Cyclins and cyclin dependent kinases both drive the cycle forward. Which one sets the timing, and how?

1. A cyclin dependent kinase only works while it is bound to a cyclin. 2. So the complex can form only when both partners are present. 3. Cdk protein levels stay roughly constant right through the cycle. 4. Cyclin levels rise and fall in a repeating pattern instead. 5. Because Cdk is always available, it is the cyclin supply that decides when the complex appears. 6. That makes cyclin the timer and Cdk the engine. 7. Saying Cdk fluctuates swaps the two roles and leaves nothing to set the timing. 8. Saying cyclin stays steady removes the very fluctuation that makes the mechanism a clock. _Source: OpenStax Biology 1e (CC BY 4.0), Chs 10 and 11, section 10.3 Control of the Cell Cycle_