FIG. 1.

Repair of a DSB in a diploid. HO endonuclease cleavage at MATa can be repaired by gene conversion (A) using the uncleavable MATα-inc allele on the homologous chromosome as the donor sequence. Gene conversion can also occur, rarely, using the interchromosomal HMLα and HMRa loci as donors (38, 40), but these would be cleaved again if HO is continually expressed. If gene conversion occurs in G₂ cells, crossing over may generate sectored colonies homozygous for THR4 and for thr4 (B). Colonies that are phenotypically identical to but genotypically different from those shown in panel B can occur if there are two independent DNA repair events, one of which is a gene conversion and the other of which is BIR (C). BIR yields Ade1⁺ Thr4⁻ cells (D). Failure to repair the DSB leads to chromosome loss and the formation of Ade1⁺ Thr4⁻ cells (E).