sentences:  At the onset of S phase, the pre-replicative complex must be activated by two S phase-specific kinases in order to form an initiation complex at an origin of replication.One kinase is the Cdc7-Dbf4 kinase called Dbf4-dependent kinase (DDK) and the other is cyclin-dependent kinase (CDK).Chromatin-binding assays of Cdc45 in yeast and "Xenopus" have shown that a downstream event of CDK action is loading of Cdc45 onto chromatin.