CDK is the enzyme. Cyclin is the switch.
- The idea
- Predict
- See it
- Why
- Check
The idea
“Cyclin is the enzyme that drives the cell cycle, and CDK levels rise and fall to time each phase.”
Cyclin's level rises and falls with the activity, so it looks like cyclin does the work. The name says otherwise. A cyclin-dependent kinase is the enzyme.
The short version
The mistake: Cyclin is the enzyme that drives the cell cycle, and CDK levels rise and fall to time each phase.
What's actually true: CDK is the kinase and its level stays steady. Cyclin rises and falls, and CDK works only while cyclin is bound to it.
- A kinase adds phosphate groups to other proteins. CDK is the kinase. The proteins it phosphorylates move the cell into the next phase.
- CDK is inactive alone. It turns on only when a cyclin binds it, which is why it's called cyclin-dependent.
- CDK levels stay fairly steady. The cell times each phase by making cyclin, then breaking it down.
On the exam: CDK is the kinase and is always there. Cyclin comes and goes and switches it on.