The signal to commence DNA replication comes from which of the following?

The signal to commence DNA replication is indeed associated with S-Cdk (S phase cyclin-dependent kinase). During the cell cycle, specifically at the G1 to S phase transition, the activity of S-Cdk is crucial for triggering the initiation of DNA replication.

S-Cdk phosphorylates several target proteins that are critical for the assembly and activation of the DNA replication machinery. This includes the phosphorylation of components of the pre-replication complex, allowing for the recruitment of DNA polymerases and other necessary proteins to the origins of replication.

Moreover, S-Cdk also plays a role in inhibiting cyclin-dependent kinase inhibitors, which helps to ensure that the cell progresses through the cycle towards DNA replication efficiently. This coordinated activity allows for the precise regulation of DNA synthesis, ensuring that replication occurs only once per cell cycle and at the correct stage when all the necessary proteins are prepared.

In contrast, while ORC (origin recognition complex) is involved in the formation of the pre-replication complex, it does not signal the commencement of replication by itself; it is the downstream action of S-Cdk that ultimately facilitates this process. The destruction of M-Cdk and Cdc6 relates to other regulatory mechanisms in cell cycle control and timing but do not