What causes changes in Cdk activity during the cell cycle?

Cdk activity during the cell cycle is primarily regulated by the levels of cyclins, which undergo significant fluctuations as the cell progresses through different phases. Cyclins are proteins that bind to cyclin-dependent kinases (Cdks), activating them to phosphorylate target proteins that drive the cell cycle forward.

As the cell cycle progresses, specific cyclins are synthesized and degraded in a tightly controlled manner. For example, cyclin D is active during the G1 phase, cyclin E is prominent during the transition from G1 to S phase, cyclin A is active during S phase and G2, and cyclin B is necessary for mitosis. The presence of specific cyclins at different points in the cycle is what allows for the precise timing of Cdk activation.

This regulation ensures that the cell cycle events occur in a specific order and are coordinated properly. When cyclin levels rise, they activate Cdks, leading to the phosphorylation of proteins necessary for cell cycle progression. Conversely, as cyclins are degraded, Dks become inactive, leading to the proper cessation of cellular processes needed for each phase. Thus, changes in cyclin levels are critical for modulating Cdk activity throughout the cell cycle, making this the correct answer to the question.