How do protein kinases and cyclins regulate the cell cycle?
There are enzymes in the cell called cyclin-dependent kinases that are present at all times but need a specific cyclin to bind to them before they become active. When they are activated by a cyclin they push the cell into the next phase of the cell cycle by modifying other enzymes. The cyclins are specific to different cell cycle checkpoints. Cyclin levels rise and fall throughout the cell cycle so they are only present at high concentrations when the cell is ready to advance to the next phase of the cell cycle. After the cell has advanced, cyclins are rapidly degraded until they are needed again.
