Recombination, Models of

Homologous recombination (HR) is a result of exchange between at least two DNAs in the region of aligned identity, detected either genetically, as formation of new combinations of markers if the exchange was between homologous chromosomes, or physically, if the exchange was between repeats within heterologous chromosomes or within a single chromosome. All models of formation of recombinant DNAs via homologous strand exchange share the three common stages: (1) formation of single-stranded DNA (ssDNA) (in either one or both participating DNAs); (2) formation of recombination intermediate driven by DNA strand complementarity; and (3) resolution of recombination intermediate to yield recombination products. There are two major types of recombinases, one catalyzing only annealing between complementary strands and the other catalyzing invasion of an ssDNA into a duplex DNA. There are three types of the elemental models of HR, differing in the extent of single strandedness of the initiating DNA, with at least two varieties of each (the annealing type vs. the invasion type). HR is detected in genetic systems with any level of complexity, from bacteriophages to eukaryotic meiosis, with outcomes being consistent with the products either of particular elemental models or of more complex models, like double-strand break repair.