Final answer:
Cyclins and cyclin-dependent kinases (Cdks) are internal signal molecules that control the cell cycle by regulating transitions through checkpoints.
Step-by-step explanation:
In the cell cycle, internal signal molecules called cyclins and cyclin-dependent kinases (Cdks) play a crucial role in controlling cell transitions through various checkpoints. These control molecules determine whether or not the cell is ready to move into the next stage of the cell cycle. Cyclins and Cdks act as positive regulators, allowing the cell to advance to the next stage, while negative regulator molecules monitor cellular conditions and can halt the cycle until specific requirements are met.