Final answer:
S phase initiation is triggered by the accumulation of proteins necessary for DNA replication, which must reach appropriate levels or become active. The G1 checkpoint controls this process.
Step-by-step explanation:
The initiation of S phase is triggered by the accumulation of certain proteins that are necessary for DNA replication, such as DNA polymerases, helicases, and primases. These proteins need to reach appropriate levels or become active before cells can enter S phase. This is controlled by the G1 checkpoint, which ensures that the cell is ready to begin replication before moving on to S phase.