Cyclins and Cyclin-Dependent Kinases
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Cyclins and cyclin-dependent kinases, or Cdks, are the cell cycle's positive regulators; cyclin levels rise and fall predictably, and only a cyclin-bound, phosphorylated Cdk pushes the cell past a checkpoint.
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A cyclin's concentration peaks shortly before its checkpoint, then drops sharply as it is degraded.
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Cdk levels stay roughly constant — cyclin concentration's rise and fall times each transition.