Prevention of Pre-replication Complex Re-assembly
During each cell cycle, it is important that the genome be completely replicated once and only once. Formation of the pre-replication complex during late M and early G1 phase is required for genome replication, but after the genome has been replicated the pre-RC must not form again until the next cell cycle. In S. cerevisiae, CDKs prevent formation of the replication complex during late G1, S, and G2 phases by excluding MCM2-7 and Cdt1 from the nucleus, targeting Cdc6 for degradation by the proteasome, and dissociating ORC1-6 from chromatin via phosphorylation. Prevention of re-replication in S. pombe is slightly different; Cdt1 is degraded by the proteasome instead of merely being excluded from the nucleus. Proteolytic regulation of Cdt1 is shared by higher eukaryotes including Caenorhabditis elegans, Drosophila melanogaster, X. laevis, and mammals. Metazoans have a fourth mechanism to prevent re-replication; during S and G2 geminin binds to Cdt1 and inhibits Cdt1 from loading MCM2-7 onto the origin of replication.
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