Cell-Cycle-Specific Repair of DNA Double-Strand Breaks in Saccharomyces cerevisiae

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Abstract
The response to ionizing radiation in yeast is cell cycle-dependent. These observations can be interpreted in terms of variations in the ability to repair DNA double-strand breaks (DSB). DSB repair was examined directly in synchronous yeast cultures irradiated in either the G2 or the G1 phase. DSB repair in G2 cells was rapid and appeared to be complete in less than 2 h. Based on the absence of DNA synthesis during repair, it was concluded that DSB repair does not require a normal round of S phase DNA replication. In G1 cells no DSB repair was observed and the nuclear DNA was degraded during incubation after irradiation. Following irradiation, the G1 cells produced buds with little delay and accumulated at the next mitosis. However, the extent of S phase DNA synthesis normally associated with budding was substantially decreased. Cell cycle-dependent changes in survival following ionizing radiation reflect differential capacity for DSB repair. The repair observed in G2 may involve recombination between sister chromatids.