Progression through the cell cycle requires a cyclin to bind to a Cdk because _________
(a) the cyclins are the molecules with the enzymatic activity in the complex.
(b) the binding of a cyclin to Cdk is required for Cdk enzymatic activity.
(c) cyclin binding inhibits Cdk activity until the appropriate time in the cell cycle.
(d) without cyclin binding, a cell-cycle checkpoint will be activated.
Choice (b) is correct. Cyclins have no enzymatic activities themselves [choice (a)], and cyclin binding to Cdk activates the Cdk [choice (c)]. As far as we know, cyclin–Cdk binding is not directly monitored by checkpoints [choice (d)].
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