Figure 4

Cdk1 and regulation of DNA replication. During G1 phase of the cell cycle, when Cdk1 is inactive, cells assemble pre-RC complexes onto their origins of replication. When Cdk1 becomes active at the end of G1 phase it phosphorylates several components of the complex, and especially phosphorylation of Sld2 and Sld3 results in origin firing and initiation of DNA replication. After origin firing, several components dissociate and cannot re-assemble into replication-competent origins until they become dephosphorylated and Cdk1 becomes inactivated during G1, thus providing a mechanism for prevention of re-replication.