Biologjia Online

Cellular Checkpoint Mechanisms Monitoring Proper Initiation of DNA

In eukaryotic cells, initiation of DNA replication is achieved by an ordered assembly of protein complexes at origins of replication. This process consists of two steps, licensing of origins in late M or early G1 phase and firing of origins at different times during S phase. Origin licensing is mediated by the assembly of pre-replicative complexes (pre-RCs) on origins. At the center of the pre-RC is the origin recognition complex (ORC), a six-subunit complex (Orc1–Orc6), which binds origins of replication (2). During late M and early G1 phase, Cdt1 and Cdc6 bind origins in an ORC-dependent manner and then cooperatively recruit a putative helicase, Mcm2–7 complex, to origins (2) (Fig. 1). Once pre-RCs are assembled, origins are licensed for replication in the subsequent S phase and ready to fire. A key event for origin firing, the second step of the initiation process, is loading of Cdc45 on origins (Fig. 1). This step is triggered by the action of two kinases, cyclin-Cdk and Cdc7-Dbf4. Although cyclin-Cdk complex works as a global activator for S phase, Cdc7-Dbf4 acts as an activator in the initiation step at the individual origins. Following Cdc45 loading on origins, the single-stranded DNA (ssDNA)-binding protein, RPA, and the primase-DNA polymerase complex are loaded on chromatin to initiate DNA replication (2).

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