Levels of Cdk activity change during the cell cycle, in part because ________________
(a) the Cdks phosphorylate each other.
(b) the Cdks activate the cyclins.
(c) Cdk degradation precedes entry into the next phase of the cell cycle.
(d) cyclin levels change during the cycle.
Choice (d) is correct. Cdks do not phosphorylate each other [choice (a)]. The Cdks do not activate the cyclins [choice (b)], and Cdks are not degraded during specific phases of the cycle [choice (c)]. The cyclins, however, are degraded in a cell-cycle-dependent fashion, and they are required for Cdk activity.
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Grain size in the intertidal is primarily determined by:
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