Final answer:
Mitotic Cdk is activated by binding to cyclin and being phosphorylated, ensuring progression at the correct cell cycle phase. Deactivation promptly occurs by cyclin degradation and inhibition from Cdk inhibitors to prevent inappropriate cell cycle advancement.
Step-by-step explanation:
Cyclin-dependent kinases (Cdks) are essential for the regulation of the cell cycle. Activation of mitotic Cdk involves a two-step process. Firstly, the Cdk must bind with a cyclin protein to form a Cdk/cyclin complex. This binding is a critical regulatory step as the levels of cyclin fluctuate throughout the cell cycle, governing when the complex is formed. The increase in cyclin concentration prompts binding with Cdk at precise points in the cell cycle to pass specific checkpoints. Secondly, for full activation, the Cdk/cyclin complex must be phosphorylated by another kinase at particular sites on the molecule, which activates the complex to phosphorylate other proteins that push the cell cycle forward.
For correct timing of Cdk activation, the cell uses cyclins, as their concentrations vary during the cell cycle stages, ensuring the Cdk/cyclin complexes form only at the appropriate times. This regulation is essential to maintain the proper sequence of cell cycle events.
Deactivation is just as critical for cell cycle regulation. It occurs quickly through the degradation of cyclins and the action of Cdk inhibitors. Cyclin levels drop as they are targeted for degradation, which disassembles the active complex. Additionally, Cdk inhibitors bind to the Cdk, preventing its activation and ensuring that the cell cycle does not progress under unfavorable conditions.