Core control of the cell cycle: At the center of regulation is Cdk (cyclin-dependent kinase), a protein that primarily does what?

Introduction / Context:
Progression through the cell cycle is driven by cyclin-dependent kinases (Cdks). These enzymes require association with cyclins to become active and to target appropriate substrates. Understanding this partnership is essential for interpreting checkpoints and anti-cancer drug mechanisms.

Given Data / Assumptions:

• Cdk proteins are constitutively expressed at relatively stable levels.
• Cyclin levels oscillate, defining cell-cycle phases.
• Activation also involves regulatory phosphorylation and dephosphorylation.

Concept / Approach:
Cdk catalytic subunits are activated by binding specific cyclins (for example, G1, S, or M cyclins). This binding changes kinase conformation, enabling substrate recognition and activity. Additional control includes phosphorylation by CAK, inhibitory phosphorylation (e.g., Wee1), and phosphatase reversal (e.g., Cdc25), but cyclin binding is the central, defining requirement.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical reconstitution shows inactive Cdk becomes active kinase only upon cyclin association and proper phosphorylation, with substrate specificity dictated by the cyclin partner.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming phosphorylation alone activates Cdks; cyclin binding is the defining activation step.

Final Answer:
Binds to different cyclins to gain substrate specificity and activity.