Final answer:
The activity of cyclin-dependent kinases (CDks) rises and falls in the cell cycle due to the fluctuation in cyclin concentrations.
Step-by-step explanation:
The activity of cyclin-dependent kinases (CDks) rises and falls because the concentrations of cyclin fluctuate throughout the cell cycle, while Cdk protein levels remain relatively stable. Cyclins and Cdks bind at specific points in the cell cycle and form complexes which regulate different checkpoints. These complexes must be phosphorylated to become fully active and activate other proteins that advance the cell cycle to the next phase.