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{{short description|Phenotypically-different organism resulting from a mutation}}
{{other uses}}
{{refimprove|date=August 2017}}
[[Image:Blue-lobster.jpg|220px|thumb|right|The [[American lobster#Coloration|blue lobster]], an example of a mutant]]
[[Image:Physcomitrella knockout mutants.JPG|thumb|right|220px|Wild-type [[Physcomitrella patens|''Physcomitrella'']] and [[knockout moss]]es: Deviating phenotypes induced in gene-disruption library transformants. ''Physcomitrella'' wild-type and transformed plants were grown on minimal Knop medium to induce differentiation and development of [[gametophore]]s. For each [[plant]], an overview (upper row, scale bar corresponds to 1 mm) and a close-up (bottom row, scale bar equals 0.5 mm) is shown. A, [[Haploid]] wild-type moss plant completely covered with leafy gametophores and close-up of wild-type leaf. B–E, Different mutants.<ref>Egener et al. BMC Plant Biology 2002 2:6 {{doi|10.1186/1471-2229-2-6}}</ref>]]
In [[biology]], and especially in [[genetics]], a '''mutant''' is an organism or a new genetic character arising or resulting from an instance of [[mutation]], which is generally an alteration of the [[DNA]] sequence of the [[genome]] or [[chromosome]] of an [[organism]]. It is a characteristic that would not be observed naturally in a specimen. The term mutant is also applied to a [[virus]] with an alteration in its [[Nucleic acid sequence|nucleotide sequence]] whose [[genome]] is in the [[Nuclear gene|nuclear genome]]. The natural occurrence of genetic mutations is integral to the process of [[evolution]]. The study of mutants is an integral part of biology; by understanding the effect that a mutation in a gene has, it is possible to establish the normal function of that gene.<ref>[http://www.pnas.org/content/68/9/2112.abstract Clock Mutants of ''Drosophila melanogaster'']</ref>
==Mutants arise by mutation==
Mutants arise by [[mutation]]s occurring in pre-existing [[genome]]s as a result of errors of [[DNA replication]] or errors of [[DNA repair]].  Errors of replication often involve [[DNA repair#translesion synthesis|translesion synthesis]] by a [[DNA polymerase#Polymerases η, ι and κ (eta, iota, and kappa)|DNA polymerase]] when it encounters and bypasses a damaged base in the template strand.<ref name="pmid19258535">{{cite journal |vauthors=Waters LS, Minesinger BK, Wiltrout ME, D'Souza S, Woodruff RV, Walker GC |title=Eukaryotic translesion polymerases and their roles and regulation in DNA damage tolerance |journal=Microbiol. Mol. Biol. Rev. |volume=73 |issue=1 |pages=134–54 |date=March 2009 |pmid=19258535 |pmc=2650891 |doi=10.1128/MMBR.00034-08 }}</ref>  A [[DNA damage (naturally occurring)|DNA damage]] is an abnormal chemical structure in DNA, such as a strand break or an oxidized base, whereas a mutation, by contrast, is a change in the sequence of standard base pairs. Errors of repair occur when repair processes inaccurately replace a damaged DNA sequence.  The DNA repair process [[microhomology-mediated end joining]] is particularly error-prone.<ref name="pmid18809224">{{cite journal |vauthors=McVey M, Lee SE |title=MMEJ repair of double-strand breaks (director's cut): deleted sequences and alternative endings |journal=Trends Genet. |volume=24 |issue=11 |pages=529–38 |date=November 2008 |pmid=18809224 |pmc=5303623 |doi=10.1016/j.tig.2008.08.007 }}</ref><ref name="pmid23610439">{{cite journal |vauthors=Truong LN, Li Y, Shi LZ, Hwang PY, He J, Wang H, Razavian N, Berns MW, Wu X |title=Microhomology-mediated End Joining and Homologous Recombination share the initial end resection step to repair DNA double-strand breaks in mammalian cells |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=110 |issue=19 |pages=7720–5 |date=May 2013 |pmid=23610439 |pmc=3651503 |doi=10.1073/pnas.1213431110 |bibcode=2013PNAS..110.7720T |doi-access=free }}</ref>
==Etymology==
Although not all mutations have a noticeable [[phenotypic]] effect, the common usage of the word "mutant" is generally a [[pejorative]] term, only used for genetically or phenotypically noticeable mutations.<ref name=":0">[http://dictionary.reference.com/browse/mutant Mutant. (n.d.). ''The American Heritage Dictionary of the English Language'', Fourth Edition. Retrieved March 05, 2008, from Dictionary.com]</ref> Previously, people used the word "'''sport'''" (related to [[wikt:spurt|spurt]]) to refer to abnormal specimens. The scientific usage is broader, referring to any organism differing from the [[wild type]]. The word finds its origin in the Latin term mūtant- (stem of mūtāns), which means "to change".<ref name=":0" />
Mutants should not be confused with organisms born with [[Developmental abnormality|developmental abnormalities]], which are caused by errors during [[morphogenesis]]. In a developmental abnormality, the DNA of the organism is unchanged and the abnormality cannot be passed on to progeny. [[Conjoined twins]] are the result of developmental abnormalities.
Chemicals that cause developmental abnormalities are called [[teratogen]]s; these may also cause mutations, but their effect on development is not related to mutations.  Chemicals that induce mutations are called [[mutagen]]s. Most mutagens are also considered to be [[carcinogen]]s.
==Epigenetic alterations==
[[Mutation]]s are distinctly different from [[epigenetics|epigenetic]] alterations, although they share some common features. Both arise as a [[chromosome|chromosomal]] alteration that can be replicated and passed on to subsequent cell generations. Both, when occurring within a gene, may silence expression of the gene.  Whereas mutant cell lineages arise as a change in the sequence of standard bases, epigenetically altered cell lineages retain the sequence of standard bases but have gene sequences with changed levels of expression that can be passed down to subsequent cell generations.  Epigenetic alterations include [[DNA methylation|methylation]] of [[CpG site#Methylation of CpG islands stably silences genes|CpG islands]] of a gene [[Promoter (genetics)|promoter]] as well as specific chromatin histone modifications.  Faulty repair of chromosomes at sites of DNA damage can give rise both to mutant cell lineages<ref name="pmid18809224">{{cite journal |vauthors=McVey M, Lee SE |title=MMEJ repair of double-strand breaks (director's cut): deleted sequences and alternative endings |journal=Trends Genet. |volume=24 |issue=11 |pages=529–38 |date=November 2008 |pmid=18809224 |pmc=5303623 |doi=10.1016/j.tig.2008.08.007 }}</ref> and/or [[DNA methylation in cancer#Methylation of CpG islands in promoters stably silences genes|epigenetically altered]] cell lineages.<ref name="pmid27259203">{{cite journal |vauthors=Dabin J, Fortuny A, Polo SE |title=Epigenome Maintenance in Response to DNA Damage |journal=Mol. Cell |volume=62 |issue=5 |pages=712–27 |date=June 2016 |pmid=27259203 |pmc=5476208 |doi=10.1016/j.molcel.2016.04.006 }}</ref>
==See also==
* [[Evolution]]
* [[Genetic engineering]]
* [[Genetically modified organism]]
* [[Mutants in fiction]]
* [[Mutationism]]
* [[Synthetic lethality]]
* [[Synthetic viability]]
==References==
{{Reflist|2}}
==External links==
{{Wikiquote}}
* [https://www.mun.ca/biology/scarr/Antennapedia_mutant.htm Antennapedia mutant]
[[Category:Evolutionary biology]]
[[Category:Classical genetics]]
[[Category:Mutation]]