Editing Genetics (section)

=== Mutations ===
{{Main|Mutation}}

[[File:Gene-duplication.png|thumb|upright|Gene duplication allows diversification by providing redundancy: one gene can mutate and lose its original function without harming the organism.]]

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can affect the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low—1 error in every 10–100&nbsp;million bases—due to the "proofreading" ability of [[DNA polymerase]]s.<ref name="griffiths2000sect2706">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2706 |chapter=Spontaneous mutations}}</ref><ref name="Kunkel">{{cite journal | vauthors = Freisinger E, Grollman AP, Miller H, Kisker C | title = Lesion (in)tolerance reveals insights into DNA replication fidelity | journal = The EMBO Journal | volume = 23 | issue = 7 | pages = 1494–1505 | date = April 2004 | pmid = 15057282 | pmc = 391067 | doi = 10.1038/sj.emboj.7600158 }}</ref> Processes that increase the rate of changes in DNA are called [[mutagenic]]: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while [[UV radiation]] induces mutations by causing damage to the DNA structure.<ref name="griffiths2000sect2727">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2727 |chapter=Induced mutations}}</ref> Chemical damage to DNA occurs naturally as well and cells use [[DNA repair]] mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence. A particularly important source of DNA damages appears to be [[reactive oxygen species]]<ref name="pmid23378590">{{cite journal | vauthors = Cadet J, Wagner JR | title = DNA base damage by reactive oxygen species, oxidizing agents, and UV radiation | journal = Cold Spring Harbor Perspectives in Biology | volume = 5 | issue = 2 | page = a012559 | date = February 2013 | pmid = 23378590 | pmc = 3552502 | doi = 10.1101/cshperspect.a012559 }}</ref> produced by [[cellular respiration|cellular aerobic respiration]], and these can lead to mutations.<ref name="pmid22750987">{{cite journal | vauthors = Jena NR | title = DNA damage by reactive species: Mechanisms, mutation and repair | journal = Journal of Biosciences | volume = 37 | issue = 3 | pages = 503–517 | date = July 2012 | pmid = 22750987 | doi = 10.1007/s12038-012-9218-2 | s2cid = 14837181 }}</ref>

In organisms that use [[chromosomal crossover]] to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence—[[Gene duplication|duplications]], [[Chromosomal inversion|inversions]], [[Gene deletion|deletions]] of entire regions—or the accidental exchange of whole parts of sequences between different chromosomes, [[chromosomal translocation]].<ref name="griffiths2000sect2844">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.2844 |chapter=Chromosome Mutation I: Changes in Chromosome Structure: Introduction}}</ref>[[File:Mutations.svg|thumb|This is a diagram showing mutations in an RNA sequence. Figure (1) is a normal RNA sequence, consisting of 4 codons. Figure (2) shows a missense, single point, non silent mutation. Figures (3 and 4) both show [[frameshift mutation]]s, which is why they are grouped together. Figure 3 shows a deletion of the second base pair in the second codon. Figure 4 shows an insertion in the third base pair of the second codon. Figure (5) shows a repeat expansion, where an entire codon is duplicated.]]