S-Cdk Initiates DNA Replication

S-Cdks are enzyme complexes activated at the onset of S-phase that trigger origin firing by phosphorylating specific initiator proteins. These phosphorylated proteins promote recruitment of helicase activator complexes, which activate DNA helicases and recruit DNA polymerase, leading to replication initiation at multiple chromosomal origins.

During early G1-phase, regulatory proteins Cdc6 and Cdt1 bind to origin recognition complexes on DNA. These proteins aid assembly of MCM proteins into inactive ring complexes on adjacent DNA, forming large multiprotein structures called pre-replicative complexes that prepare origins for replication.

S-Cdks prevent re-replication by phosphorylating Cdc6 and Cdt1 proteins, promoting their release from origin recognition complexes and triggering their degradation. This disassembles pre-replicative complexes, converting origins to a post-replicative state where high Cdk activity persists throughout S-phase and mitosis, ensuring DNA replicates only once.

Low Cdk activity at the end of M-phase permits pre-replicative complex assembly, creating a replication-competent state. During the G1-S transition, increased Cdk activity triggers replication initiation and disassembles pre-replicative complexes. Persistent high Cdk activity throughout S-phase prevents re-assembly until mitosis ends and activity reduces.

MCM proteins function as DNA helicases during replication. After replication completes and helicases disengage from the DNA strand, S-Cdks phosphorylate these helicases, triggering their export from the nucleus and preventing them from re-initiating replication at the same origin during the cell cycle.

Origins exist in two states to regulate replication timing and prevent re-replication. The pre-replicative complex state in G1-phase creates a replication-competent origin. The post-replicative complex state from S-phase through M-phase, maintained by high Cdk activity, prevents re-replication and ensures DNA duplicates only once per cell cycle.

Origin recognition complexes bind to DNA at multiple chromosomal locations and serve as docking sites for regulatory proteins like Cdc6 and Cdt1. These complexes anchor the assembly of pre-replicative complexes, which become competent for replication when S-Cdks are activated and phosphorylate initiator proteins during S-phase.