Final answer:
Cyclin-CDK complexes are modulated by fluctuating levels of cyclins and require phosphorylation to be fully active. Cyclins determine when these complexes form, enabling the progress through the cell cycle checkpoints. Cdk inhibitors also regulate these complexes by halting activation until specific cell cycle events are completed.
Step-by-step explanation:
Activities Modulated by Cyclin-CDK Complexes
The activities of cyclin-CDK complexes are crucial for the regulation of the cell cycle and are modulated by a few different mechanisms. The levels of cyclin proteins fluctuate throughout the cell cycle, dictating when Cdk/cyclin complexes form. For these complexes to be fully active, not only is a tight binding of Cdks and cyclins required, but the complex must also be phosphorylated at specific sites. This phosphorylation changes the shape of the protein, leading to its activation.
Cdks, being kinases, phosphorylate other proteins that are involved in progressing the cell to the next phase of the cycle. While concentrations of Cdk proteins remain relatively stable, the concentrations of cyclin determine the formation and activation of the Cdk/cyclin complexes at different checkpoints. Moreover, the cyclic fluctuations of cyclin levels are based on cell cycle timing, and these complexes are necessary for the cell cycle to move past its checkpoints.
Aside from cyclins themselves, Cdk inhibitors play a role in negative regulation by preventing the full activation of the Cdks until certain cell cycle events are completed. These inhibitors can halt the cell cycle progression, acting as a control mechanism to ensure proper cell cycle events.