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Homologous chromosomes are represented in red and blue, and the two sister chromatids of each homolog are represented as pairs of double stranded DNA molecules indicated by two parallel lines in close proximity. Proteins required for specific steps of meiotic recombination are indicated on the left and right hand side of the diagram. Note that the existence of some of the recombination intermediates represented in this diagram has not been directly demonstrated in C. elegans, but are inferred from studies in yeast. The molecular events of recombination are represented in temporal progression starting at the top of the diagram with the formation of a DSB by SPO-11 in a single chromatid of one of the homologs. Resection of DNA ends and RAD-51 loading promote the invasion of a chromatid from the homologous chromosome, the formation of a D loop, and the start of DNA synthesis. These intermediates can be destabilized by the activity of RTEL-1, which leads to repair as NCO products, or they can be stabilized by CO promoting factors that promote the formation of double Holliday junctions. Note that although MSH-4, MSH-5, ZHP-3, and COSA-1 are all required for CO formation and eventually become associated with CO-fated recombination events, there are clear differences in the timing of loading of these proteins that are not depicted on this model (see Section 4.2.3 for a detailed description). The asymmetric cleavage of double Holliday junctions by different endonucleases promotes the formation of inter-homolog CO events.