Cyclins and CDKs are what?

• A. Hormones that regulate metabolism.
• B. Enzymes that digest carbohydrates.
• C. Structural components of the cytoskeleton.
• D. Regulatory proteins involved in cell cycle control.

Answer: (D)

Cyclins and CDKs regulate the cell cycle. CDKs are kinases that phosphorylate target proteins to push the cell from one phase to the next, but they’re only active when bound to a cyclin partner. Cyclin levels rise and fall during the cycle, so different cyclin-CDK pairs activate at specific times to drive transitions such as G1 to S and G2 to M. For example, cyclin B bound to CDK1 promotes entry into mitosis. This regulatory pairing coordinates cell division, whereas hormones, carbohydrate-digesting enzymes, or cytoskeletal structural components do not perform this control function.