Final answer:
True, Cdks must be bound to cyclins to regulate the cell cycle, and the Cdk/cyclin complex must be phosphorylated to be fully active. The combination of Cdk stability and cyclin fluctuation is crucial for cell cycle progression.
Step-by-step explanation:
The student's question is whether cyclin-dependent kinases (Cdks) must be bound to cyclins for activity. The answer to this question is true. Cdks require binding to cyclins for their regulatory activity in the cell cycle. Moreover, the Cdk/cyclin complex must also be phosphorylated at specific sites to be fully active. This phosphorylation alters the shape of the protein it targets, resulting in activation. The levels of Cdk proteins remain relatively stable, whereas cyclin concentrations fluctuate, determining the timing of the formation of the Cdk/cyclin complexes. These complexes are essential for advancing the cell through various checkpoints of the cell cycle.