Homologs are shown in red and black, respectively; dashed lines indicate nascent DNA. Solid black arrows indicate pathways in wild-type cells. Dashed arrows indicate pathways in mutants. The crossover or non-crossover decision follows pairing and strand-invasion by one DSB-end to form a nascent D-loop (steps 1 and 2; ; ). Along the crossover pathway, ZMM proteins convert the nascent JM into a SEI, which is then stabilized by Msh4-Msh5 and Mlh1-Mlh3 antagonizing Sgs1 (steps 3B, 4B and 6–8A; also see ). Along the non-crossover pathway, the initial D-loop is not stabilized by ZMMs and ultimately disassembles even in the absence of Sgs1 (steps 3–8D). When homologs have successfully paired and synapsed, the sister-chromatid (or any second homologous template) may be invaded by the second DSB-end, e.g. steps 3D and 4B. Following extension by DNA synthesis, this end undergoes one of two annealing reactions with the first DSB-end. At a crossover-designated site, the second DSB-end anneals with the SEI to form a canonical dHJ, which is then resolved into a crossover (steps 6–8A). At a non-crossover site, the two DSB-ends anneal to seal the break (steps 6–8D). Along both pathways, the helicase activity of Sgs1 (± Top3 strand-passage activity) ensures that the second DSB-end completely dissociates from the template duplex. This could occur early, by disrupting the D-loop intermediate, or late by dissolving dHJs formed by the second DSB-end. In sgs1Δ-C795 cells, the second DSB-end does not efficiently disengage from its template and forms a stable dHJ independently of the first DSB-end, e.g. steps 5C, 6B, 6E. Along the crossover pathway, this will lead to formation of a ternary JM, which may be resolved into adjacent interhomolog (IH) and intersister (IS) crossovers (steps 6–8B). Successive invasion of two templates by a DSB-end will give a quaternary JM, whose resolution can produce a four-chromatid double crossover (steps 5–8C). Along the non-crossover pathway, stable dHJ formation by the second DSB-end will produce an IH-non-crossover associated with an intersister exchange (steps 6–8E). D-loop migration and “end-first” strand-displacement is proposed to be a common step that precedes strand-annealing to form mature JMs. This mechanism readily accommodates the formation of multi-chromatid JMs. Strand-displacement was previously proposed to explain the occurrence of DSB-distal JMs that lack intervening heteroduplex DNA ().