Why do levels of Cdk activity change during the cell cycle?

The correct answer highlights the essential role of cyclins in regulating Cdk (cyclin-dependent kinase) activity throughout the cell cycle. Cyclins are regulatory proteins whose levels fluctuate at specific points during the cell cycle. These fluctuations are crucial because Cdk activity is dependent on the binding of cyclins; without the appropriate cyclin, Cdk remains inactive.

During different phases of the cell cycle, specific cyclins are synthesized and degraded, corresponding to particular actions required for cell progression. For instance, certain cyclins are present during the G1 phase, activating Cdks to promote transition to the S phase, where DNA synthesis occurs. As the cycle progresses, cyclins that are active in one phase are tagged for degradation, and new cyclins are produced for the next phase, allowing for precise control over the cell cycle. This cyclical synthesis and degradation of cyclins ensure that the cyclin-Cdk complexes are activated at the appropriate times and that the cycle progresses in a regulated manner.

In contrast, the other options do not accurately capture the mechanism by which Cdk activity changes. The notion that Cdks phosphorylate each other does not explain the fluctuations in Cdk activity on its own, nor does the idea that Cdks activate cyclins; rather,