What must happen for a Cdk to be active?

For a cyclin-dependent kinase (Cdk) to be active, it is essential for it to bind to its cyclin partner. This binding is a critical regulatory mechanism that enables the Cdk to undergo a conformational change, allowing it to become catalytically active. Cyclins are specific proteins that regulate the cell cycle, and their levels fluctuate at different phases of the cycle. When a Cdk is associated with its corresponding cyclin, the resulting complex can phosphorylate target proteins, which facilitates the progression of the cell cycle.

The necessity of this interaction emphasizes the importance of cyclins in controlling Cdk activity and ensuring proper cell cycle regulation. Without the binding to a cyclin, a Cdk remains inactive and cannot perform its function in cell cycle regulation.