Final answer:
Regulatory proteins play a crucial role in controlling the cell cycle and ensuring that the cell proceeds only when ready.
Step-by-step explanation:
Tight control of the activity of regulatory proteins ensures that the cell proceeds through the cell cycle only when it is ready. These regulatory proteins control the cycle by signaling the cell to either start or delay the next phase of the cycle. They ensure that the cell completes the previous phase before moving on. There are specific checkpoints in the cell cycle, such as the G₁ checkpoint, the G₂/M transition, and metaphase, where these regulatory proteins monitor cellular conditions and can halt the cycle until specific requirements are met.