| Eukaryotic cells irradiated with high doses of UV light exhibit cell cycle delays referred to as G1/S, intra-S, and G2/M checkpoints. Although the physiological role of these responses is unclear, the checkpoint genes that mediate them are important for maintaining viability in the presence of DNA damage and are thought to prevent cancer in humans. To study the cell cycle response to UV-irradiation, a computer-aided timelapse microscopy method was developed to facilitate the tracking of yeast cells through multiple division cycles. Surprisingly, none of the canonical checkpoint responses were found to occur after a low dose of UV that approximates sunlight exposure. This observation calls into question the function of the checkpoint genes required for those responses. Although there is not a G1/S, intra-S, or G2/M checkpoint response after a physiological UV dose, a previously uncharacterized cell cycle response was detected during the second cycle after irradiation. Yeast cells at all stages of the cell cycle exhibit a single UV checkpoint response. Cells carry lesions into S-phase and delay cell cycle progression for hours after the completion of bulk DNA synthesis. The timing of the response suggests that it may provide time for post-replication repair rather than nucleotide excision repair (NER). Post-replication delay requires the initiation of DNA replication and the checkpoint kinase Chk1, indicating that it is a physiological response of the cell to the replication of damaged DNA. UV-irradiated chk1 null cells lose viability and exhibit nuclear abnormalities only after cells have passed through S-phase, therefore cell cycle progression in the presence of UV lesions is not dangerous to the cell until DNA replication has occurred. In contrast to current models of UV lesion recognition, these results indicate that the DNA damage checkpoint monitors the presence of post-replication gaps rather than stalled replication forks, NER intermediates, or DNA photoproducts. |
