What mechanism ensures that DNA replication occurs only once per cell cycle?

Cdc6 plays an essential role in DNA replication by aiding in the loading of the MCM helicase onto DNA, which is a critical step in the formation of the prereplicative complex that initiates DNA replication. During the cell cycle, particularly in the transition from G1 to S phase, Cdc6 is tightly regulated to ensure that DNA replication occurs only once.

The phosphorylation and subsequent inactivation of Cdc6 act as a crucial checkpoint mechanism. As cells enter the S phase, Cdc6 is phosphorylated by cyclin-dependent kinases (CDKs), leading to its inactivation. This inactivation prevents Cdc6 from recycling and participating in forming new prereplicative complexes on the DNA, thus ensuring that replication does not initiate again until the next cell cycle is underway. By regulating Cdc6 in this manner, the cell maintains strict control over the replication process, preventing over-replication or potential genomic instability that could arise from multiple rounds of DNA synthesis occurring within a single cell cycle.

The other options reflect mechanisms that are either less directly related to this specific regulatory process or describe functions that do not effectively prevent multiple rounds of replication per cell cycle. Therefore, the phosphorylation and inactivation of Cdc6 stands out as