Recombinational repair of spontaneous double-strand breaks (DSBs) exhibits sister bias. DSB-initiated meiotic recombination exhibits homolog bias. Physical analysis in yeast reveals that, in both cases, the recombination reaction intrinsically gives homolog bias. From this baseline default, cohesin intervenes to confer sister bias, likely independent of cohesion. In meiosis, cohesin's sister-biasing effect is counteracted by RecA homolog Rad51 and its mediators, plus meiotic RecA homolog Dmc1, which thereby restore intrinsic homolog bias. Meiotic axis complex Red1/Mek1/Hop1 participates by cleanly switching recombination from mitotic to meiotic mode, concomitantly activating Dmc1. We propose that a Rad51/DNA filament at one DSB end captures the intact sister, creating an anchor pad. This filament extends across the DSB site on the intact partner, precluding intersister strand exchange, thus forcing use of the homolog. Cohesin and Dmc1 interactively modulate this extension, with program-appropriate effects. In accord with this model, Rad51-mediated recombination in vivo requires the presence of a sister.
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