Describe the double-strand break model of homologous recombination.

Ans: One of the two homologs, which will be the recipient duplex, undergoes a double-strand break. Exonucleases form 3 single-stranded ends that invade the donor duplex and displace a strand from it, forming a D-loop. The D-loop is extended by DNA synthesis, and it will capture the second double-strand break end by pairing with it. Further DNA synthesis and ligation result in two Holliday junctions flanking a region of heteroduplex DNA. The Holliday junctions can be resolved by nicking two connecting strands in a way that results in either crossover products or noncrossover products.