Editing Genetic recombination (section)

==Meiotic recombination==
A molecular model for the mechanism of meiotic recombination presented by Anderson and Sekelsky<ref>{{cite journal | vauthors = Andersen SL, Sekelsky J | title = Meiotic versus mitotic recombination: two different routes for double-strand break repair: the different functions of meiotic versus mitotic DSB repair are reflected in different pathway usage and different outcomes | journal = BioEssays | volume = 32 | issue = 12 | pages = 1058–66 | date = December 2010 | pmid = 20967781 | pmc = 3090628 | doi = 10.1002/bies.201000087 }}</ref> is outlined in the first figure in this article. Two of the four chromatids present early in meiosis (prophase I) are paired with each other and able to interact. Recombination, in this model, is initiated by a double-strand break (or gap) shown in the DNA molecule (chromatid) at the top of the figure. Other types of DNA damage may also initiate recombination. For instance, an inter-strand cross-link (caused by exposure to a cross-linking agent such as mitomycin C) can be repaired by HRR.

Two types of recombinant product are produced. Indicated on the right side is a "crossover" (CO) type, where the flanking regions of the chromosomes are exchanged, and on the left side, a "non-crossover" (NCO) type where the flanking regions are not exchanged. The CO type of recombination involves the intermediate formation of two "Holliday junctions" indicated in the lower right of the figure by two X-shaped structures in each of which there is an exchange of single strands between the two participating chromatids. This pathway is labeled in the figure as the DHJ (double-Holliday junction) pathway.

The NCO recombinants (illustrated on the left in the figure) are produced by a process referred to as "synthesis dependent strand annealing" (SDSA). Recombination events of the NCO/SDSA type appear to be more common than the CO/DHJ type.<ref>{{Cite journal|pmc=1657055|year=2006|last1=Mehrotra|first1=S.|last2=McKim|first2=K. S.|title=Temporal Analysis of Meiotic DNA Double-Strand Break Formation and Repair in Drosophila Females|journal=PLOS Genetics|volume=2|issue=11|pages=e200|doi=10.1371/journal.pgen.0020200|pmid=17166055 |doi-access=free }}</ref> The NCO/SDSA pathway contributes little to genetic variation, since the arms of the chromosomes flanking the recombination event remain in the parental configuration. Thus, explanations for the adaptive function of meiosis that focus exclusively on crossing-over are inadequate to explain the majority of recombination events.