Final answer:
The cell cycle is timed by rhythmic fluctuations in the activity of cyclin-dependent kinases (Cdks). Cdks phosphorylate and activate proteins that advance the cell to the next phase. Cyclins bind to Cdks and promote their activity.
Step-by-step explanation:
The cell cycle is timed by rhythmic fluctuations in the activity of cyclin-dependent kinases (Cdks). Cdks are enzymes that phosphorylate other proteins, activating them and advancing the cell to the next phase of the cell cycle. Cyclins, on the other hand, regulate the cell cycle by tightly binding to Cdks and promoting their activity.
The concentration of cyclin proteins fluctuates throughout the cell cycle, with different cyclins being active at specific points in the cycle. When the concentration of a specific cyclin reaches a certain threshold, it binds to its corresponding Cdk, forming a complex. This complex is then phosphorylated, fully activating it and allowing it to phosphorylate other proteins, advancing the cell cycle.
Overall, the rhythmic fluctuations in cyclin levels and the activity of Cdks provide a precise timing mechanism for the progression of the cell cycle, ensuring that each step is properly regulated.