<P> During the G stage of the cell cycle, the replication initiation factors, origin recognition complex (ORC), Cdc6, Cdt1, and minichromosome maintenance (Mcm) protein complex, bind sequentially to DNA to form the pre-replication complex (pre-RC). At the transition of the G stage to the S phase of the cell cycle, S phase--specific cyclin - dependent protein kinase (CDK) and Cdc7 / Dbf4 kinase (DDK) transform the pre-RC into an active replication fork . During this transformation, the pre-RC is disassembled with the loss of Cdc6, creating the initiation complex . In addition to the binding of the Mcm proteins, cell division cycle 45 (Cdc45) protein is also essential for initiating DNA replication . Studies have shown that Mcm is critical for the loading of Cdc45 onto chromatin and this complex containing both Mcm and Cdc45 is formed at the onset of the S phase of the cell cycle . Cdc45 targets the Mcm protein complex, which has been loaded onto the chromatin, as a component of the pre-RC at the origin of replication during the G stage of the cell cycle . </P> <P> Cell division cycle 45 (Cdc45) protein is a critical component for the conversion of the pre-replicative complex to the initiation complex . The Cdc45 protein assembles at replication origins before initiation and is required for replication to begin in Saccharomyces cerevisiae, and has an essential role during elongation . Thus, Cdc45 has central roles in both initiation and elongation phases of chromosomal DNA replication . </P> <P> Cdc45 associates with chromatin after the beginning of initiation in late G stage and during the S phase of the cell cycle . Cdc45 physically associates with Mcm5 and displays genetic interactions with five of the six members of the Mcm gene family and the ORC2 gene . The loading of Cdc45 onto chromatin is critical for loading other various replication proteins, including DNA polymerase α, DNA polymerase ε, replication protein A (RPA) and proliferating cell nuclear antigen (PCNA) onto chromatin . Within a Xenopus nucleus - free system, it has been demonstrated that Cdc45 is required for the unwinding of plasmid DNA . The Xenopus nucleus - free system also demonstrates that DNA unwinding and tight RPA binding to chromatin occurs only in the presence of Cdc45 . </P> <P> Binding of Cdc45 to chromatin depends on Clb - Cdc28 kinase activity as well as functional Cdc6 and Mcm2, which suggests that Cdc45 associates with the pre-RC after activation of S - phase cyclin - dependent kinases (CDKs). As indicated by the timing and the CDK dependence, binding of Cdc45 to chromatin is crucial for commitment to initiation of DNA replication . During S phase, Cdc45 physically interacts with Mcm proteins on chromatin; however, dissociation of Cdc45 from chromatin is slower than that of Mcm's, which indicates that the proteins are released by different mechanisms . </P>

Where does dna replication occur in the eukaryotic cells