Rhythmic fluctuations in the abundance and activity of cell-cycle control molecules pace the events of the cell cycle.
• Kinase - a protein which activates or deactivates another protein by phosphorylating them.
• Kinases give the go-ahead signals at the G1 and G2 checkpoints
• The Kinases that drive these checkpoints must themselves be activated
o The activating molecule is a cyclin, a protein that derives its name from its cyclically fluctuating concentration in the cell
o Because of this requirement, these kinases are called cyclin-dependent kinases, or Cdk's