Fig 6. Model of D-loop progression.

Single-stranded 3´ overhangs at DSB ends become loaded with Dmc1 (green). One end can invade a DNA molecule to probe for homology and form a Dmc1‒heteroduplex strand. Early joint molecules (JMs) will mainly be formed between sister DNA molecules and are unstable. They can reject the invading strand with the help of Sgs1 and then re-use it in subsequent rounds of homology testing. In diplonema, the heteroduplex is stripped of Dmc1, allowing DNA synthesis (dotted line) to begin at the OH-end of the invading strand. This will extend the D-loop. The extended strand can then be displaced, pair with the other end of the DSB, fill the gaps (via synthesis-dependent strand annealing ‒ SDSA), and produce a noncrossover (NCO). Alternatively, the extended D-loop may capture the second DSB end and form a double Holliday junction (dHJ). This intermediate may either become a NCO by dissolution or mature into a crossover (CO). In the absence of Sgs1, early JMs will persist into diplonema and be then transformed into (mainly intersister) COs. In the absence of Mcmd1‒Pamd1, early JMs will be immediately converted to extended D-loops and also result primarily in intersister exchange.