Abstract

An additional cell cycle checkpoint Cell division is controlled by checkpoints that regulate the temporal order of the cell cycle phases, including the G 1 /S, G 2 /M, and metaphase/anaphase transitions. Yet there are no known control mechanisms for a fourth fundamental transition—the S/G 2 transition. Saldivar et al. report a switchlike control mechanism that regulates the S/G 2 transition. The checkpoint kinase ATR senses ongoing DNA replication in S phase and represses the mitotic transcriptional network, ensuring that DNA replication in S phase is completed before mitosis. Science , this issue p. 806