Eukaryotic cells irradiated with high doses of UV exhibit cell-cycle responses referred to as G 1͞S, intraS, and G2͞M 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 ⌬chk1 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.cell cycle ͉ chk1 ͉ nucleotide excision repair ͉ yeast U V radiation from sunlight exposure induces DNA damage that is potentially lethal to cells and is carcinogenic to animals (1). Studies in fission and budding yeasts have demonstrated that genes that promote survival in UV light encode both DNA repair factors (2, 3) and checkpoint proteins that regulate the cell cycle in response to DNA damage (4, 5). These genes are conserved among eukaryotes, so diverse model systems may be used to address questions of molecular mechanism (6).The pyrimidine dimer is the most abundant form of DNA damage known to be induced by UV (7). This lesion is removed from duplex DNA by nucleotide excision repair (NER). When present during S phase, pyrimidine dimers cause gaps in the daughter DNA strand (8-12). The repair of these gaps is referred to as postreplication repair and involves bypass polymerases, homologous recombination, and other processes (13).Several eukaryotic cell-cycle responses have been observed after high-dose UV irradiation. Fission yeast UV-irradiated in G 2 delay the onset of mitosis, a response conserved in human cells that has been termed a G 2 ͞M DNA damage checkpoint (5, 14-16). It is posited that the UV sensitivity of fission yeast G 2 ͞M checkpoint mutants results from a failure to make time for the repair of UV-induced DNA lesions before mitosis (5). Budding yeast exhibit a G 1 ͞S checkpoint response that delays S phase entry after UV irradiation in G 1 (17). Because activation of the G 1 ͞S checkpoint requires NER genes, it has been suggested that the checkpoint monitors the presence of NER intermediates (12,(18)(19)(20). Mammalian cells exhibit a p53-dependent G 1 ͞S DNA damage checkpoint response that can eliminate damaged cells through apoptosis, delay entry into S phase, and facilitate NER (21). Like the G 1 ͞S checkpoint of yeast, the induction of p53 in G 1 requires NER and is independent of DNA replication (22). A NER-independent checkpoint is activated when UV lesions are present during S phase in Xenopus egg extracts and budding yeast NER mutants (18,23). In both model systems, UV lesions cause an arrest in S phase; however, this arrest appears to be ch...