A cyclin
A) activates a Cdk molecule when its concentration is decreased.
B) is present in similar concentrations throughout the cell cycle.
C) activates a Cdk molecule when it is in sufficient concentration.
D) decreases in concentration when M phase-promoting factor MPF) activity increases.
E) is activated by binding to microtubules.

Question

Quizplus · Accepted Answer

Cyclins activate Cdk molecules when they are in sufficient concentration, not necessarily when their concentration is decreased. The concentration of cyclins varies throughout the cell cycle, increasing and then decreasing. It is the activity of cyclins that correlates with the activity of Cdks, not their concentration in the cell. The concentration of cyclins generally increases as cells prepare to enter M phase, and this increase is necessary to activate M phase-promoting factor (MPF), not the other way around. Cyclins do not bind to microtubules, but rather bind and activate Cdks to regulate the cell cycle.

A Cyclin