Fig. 1.

Schematic of S. cerevisiae mating type loci on chromosome III and the SDSA model of MAT switching. (A) Chromosome III contains three mating type genes, the expressed MAT locus and HML and HMR, which are heterochromatic and silent. MATa cells switch using HMLα, whereas MATα cells recombine with HMRa. This preference is controlled by the RE element. (B) Individual steps of the DSB repair mechanism are numbered on the left. 1: MAT switching is initiated by an HO-induced DSB at the Ya-Z1 junction within MATa. 2: 5′–3′ resection creates ssDNA with 3′ ends to which Rad51 (yellow circles) is recruited. 3: The Rad51 filament searches for homologous dsDNA. 4: Upon MAT-HML synapsis, new DNA synthesis is primed from the free 3′ end of the invading strand. 5: The nascent strand is dissociated from the donor and anneals with homologous sequences on the left side of the break. 6: The nonhomologous 3′ tail is clipped off, and new DNA synthesis begins at the free 3′ ends to fill in the single-stranded gaps. 7: DSB repair is completed by ligation of the filled-in ends. In the SDSA model, all newly synthesized DNA appears at the repaired locus, whereas the donor locus remains unmodified.