During the cell cycle, which of the following is true of the cyclin:CDK complex specific for one phase of the cell cycle?

Question

a. it is present only during that phase

b. it is constantly present at high levels in all phases

c. it is constantly present at low levels in all phases

Answer 1

Technically, none of these answers are correct, but A is the best answer. CDKs are present, more or less, at a constant level no matter the cell cycle; it's the cyclins that, well, cycle, depending on which phase the cell is in. The simple interpretation is that each cycle has a "master" cyclin:CDK complex that's calling all the shots, and the fall of one specific cyclin:CDK pair and rise of a different pair ushers in a new phase of the cycle. The only cyclin thats regulated extrecellularly is G1/S (cyclin D); all others are on "automatic pilot" provided the conditions are met to proceed to the next phase.