Recombinational DNA repair at a collapsed replication fork. (a) When a replication fork encounters a template strand break, one arm of the fork is lost and the fork collapses. The 5′ end at the break is processed to create a 3′ single-stranded extension, which is used in a strand invasion reaction. Migration of the branch can create a Holliday intermediate. Resolution of the Holliday intermediate, followed by ligation, restores a viable replication fork. The replisome is reloaded onto this restored fork (not shown), and replication continues. (b) Details of branch migration. In this process, the branch moves, but the net amount of duplex DNA does not change. Base pairs separated on one side of the branch are replaced on the other side.