What role does cyclin play in the cell cycle with respect to Cdk activity?

Cyclins play a crucial role in regulating the activity of cyclin-dependent kinases (Cdks), which are essential for the progression through various phases of the cell cycle. Specifically, the binding of a cyclin to a Cdk is necessary for the activation of the Cdk enzyme. This binding induces a conformational change in the Cdk, allowing it to achieve an active form that can phosphorylate target substrates, which in turn drives the cell cycle forward by promoting key transitions such as from G1 to S phase or from G2 to M phase.

Cyclin levels fluctuate throughout the cell cycle, rising and falling to ensure that Cdks are activated only at specific points, thereby ensuring proper cell cycle regulation and preventing premature or erroneous cell division. This highlights the regulatory nature of cyclins in the orchestration of cell cycle events, marking their importance in cellular control mechanisms.