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Mutation
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=== Induced mutation === Induced mutations are alterations in the gene after it has come in contact with mutagens and environmental causes. ''Induced mutations'' on the molecular level can be caused by: * Chemicals ** [[Hydroxylamine]] ** [[Base analogue]]s (e.g., [[Bromodeoxyuridine]] (BrdU)) ** [[Alkylation|Alkylating agent]]s (e.g., [[ENU|''N''-ethyl-''N''-nitrosourea]] (ENU). These agents can mutate both replicating and non-replicating DNA. In contrast, a base analogue can mutate the DNA only when the analogue is incorporated in replicating the DNA. Each of these classes of chemical mutagens has certain effects that then lead to [[transition (genetics)|transition]]s, [[transversion]]s, or deletions. ** Agents that form [[DNA adduct]]s (e.g., [[ochratoxin A]])<ref>{{cite journal | vauthors = Pfohl-Leszkowicz A, Manderville RA | title = Ochratoxin A: An overview on toxicity and carcinogenicity in animals and humans | journal = Molecular Nutrition & Food Research | volume = 51 | issue = 1 | pages = 61–99 | date = January 2007 | pmid = 17195275 | doi = 10.1002/mnfr.200600137 }}</ref> ** DNA [[Intercalation (biochemistry)|intercalating]] agents (e.g., [[ethidium bromide]]) ** [[Crosslinking of DNA|DNA crosslinkers]] ** [[Oxidative stress|Oxidative damage]] ** [[Nitrous acid]] converts amine groups on A and C to [[diazo]] groups, altering their hydrogen bonding patterns, which leads to incorrect base pairing during replication. * Radiation ** [[Ultraviolet]] light (UV) (including [[non-ionizing radiation]]). Two nucleotide bases in DNA—[[cytosine]] and thymine—are most vulnerable to radiation that can change their properties. UV light can induce adjacent [[pyrimidine]] bases in a DNA strand to become covalently joined as a [[pyrimidine dimer]]. UV radiation, in particular longer-wave UVA, can also cause [[DNA oxidation|oxidative damage to DNA]].<ref name="Kozmin">{{cite journal | vauthors = Kozmin S, Slezak G, Reynaud-Angelin A, Elie C, de Rycke Y, Boiteux S, Sage E | title = UVA radiation is highly mutagenic in cells that are unable to repair 7,8-dihydro-8-oxoguanine in Saccharomyces cerevisiae | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 38 | pages = 13538–43 | date = September 2005 | pmid = 16157879 | pmc = 1224634 | doi = 10.1073/pnas.0504497102 | bibcode = 2005PNAS..10213538K | doi-access = free }}</ref> ** [[Ionizing radiation]]. Exposure to ionizing radiation, such as [[Gamma ray|gamma radiation]], can result in mutation, possibly resulting in cancer or death. Whereas in former times mutations were assumed to occur by chance, or induced by mutagens, molecular mechanisms of mutation have been discovered in bacteria and across the tree of life. As S. Rosenberg states, "These mechanisms reveal a picture of highly regulated mutagenesis, up-regulated temporally by stress responses and activated when cells/organisms are maladapted to their environments—when stressed—potentially accelerating adaptation."<ref name="Fitzgerald-2019">{{cite journal | vauthors = Fitzgerald DM, Rosenberg SM | title = What is mutation? A chapter in the series: How microbes "jeopardize" the modern synthesis | journal = PLOS Genetics | volume = 15 | issue = 4 | pages = e1007995 | date = April 2019 | pmid = 30933985 | doi = 10.1371/journal.pgen.1007995 | pmc = 6443146 | doi-access = free }}</ref> Since they are self-induced mutagenic mechanisms that increase the adaptation rate of organisms, they have some times been named as adaptive mutagenesis mechanisms, and include the SOS response in bacteria,<ref>{{cite journal | vauthors = Galhardo RS, Hastings PJ, Rosenberg SM | title = Mutation as a stress response and the regulation of evolvability | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 42 | issue = 5 | pages = 399–435 | date = 1 January 2007 | pmid = 17917874 | pmc = 3319127 | doi = 10.1080/10409230701648502 }}</ref> ectopic intrachromosomal recombination<ref>{{cite journal | vauthors = Quinto-Alemany D, Canerina-Amaro A, Hernández-Abad LG, Machín F, Romesberg FE, Gil-Lamaignere C | title = Yeasts acquire resistance secondary to antifungal drug treatment by adaptive mutagenesis | journal = PLOS ONE | volume = 7 | issue = 7 | pages = e42279 | date = 31 July 2012 | pmid = 22860105 | doi = 10.1371/journal.pone.0042279 | veditors = Sturtevant J | pmc = 3409178 | bibcode = 2012PLoSO...742279Q | doi-access = free }}</ref> and other chromosomal events such as duplications.<ref name="Fitzgerald-2019" />
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