UV irradiation induces a postreplication DNA damage checkpoint

Eukaryotic cells irradiated with high doses of UV exhibit cell-cycle responses referred to as G₁/S, intraS, and G₂/M checkpoints. After a moderate UV dose that approximates sunlight exposure and is lethal to fission yeast checkpoint mutants, we found unexpectedly that these cell-cycle responses do not occur. Instead, cells at all stages of the cell cycle carry lesions into S phase and delay cell-cycle progression for hours after the completion of bulk DNA synthesis. Both DNA replication and the checkpoint kinase, Chk1, are required to generate this cell-cycle response. UV-irradiation of Δchk1 cells causes chromosome damage and loss of viability only after cells have replicated irradiated DNA and entered mitosis. These data suggest that an important physiological role of the cell-cycle response to UV is to provide time for postreplication repair.