doc_id": 1560288069817212400,
"text": " phenotype is favoured. · Graph 2 depicts stabilizing selection, where the intermediate phenotype is favoured over the extreme traits. · Graph 3 shows disruptive selection, in which the extreme phenotypes are favoured over the intermediate.\nNatural selection within... | [
{
"doc_id": 4918649367885492000,
"text": " generations, can lead to the re-occurrence of traits thought to be lost like hindlegs in dolphins, teeth in chickens, wings in wingless stick insects, tails and additional nipples in humans etc. \"Throwbacks\" such as these are known as atavisms.\nNatural selection... |
I'm working on a project about the impact of plant viruses on agriculture, and I'm particularly interested in the potato virus Y (PVY). How does PVY spread among potato crops and related species like tomatoes and peppers? What are the most effective methods for controlling PVY, especially in perennial fruits where econ... | [
{
"doc_id": 6421948320632123000,
"text": " acquired economical importance when it proved difficult to control in seed potato crops. Transmitted by aphids, this virus can reduce crop yields by up to 80 per cent, causing significant losses to potato yields.\nPlants have elaborate and effective defence mechani... | [
{
"doc_id": 6221863575403593000,
"text": "negative plant pathogens such as Ralstonia solanacearum and Xylella fastidiosa."
},
{
"doc_id": 4131694095423802000,
"text": ", such as budding and grafting these techniques lead to plant wounds. Wounds are the primary location of bacterial entry into th... |
I'm curious about the mechanisms of viral transmission, especially in the context of respiratory viruses. How does the size of droplets affect the spread of these viruses? Specifically, what is the critical size below which droplets evaporate faster than they settle, and how does this contribute to the formation of air... | [
{
"doc_id": 2778613646658479000,
"text": " pharynx reaches peak viral load approximately four days after infection or in the first week of symptoms and declines thereafter. The duration of SARS-CoV-2 RNA shedding is generally between 3 and 46 days after symptom onset.\nA study by a team of researchers from ... | [
{
"doc_id": 577048956055121400,
"text": " However, infection can occur over longer distances, particularly indoors.\nThe transmission of the virus is carried out through virus-laden fluid particles, or droplets, which are created in the respiratory tract, and they are expelled by the mouth and the nose. The... |
I'm curious about the structural and functional differences between sperm and ovum in the human reproductive system. How do these differences contribute to the fertilization process? What specific roles do the motility of sperm and the structure of the ovum play in ensuring successful fertilization and formation of a z... | [
{
"doc_id": 5381504249992746000,
"text": " into sperm or ova. The ova mature in the ovaries of females, and the sperm develop in the testes of males. Each sperm cell, or spermatozoon, is small and motile. The spermatozoon has a flagellum, which is a tail-shaped structure that allows the cell to propel and m... | [
{
"doc_id": 5097667322802003000,
"text": "The two important Cells of the Human Reproductive system are Sperm and Ovum, the former being male Reproductive Cell and the latter being a female Reproductive Cell. Both of these Cells are responsible to undergo Fertilisation through fusion and formation of zygote.... |
I'm curious about the differences in cellular components between sperm cells and egg cells, particularly focusing on mitochondria, cytoplasm, and chromosomes. How does the distribution and amount of mitochondria in sperm cells compare to that in egg cells? Additionally, why is there a significant difference in the amou... | [
{
"doc_id": 8747368678985454000,
"text": " Mitochondria is centrally located in this cell. Mitochondria is scattered in the cytoplasm of the cell. Amount of Cytoplasm Cytoplasm is present in very small amounts in sperm cells. Cytoplasm is present in large amounts in the egg cell or the ovum. Nucleoplasm pre... | [
{
"doc_id": 1686872585215484000,
"text": " present in large amounts in the egg cell or the ovum. Nucleoplasm present/absent Nucleoplasm is absent in the cell. Nucleoplasm or the germinal vesicle is present in the egg cell. Type of Chromosomes Sperm cells contain X or Y chromosomes. Egg cells contain only X ... |
I'm curious about the factors that contribute to low birth weight in developing countries and how this issue impacts infant mortality rates. Could someone provide insights into the biological and environmental factors that lead to low birth weight, and discuss the specific challenges faced in developing regions that ex... | [
{
"doc_id": 85825616833777100,
"text": " needs to be born earlier for medical reasons. In developed countries, infants are typically 3–4 kg (7–9 lb) in weight and 47–53 cm (19–21 in) in height at birth. However, low birth weight is common in developing countries, and contributes to the high levels of infant... | [
{
"doc_id": 8964374264023119000,
"text": " birth and low birth weights, especially when under 3.5 pounds (1.6 kg), can increase the risk of several health-related issues, such as vision and hearing loss, mental disabilities, and cerebral palsy. There is an increased possibility of potential complications as... |
I'm fascinated by the recent advancements in genetic engineering and the possibility of reviving extinct species. With the successful cloning of Dolly the sheep in 1996, it's clear that organisms can be created from the DNA found in a single cell. Museums around the world house numerous specimens of extinct animals, ma... | [
{
"doc_id": 1628279757158979800,
"text": " engineering have raised questions about bringing extinct species back to life. Since Dolly the sheep was cloned in 1996, scientists know it is possible to create an organism from the DNA in a single cell. Stored in museum collections throughout the world are specim... | [
{
"doc_id": 5273563624654203000,
"text": " back to life. Since Dolly the sheep was cloned in 1996, scientists know it is possible to create an organism from the DNA in a single cell. Stored in museum collections throughout the world are specimens of extinct animals containing DNA. The idea of using DNA to r... |
I'm curious about the evolutionary reasons behind human hairlessness compared to other mammals. In particular, how does hair density relate to body mass in mammals, and what evidence supports the theories of human hairlessness evolution? Could someone provide insights or studies that explore these aspects in detail? | [
{
"doc_id": 8021541184269791000,
"text": "\". Science. 334 (6052): 89–94. Bibcode : 2011Sci...334...89A. doi : 10.1126/science.1209202. PMC 3677943. PMID 21868630. ^ Sandel, Aaron A. (30 July 2013). \"Brief communication: Hair density and body mass in mammals and the evolution of human hairlessness\". Ameri... | [
{
"doc_id": 9025639265771660000,
"text": "94. Bibcode : 2011Sci...334...89A. doi : 10.1126/science.1209202. PMC 3677943. PMID 21868630. ^ Sandel, Aaron A. (30 July 2013). \"Brief communication: Hair density and body mass in mammals and the evolution of human hairlessness\". American Journal of Physical Anth... |
I'm studying the concept of ecosystems and came across Eugene Odum's definition, which describes an ecosystem as a unit that includes all organisms in a given area interacting with the physical environment, leading to a flow of energy, trophic structure, biotic diversity, and material cycles. Could someone elaborate on... | [
{
"doc_id": 4976187155010209000,
"text": " to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Eugene Odum, a founder of ecology, defined an ecosystem as: \"Any unit that includes all of the organisms...in a giv... | [
{
"doc_id": 77160317237409300,
"text": "### Ecosystem Roles"
},
{
"doc_id": 5085700098625718000,
"text": "Ecosystems are profoundly local, based on individual interactions of\nindividual organisms. It may be debatable how much it matters to nature how\nmany species there are on the planet as a ... |
I'm reading about the discovery of an important compound in the history of nutrition and biochemistry. Albert Szent-Györgyi and Joseph L. Svirbely's Hungarian team, along with Charles Glen King's American team, were pivotal in identifying the anti-scorbutic factor, which is crucial for preventing scurvy. Szent-Györgyi ... | [
{
"doc_id": 322629905076492200,
"text": "ly's Hungarian team, and Charles Glen King's American team, identified the anti-scorbutic factor. Szent-Györgyi isolated hexuronic acid from animal adrenal glands, and suspected it to be the antiscorbutic factor. In late 1931, Szent-Györgyi gave Svirbely the last of ... | [
{
"doc_id": 649706145039953500,
"text": "izing the vitamin in bulk by what is now called the Reichstein process. This made possible the inexpensive mass-production of vitamin C. In 1934, Hoffmann–La Roche bought the Reichstein process patent, trademarked synthetic vitamin C under the brand name Redoxon, and... |
In the field of molecular biology, the dynamic nature of gene expression and protein levels within cells is a topic of intense interest. A recent discussion in my study group revolved around the question of whether cell states should be considered discrete or continuous. We also pondered how the method of categorizing ... | [
{
"doc_id": 694647284470496300,
"text": "Finally, intrinsic cell dynamics, such as cell cycle or circadian rhythms, and dynamic interactions between the cell and its environment, can further complicate the distinction between discrete and continuous cell states. Reliable information on the dynamics of cell ... | [
{
"doc_id": 9159636030409312000,
"text": "Discrete versus continuous cell states\nDefining the state of a cell based on its constituents is already a challenge in terminally differentiated tissues or tissues with limited turnover (e.g. the cerebral cortex). In such systems, cells typically maintain stable p... |
I'm currently studying neurotransmitters and their roles in different parts of the nervous system. Specifically, I'm interested in understanding how acetylcholine (ACh) functions as an excitatory neurotransmitter at the neuromuscular junction and in autonomic ganglia, and how ACh is synthesized in the basal nucleus of ... | [
{
"doc_id": 3065246000743737300,
"text": "* Acetylcholine (ACh): One of the most important neurotransmitters found in multiple synapses in the body, including, but not limited to, the neuromuscular junction, autonomic ganglia, caudate nucleus, and the limbic system. Generally, ACh is an excitatory neurotran... | [
{
"doc_id": 8320002239243868000,
"text": "[ 4 years ago Posted 4 years ago. Direct link to Ivana - Science trainee's\npost “Acetylcholine interacts w...” ](/science/biology/human-biology/neuron-\nnervous-system/a/the-\nsynapse?qa_expand_key=ag5zfmtoYW4tYWNhZGVteXJACxIIVXNlckRhdGEiHWthaWRfNzQ3MjAzOTc2NDM2M... |
I'm studying the effects of eliminating programmed cell death on neural migration and olfactory function in the brain. Specifically, I'm interested in how the rostral migratory stream is impacted when programmed cell death is inhibited. Does anyone have insights or research findings that discuss whether impaired migrat... | [
{
"doc_id": 1888180663439848200,
"text": "7. Kim WR, Kim Y, Eun B, Park OH, Kim H, Kim K _et al_. Impaired migration in the rostral migratory stream but spared olfactory function after the elimination of programmed cell death in Bax knock-out mice. _J Neurosci_ 2007; **27** : 14392–14403."
},
{
"doc... | [
{
"doc_id": 2594688865970449400,
"text": "20migration%20and%20positioning%20in%20the%20olfactory%20bulb%20and%20the%20exit%20of%20neuroblasts%20from%20the%20subventricular%20zone&journal=Eur%20J%20Neurosci&doi=10.1111%2Fj.1460-9568.2009.06870.x&volume=30&pages=742-755&publication_year=2009&author=Hurtado-\n... |
I'm studying the process of fertilization and early embryo development in humans. From what I understand, sperm cells, which are produced in the testes, begin a journey to reach the egg cell, or ovum, which is produced in the ovaries. After entering the female reproductive tract, sperm cells travel through the cervix, ... | [
{
"doc_id": 8852778367704147000,
"text": " fallopian tube that links an ovary to the uterus. If the fertilized egg successfully travels down the fallopian tube and implants in the uterus, an embryo starts growing. Ans. Sperm are male reproductive cells or male gametes in the male reproductive organs known a... | [
{
"doc_id": 3196427959912516600,
"text": " find their way into the cervix. The sperm then begin their long journey towards the egg. Leaving the cervix they enter the womb. Here, they swim toward the Fallopian tubes.\n\nAns. Fertilisation is the fusion of male and female gametes to rise to a new individual. ... |
I'm curious about the effects of ultraviolet (UV) radiation on human skin and how these effects vary among different individuals. Specifically, I've read that some people might be more susceptible to skin damage from the sun than others. Could you explain why this is the case and what factors contribute to increased se... | [
{
"doc_id": 6544018315048092000,
"text": " or have been sunburned. Have light-color skin, hair, and eyes. Take some types of oral and topical medicines, such as antibiotics, birth control pills, and benzoyl peroxide products, as well as some cosmetics, may increase skin and eye sensitivity to UV in all skin... | [
{
"doc_id": 6654111313984970000,
"text": ", but the risks increase in people who: Spend a lot of time in the sun or have been sunburned. Have light-color skin, hair, and eyes. Take some types of oral and topical medicines, such as antibiotics, birth control pills, and benzoyl peroxide products, as well as s... |
I'm reading up on the role of Bcl-x in the development of the mouse substantia nigra and came across a study indicating that Bcl-x is essential for proper development. The study also mentions that pro-apoptotic proteins, particularly Bax and Bak, play a significant role in promoting neuronal death. How do these protein... | [
{
"doc_id": 4726265538697730000,
"text": " ](http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6573662) [\nGoogle Scholar\n](http://scholar.google.com/scholar_lookup?&title=Bax%20deficiency%20prevents%20the%20increased%20cell%20death%20of%20immature%20neurons%20in%20bcl-\nx-\ndeficient%20mice&journal=J%20Neurosci... | [
{
"doc_id": 1577163544796728600,
"text": ", Roth KA, Sawa H,\nNakayama K, Nakayama K et al. Massive cell death of immature hematopoietic\ncells and neurons in Bcl-x-deficient mice. Science 1995; 267: 1506–1510.\"), [\n87 ](/articles/cddis201764#ref-CR87 \"Shindler KS, Latham CB, Roth KA. Bax\ndeficiency pre... |
I'm curious about the composition and function of the vitreous in the eye. How does the vitreous, a transparent, gelatinous mass that fills the space between the lens and the retina, contribute to the overall health and function of the eye? Additionally, what role do the phagocytes and hyalocytes of Balazs play in main... | [
{
"doc_id": 272328679096451650,
"text": " acid), no blood vessels, and 98–99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibres with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in mic... | [
{
"doc_id": 4089613856477298700,
"text": "99. Jin M, Kashiwagi K, Iizuka Y, Tanaka Y, Imai M, Tsukahara S. Matrix metalloproteinases in human diabetic and nondiabetic vitreous. Retina. 2001;21:28–33. [PubMed] [Google Scholar]\n100. Descamps FJ, Martens E, Kangave D, Struyf S, Geboes K, Van Damme J, Opdenakk... |
I've been reading about vitamin C and its importance in human health. Linus Pauling, a notable figure in the field, suggested in 1970 that humans should consume at least 2,300 mg/day of vitamin C to achieve optimal health, compensating for the inability to synthesize the vitamin internally, unlike many other mammals. T... | [
{
"doc_id": 498406216741366300,
"text": " should be consuming at least 2,300 mg/day to compensate for the inability to synthesize vitamin C. The recommendation also fell into the consumption range for gorillas – a non-synthesizing near-relative to humans. A second argument for high intake is that serum asco... | [
{
"doc_id": 3543008283842064000,
"text": " 50 µmol/L, so 'normal' is about 25% of what can be achieved when oral consumption is in the proposed megadose range.\nPauling popularized the concept of high dose vitamin C as prevention and treatment of the common cold in 1970. A few years later he proposed that v... |
I'm reading up on the genetic factors that influence human resistance to malaria and came across a couple of research papers. One by Hedrick in 2011 discusses the population genetics of malaria resistance, and another by Weatherall in 2008 explores how genetic variation in red cells affects susceptibility to malaria. C... | [
{
"doc_id": 8660851524417328000,
"text": "Role in disease[edit]\nThe survival of many serious genetic disorders in our long evolutionary history has led researchers to reassess the role of antagonistic pleiotropy in disease. If genetic disorders are defined by the existence of deleterious alleles, then natu... | [
{
"doc_id": 3717481448095056000,
"text": " S2CID 20415396. ^ Hedrick PW (October 2011). \"Population genetics of malaria resistance in humans\". Heredity. 107 (4): 283–304. doi : 10.1038/hdy.2011.16. PMC 3182497. PMID 21427751. ^ Weatherall DJ (May 2008). \"Genetic variation and susceptibility to infection:... |
I'm studying the life cycle and transmission of malaria and have a specific question regarding the role of mosquitoes in spreading the disease. How do mosquitoes act as vectors for the *Plasmodium* parasite, and what mechanisms allow *Plasmodium* to survive and multiply within the mosquito before being transmitted to h... | [
{
"doc_id": 4161775671760185300,
"text": "[ https://www.ncbi.nlm.nih.gov/books/NBK27911/\n](https://www.ncbi.nlm.nih.gov/books/NBK27911/)"
}
] | [
{
"doc_id": 7579552577826821000,
"text": "Riehle, M. A., _et al_. Using bacteria to express and display anti-\n_Plasmodium_ molecules in the mosquito midgut. _International Journal of\nParasitology_ **37**, 595–603 (2007)"
},
{
"doc_id": 712862380789366700,
"text": "* Malaria. (2022).\n[ https:/... |
I'm reading about the evolution of senescence and came across the paper by George C. Williams from 1957 titled "Pleiotropy, Natural Selection, and the Evolution of Senescence." Williams discusses how pleiotropic genes, which have multiple effects on different traits, can influence the aging process through natural sele... | [
{
"doc_id": 7033329867091455000,
"text": "Adaptivity and senescence[edit]\nAn antagonistically pleiotropic gene can be selected for if it has beneficial effects in early life while having its negative effects in later life because genes tend to have larger impacts on fitness in an organism's prime than in t... | [
{
"doc_id": 488661444248667840,
"text": " result from selection for pleiotropic genes that are beneficial early in life but harmful later on. In this case, selection pressure is relatively high when Fisher's reproductive value is high and relatively low when Fisher's reproductive value is low.\nCancer versu... |
In the context of human vision, myopia, or nearsightedness, is a condition where the eye can see nearby objects clearly, but distant objects appear blurry. This is due to a refractive error where the focal point of the light entering the eye falls in front of the retina instead of directly on it. However, I'm curious a... | [
{
"doc_id": 4660484379006952000,
"text": "For a myope, the refraction power is already too high to focus distant object. As the object gets closer, there will be point where the (excess for distance) refraction is just right. Objects beyond this point will be out-of-focus. At & within this point, the eye"
... | [
{
"doc_id": 5277346963269157000,
"text": "If you have myopia, your eyes are focused up close"
},
{
"doc_id": 6483076863330530000,
"text": "![A blurry street sign with myopia \\(nearsighted\\) vs. a clear street sign\nwith normal\nvision.](https://my.clevelandclinic.org/-/scassets/images/org/heal... |
I've been reading about hybridization in nature and came across an interesting case involving the gelada and the hamadryas baboon. Despite these two species belonging to different genera and having diverged millions of years ago, interbreeding still occurs in the wild. What are the potential evolutionary implications o... | [
{
"doc_id": 8889518085160947000,
"text": "Post-zygotic isolation[edit]\nA number of mechanisms which act after fertilization preventing successful inter-population crossing are discussed below.\nZygote mortality and non-viability of hybrids[edit]\nA type of incompatibility that is found as often in plants a... | [
{
"doc_id": 2028903019636597200,
"text": "Genetics[edit]\nPre-copulatory mechanisms in animals[edit]\nThe genetics of ethological isolation barriers will be discussed first. Pre-copulatory isolation occurs when the genes necessary for the sexual reproduction of one species differ from the equivalent genes o... |
I've been reading about vitamin C and came across some interesting points regarding its metabolism and excretion. Vitamin C, or ascorbic acid, is known to be highly concentrated in certain organs, which leads me to wonder how effective plasma vitamin C levels are as an indicator of the overall body status. Moreover, I ... | [
{
"doc_id": 1840698153413007000,
"text": "al glands can exceed 2,000 µmol/L, and muscle is at 200–300 µmol/L. The known coenzymatic functions of ascorbic acid do not require such high concentrations, so there may be other, as yet unknown functions. A consequence of all this high concentration organ content ... | [
{
"doc_id": 3659852936339318300,
"text": ", but in solutions above pH 5 is predominantly found in the ionized form, ascorbate.\nNumerous analytical methods have been developed for ascorbic acid detection. For example, vitamin C content of a food sample such as fruit juice can be calculated by measuring the ... |
I'm studying the concept of adaptedness in organisms and how adaptive traits contribute to survival and reproduction. Could someone provide a detailed explanation of how adaptation can lead to both the gain of new features and the loss of ancestral ones? For instance, how does bacterial adaptation to antibiotic selecti... | [
{
"doc_id": 1521164811212119300,
"text": " cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or ... | [
{
"doc_id": 7011957731131189000,
"text": " bacterium Sphingobium evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and ... |
I recently read about a study where researchers found that all *Klebsiella pneumoniae* isolated from car door handles were resistant to Augmentin and Ceftriaxone. Given that these surfaces can act as a reservoir for resistant bacteria, how significant is the risk of infection from touching such surfaces in public setti... | [
{
"doc_id": 6669481434202111000,
"text": " µg), Streptomycin (30 µg) and Cloxacillin (30 µg). The results showed a significant frequency of occurrence of Staphylococcus aureus at 35% and Klebsiella pneumoniae having least at 5%. From car door handles, S. epidermidis recorded 37% while K. pneumoniae recorded... | [
{
"doc_id": 1251800354590232000,
"text": "Hafiza YahyaNorlelawati Ariffin\nHanis Nadia Yahya\nView\n... Of all the human organs, the hands are often more exposed to environmental contamination which makes them a means for transmitting microorganisms from one person to another as they are able to harbour tra... |
I'm currently studying the role of ATP (Adenosine Triphosphate) in cellular processes and have a specific question. How does the structure of ATP contribute to its function as the primary energy currency in biological systems? What are the key chemical properties of ATP that allow it to efficiently store and release en... | [
{
"doc_id": 9077804600915274000,
"text": "wiki/Dephosphorylation \"Dephosphorylation\") of ATP\nby enzymes known as [ ATPases ](/wiki/ATPase \"ATPase\"). The cleavage of a\nphosphate group from ATP results in the coupling of energy to metabolic\nreactions and a by-product of ADP. [1] ATP is continually re... | [
{
"doc_id": 4296125947924500000,
"text": "## ATP Hydrolysis and Synthesis"
},
{
"doc_id": 209756938577391460,
"text": "## ATP and Energy Coupling"
},
{
"doc_id": 1187127491154726100,
"text": "Breaking one of ATP's phosphorus bonds generates approximately 30.5 [\nkilojoules ](/wiki/... |
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