Coupling of mitosis to the completion of S phase through Cdc34-mediated degradation of Wee 1

The dependence of mitosis on the completion of the period of DNA replicatio n in the cell cycle [synthesis (S) phase] ensures that chromosome segregati on occurs only after the genome has been fully duplicated. A key negative r egulator of mitosis, the protein kinase Wee1, was degraded in a Cdc34-depen dent fashion in Xenopus egg extra cts. Th is proteolysis event was required for a timely entrance into mitosis and was inhibited when DNA replication was blocked. Therefore, the DNA replication checkpoint can prevent mitosis by suppressing the proteolysis of Wee1 during 5 phase.