Editing Chromosomal crossover (section)

==Origins==
There are two popular and overlapping theories that explain the origins of crossing-over, coming from the different theories on the origin of [[meiosis]]. The first theory rests upon the idea that meiosis evolved as another method of [[DNA repair]], and thus crossing-over is a novel way to replace possibly damaged sections of DNA.<ref name="ReferenceA"/> The second theory comes from the idea that meiosis evolved from [[bacterial transformation]], with the function of propagating diversity.<ref name="ReferenceA">{{cite journal |vauthors=Bernstein H, Bernstein C |title=Evolutionary origin of recombination during meiosis |journal=BioScience |date=2010 |volume=60 |issue=7 |pages=498–505 |doi=10.1525/bio.2010.60.7.5 |s2cid=86663600}}<!--|access-date=10 March 2015--></ref>

In 1931, Barbara McClintock discovered a triploid maize plant. She made key findings regarding corn's karyotype, including the size and shape of the chromosomes. McClintock used the prophase and metaphase stages of mitosis to describe the morphology of corn's chromosomes, and later showed the first ever cytological demonstration of crossing over in meiosis. Working with student Harriet Creighton, McClintock also made significant contributions to the early understanding of codependency of linked genes.{{cn|date=December 2024}}

===DNA repair theory===
Crossing over and DNA repair are very similar processes, which utilize many of the same protein complexes.<ref>{{cite journal | vauthors = Dangel NJ, Knoll A, Puchta H | title = MHF1 plays Fanconi anaemia complementation group M protein (FANCM)-dependent and FANCM-independent roles in DNA repair and homologous recombination in plants | journal = The Plant Journal | volume = 78 | issue = 5 | pages = 822–833 | date = June 2014 | pmid = 24635147 | doi = 10.1111/tpj.12507 | doi-access = free | bibcode = 2014PlJ....78..822D }}</ref><ref>{{cite journal | vauthors = Saponaro M, Callahan D, Zheng X, Krejci L, Haber JE, Klein HL, Liberi G | title = Cdk1 targets Srs2 to complete synthesis-dependent strand annealing and to promote recombinational repair | journal = PLOS Genetics | volume = 6 | issue = 2 | pages = e1000858 | date = February 2010 | pmid = 20195513 | pmc = 2829061 | doi = 10.1371/journal.pgen.1000858 | doi-access = free }}</ref>
In her report, "The Significance of Responses of the Genome to Challenge", McClintock studied corn to show how corn's genome would change itself to overcome threats to its survival. She used 450 self-pollinated plants that received from each parent a chromosome with a ruptured end. She used modified patterns of gene expression on different sectors of leaves of her corn plants to show that transposable elements ("controlling elements") hide in the genome, and their mobility allows them to alter the action of genes at different loci. These elements can also restructure the genome, anywhere from a few nucleotides to whole segments of chromosome.
Recombinases and primases lay a foundation of nucleotides along the DNA sequence. One such particular protein complex that is conserved between processes is [[RAD51]], a well conserved recombinase protein that has been shown to be crucial in DNA repair as well as cross over.<ref>{{cite journal | vauthors = Esposito MS | title = Evidence that spontaneous mitotic recombination occurs at the two-strand stage | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 75 | issue = 9 | pages = 4436–4440 | date = September 1978 | pmid = 360220 | pmc = 336130 | doi = 10.1073/pnas.75.9.4436 | doi-access = free | bibcode = 1978PNAS...75.4436E }}<!--|access-date=20 March 2015--></ref> 

Several other genes in ''D. melanogaster'' have been linked as well to both processes, by showing that mutants at these specific loci cannot undergo DNA repair or crossing over. Such genes include mei-41, mei-9, hdm, {{not a typo|spnA}}, and brca2.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}  This large group of conserved genes between processes supports the theory of a close evolutionary relationship.
Furthermore, DNA repair and crossover have been found to favor similar regions on chromosomes. In an experiment using [[radiation hybrid mapping]] on wheat's (''Triticum aestivum L.'') 3B chromosome, crossing over and DNA repair were found to occur predominantly in the same regions.<ref>{{cite journal | vauthors = Kumar A, Bassi FM, Paux E, Al-Azzam O, de Jimenez MM, Denton AM, Gu YQ, Huttner E, Kilian A, Kumar S, Goyal A, Iqbal MJ, Tiwari VK, Dogramaci M, Balyan HS, Dhaliwal HS, Gupta PK, Randhawa GS, Feuillet C, Pawlowski WP, Kianian SF | display-authors = 6 | title = DNA repair and crossing over favor similar chromosome regions as discovered in radiation hybrid of Triticum | journal = BMC Genomics | volume = 13 | issue = 339 | pages = 339 | date = July 2012 | pmid = 22827734 | pmc = 3443642 | doi = 10.1186/1471-2164-13-339 | doi-access = free }}</ref> Furthermore, crossing over has been correlated to occur in response to stressful, and likely DNA damaging, conditions.<ref>{{cite journal | vauthors = Steinboeck F, Hubmann M, Bogusch A, Dorninger P, Lengheimer T, Heidenreich E | title = The relevance of oxidative stress and cytotoxic DNA lesions for spontaneous mutagenesis in non-replicating yeast cells | journal = Mutation Research | volume = 688 | issue = 1–2 | pages = 47–52 | date = June 2010 | pmid = 20223252 | doi = 10.1016/j.mrfmmm.2010.03.006 | bibcode = 2010MRFMM.688...47S }}<!--|access-date=14 March 2015--></ref><ref>{{cite journal | vauthors = Nedelcu AM, Marcu O, Michod RE | title = Sex as a response to oxidative stress: a twofold increase in cellular reactive oxygen species activates sex genes | journal = Proceedings. Biological Sciences | volume = 271 | issue = 1548 | pages = 1591–1596 | date = August 2004 | pmid = 15306305 | pmc = 1691771 | doi = 10.1098/rspb.2004.2747 }}</ref>

===Links to bacterial transformation===
The process of bacterial transformation also shares many similarities with chromosomal cross over, particularly in the formation of overhangs on the sides of the broken DNA strand, allowing for the annealing of a new strand. Bacterial transformation itself has been linked to DNA repair many times.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}  The second theory comes from the idea that meiosis evolved from [[bacterial transformation]], with the function of propagating genetic diversity.<ref name="ReferenceA"/><ref>{{cite journal | vauthors = Charpentier X, Kay E, Schneider D, Shuman HA | title = Antibiotics and UV radiation induce competence for natural transformation in Legionella pneumophila | journal = Journal of Bacteriology | volume = 193 | issue = 5 | pages = 1114–1121 | date = March 2011 | pmid = 21169481 | pmc = 3067580 | doi = 10.1128/JB.01146-10 }}</ref>  Thus, this evidence suggests that it is a question of whether cross over is linked to DNA repair or bacterial transformation, as the two do not appear to be mutually exclusive. It is likely that crossing over may have evolved from bacterial transformation, which in turn developed from DNA repair, thus explaining the links between all three processes. {{cn|date=December 2024}}