Each cyclin-CDK pair runs its own step
- The idea
- Predict
- See it
- Why
- Check
The idea
“Cyclin-CDK is one switch for the whole cycle, so losing any cyclin stops everything at the start.”
Diagrams often show one cyclin and one CDK, so it looks like a single master switch. Cells actually make several cyclins, each timed for its own step.
The short version
The mistake: Cyclin-CDK is one switch for the whole cycle, so losing any cyclin stops everything at the start.
What's actually true: Each cyclin pairs with CDK to drive one transition, so losing one cyclin stops the cycle only at its own step.
- A CDK, or cyclin-dependent kinase, is an enzyme that phosphorylates target proteins. It's active only when a cyclin is bound to it.
- CDK levels stay fairly steady. Cyclins rise and fall at different times, so the cyclin present decides which step happens.
- One pair triggers the G1/S transition and DNA replication. Another, often called MPF, pushes the cell from G2 into mitosis.
On the exam: Find the step the missing cyclin drives. The cell gets that far and stops there.