Molecular and Cellular Biology Exam 1 Practice

Cyclin-CDK complexes control when the cell moves from one phase to the next and pass the checkpoints. Cyclins are produced and degraded in a timed pattern, and when a cyclin binds a CDK, it activates the kinase. That activated complex then phosphorylates specific target proteins to drive transitions such as G1 to S, S to G2, and G2 to M, while at checkpoints it helps ensure conditions are right (enough size, adequate DNA replication, proper spindle formation) before allowing the cycle to proceed. If problems are detected, CDK activity can be restrained, delaying progression until the issues are resolved.

This is why the option stating that cyclins activate CDKs and that cyclin-CDK activity governs progression through the checkpoints is the best fit. It captures both the regulatory role of cyclins in activating CDKs and the checkpoints they help regulate. The idea that CDKs degrade cyclins at checkpoints is not the main mechanism, and checkpoints do not cause the cell to skip S phase. And saying progression isn’t regulated by cyclins contradicts the central control by these complexes.