Editing Mitochondrial DNA (section)

== Origin ==
Nuclear and mitochondrial DNA are thought to have separate [[evolution]]ary origins, with the mtDNA derived from the circular genomes of [[bacteria]] engulfed by the ancestors of modern eukaryotic cells. This theory is called the [[endosymbiotic theory]]. In the cells of extant organisms, the vast majority of the proteins in the mitochondria (numbering approximately 1500 different types in [[mammal]]s) are coded by [[nuclear DNA]], but the genes for some, if not most, of them are thought to be of bacterial origin, having been transferred to the [[eukaryotic]] nucleus during [[evolution]].<ref name="Johnston, I. G. and Williams, B. P. 2016">{{Cite journal |vauthors=Johnston IG, Williams BP |date=February 2016 |title=Evolutionary Inference across Eukaryotes Identifies Specific Pressures Favoring Mitochondrial Gene Retention |url=http://pure-oai.bham.ac.uk/ws/files/27980093/for_xiv.pdf |journal=Cell Systems |volume=2 |issue=2 |pages=101–111 |doi=10.1016/j.cels.2016.01.013 |pmid=27135164 |doi-access=free}}</ref>

The reasons mitochondria have retained some genes are debated. The existence in some species of mitochondrion-derived organelles lacking a genome<ref>{{Cite book |title=A Survey of Cell Biology |vauthors=van der Giezen M, Tovar J, Clark CG |year=2005 |isbn=978-0-12-364648-4 |series=International Review of Cytology |volume=244 |pages=175–225 |chapter=Mitochondrion-Derived Organelles in Protists and Fungi |doi=10.1016/S0074-7696(05)44005-X |pmid=16157181}}</ref> suggests that complete gene loss is possible, and transferring mitochondrial genes to the nucleus has several advantages.<ref>{{Cite journal |vauthors=Adams KL, Palmer JD |date=December 2003 |title=Evolution of mitochondrial gene content: gene loss and transfer to the nucleus |journal=Molecular Phylogenetics and Evolution |volume=29 |issue=3 |pages=380–395 |bibcode=2003MolPE..29..380A |doi=10.1016/S1055-7903(03)00194-5 |pmid=14615181}}</ref> The difficulty of targeting remotely produced hydrophobic protein products to the mitochondrion is one hypothesis for why some genes are retained in mtDNA;<ref>{{Cite journal |author-link5=Siv G. E. Andersson |vauthors=Björkholm P, Harish A, Hagström E, Ernst AM, Andersson SG |date=August 2015 |title=Mitochondrial genomes are retained by selective constraints on protein targeting |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=112 |issue=33 |pages=10154–10161 |bibcode=2015PNAS..11210154B |doi=10.1073/pnas.1421372112 |pmc=4547212 |pmid=26195779 |doi-access=free}}</ref> [[CoRR hypothesis|colocalisation for redox regulation]] is another, citing the desirability of localised control over mitochondrial machinery.<ref>{{Cite journal |vauthors=Allen JF |date=August 2015 |title=Why chloroplasts and mitochondria retain their own genomes and genetic systems: Colocation for redox regulation of gene expression |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=112 |issue=33 |pages=10231–10238 |bibcode=2015PNAS..11210231A |doi=10.1073/pnas.1500012112 |pmc=4547249 |pmid=26286985 |doi-access=free}}</ref> Recent analysis of a wide range of mtDNA genomes suggests that both these features may dictate mitochondrial gene retention.<ref name="Johnston, I. G. and Williams, B. P. 2016" />