Which protein is maintained at constant levels through the cell cycle: CDK or cyclin?

What this question is testing

You are being asked to fill in a missing phrase about how the concentration of certain cell-cycle control proteins changes over time.

CDKs vs cyclins: which one stays constant?

• CDKs are enzymes (kinases) that are usually present at fairly constant amounts in the cell.
• Cyclins are regulatory proteins whose concentrations change cyclically; they are made and then broken down at specific points in the cycle.

Filling in the blank

So the statement is:

• CDK is a protein maintained at constant levels through the cell cycle.

Quick memory tip

Think: Cyclins cycle, CDKs do not. Cyclins turn CDKs on and off by binding to them.