Figure 35.15 Repair of double-strand break by using recombination. 1. A 5′ exonuclease generates single-strand DNA at the site of the break. 2. Strand invasion takes place when the strand on the damaged DNA base-pairs with the complementary strand on the undamaged DNA, forming a displacement loop, or D-loop. 3. DNA synthesis takes place. 4. A second strand invasion takes place, resulting in the formation of two Holliday junctions. 5. The Holliday junctions are cleaved and ligated, forming two intact molecules of DNA.