Migrating D-loop model of recombination. After DSB formation the ends are processed to reveal protruding 3′ single-stranded tails. One tail invades a homologous duplex to form a D-loop that migrates concomitant with DNA synthesis. With collapse of the D-loop as shown on the left side, the newly synthesized strand may anneal with the complementary protruding single-stranded tail on the other side of the break. Further fill-in synthesis will result in a non-crossover product. If the D-loop is captured by the protruding strand from the other side of the DSB, as shown on the right side, it may be processed to form an intermediate with one or possibly two Holliday junctions. Resolution of the Holliday junction by nicking the top and bottom strands will yield crossover products, while nicking the two inner strands will yield non-crossover products. The U. maydis gene products that serve in recombination are shown on the right side of the diagram at the approximate step in the pathway where they are thought to function.