Editing DNA (section)

== Genetic recombination ==
{{further|Genetic recombination}}
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<div style="border: none; width:250px;"><div class="thumbcaption">Structure of the [[Holliday junction]] intermediate in [[genetic recombination]]. The four separate DNA strands are coloured red, blue, green and yellow.<ref>{{Cite web| vauthors = Thorpe JH, Gale BC, Teixeira SC, Cardin CJ |title=RCSB PDB – 1M6G: Structural Characterisation of the Holliday Junction TCGGTACCGA|url=https://www.rcsb.org/structure/1M6G|access-date=2023-03-27|website=www.rcsb.org|language=en-US}}</ref></div></div></div>
[[File:Homologous Recombination.jpg|thumb|300px| A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.]]

A DNA helix usually does not interact with other segments of DNA, and in human cells, the different chromosomes even occupy separate areas in the nucleus called "[[chromosome territories]]".<ref>{{cite journal | vauthors = Cremer T, Cremer C | title = Chromosome territories, nuclear architecture and gene regulation in mammalian cells | journal = Nature Reviews Genetics | volume = 2 | issue = 4 | pages = 292–301 | date = April 2001 | pmid = 11283701 | doi = 10.1038/35066075 | s2cid = 8547149 }}</ref> This physical separation of different chromosomes is important for the ability of DNA to function as a stable repository for information, as one of the few times chromosomes interact is in [[chromosomal crossover]] which occurs during [[sexual reproduction]], when [[genetic recombination]] occurs. Chromosomal crossover is when two DNA helices break, swap a section and then rejoin.

Recombination allows chromosomes to exchange genetic information and produces new combinations of genes, which increases the efficiency of [[natural selection]] and can be important in the rapid evolution of new proteins.<ref>{{cite journal | vauthors = Pál C, Papp B, Lercher MJ | title = An integrated view of protein evolution | journal = Nature Reviews Genetics | volume = 7 | issue = 5 | pages = 337–48 | date = May 2006 | pmid = 16619049 | doi = 10.1038/nrg1838 | s2cid = 23225873 }}</ref> Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks.<ref>{{cite journal | vauthors = O'Driscoll M, Jeggo PA | title = The role of double-strand break repair – insights from human genetics | journal = Nature Reviews Genetics | volume = 7 | issue = 1 | pages = 45–54 | date = January 2006 | pmid = 16369571 | doi = 10.1038/nrg1746 | s2cid = 7779574 }}</ref>

The most common form of chromosomal crossover is [[homologous recombination]], where the two chromosomes involved share very similar sequences. [[Non-homologous recombination]] can be damaging to cells, as it can produce [[chromosomal translocation]]s and genetic abnormalities. The recombination reaction is catalyzed by enzymes known as [[recombinase]]s, such as [[RAD51]].<ref>{{cite journal | vauthors = Vispé S, Defais M | title = Mammalian Rad51 protein: a RecA homologue with pleiotropic functions | journal = Biochimie | volume = 79 | issue = 9–10 | pages = 587–92 | date = October 1997 | pmid = 9466696 | doi = 10.1016/S0300-9084(97)82007-X }}</ref> The first step in recombination is a double-stranded break caused by either an [[endonuclease]] or damage to the DNA.<ref>{{cite journal | vauthors = Neale MJ, Keeney S | title = Clarifying the mechanics of DNA strand exchange in meiotic recombination | journal = Nature | volume = 442 | issue = 7099 | pages = 153–58 | date = July 2006 | pmid = 16838012 | doi = 10.1038/nature04885 | bibcode = 2006Natur.442..153N | pmc = 5607947 }}</ref> A series of steps catalyzed in part by the recombinase then leads to joining of the two helices by at least one [[Holliday junction]], in which a segment of a single strand in each helix is annealed to the complementary strand in the other helix. The Holliday junction is a tetrahedral junction structure that can be moved along the pair of chromosomes, swapping one strand for another. The recombination reaction is then halted by cleavage of the junction and re-ligation of the released DNA.<ref>{{cite journal | vauthors = Dickman MJ, Ingleston SM, Sedelnikova SE, Rafferty JB, Lloyd RG, Grasby JA, Hornby DP | s2cid = 39505263 | title = The RuvABC resolvasome | journal = European Journal of Biochemistry | volume = 269 | issue = 22 | pages = 5492–501 | date = November 2002 | pmid = 12423347 | doi = 10.1046/j.1432-1033.2002.03250.x | doi-access = free }}</ref> Only strands of like polarity exchange DNA during recombination. There are two types of cleavage: east-west cleavage and north–south cleavage. The north–south cleavage nicks both strands of DNA, while the east–west cleavage has one strand of DNA intact. The formation of a Holliday junction during recombination makes it possible for genetic diversity, genes to exchange on chromosomes, and expression of wild-type viral genomes.