Cells use cyclins and cyclin-dependent kinases to control the cell cycle. Different combinations of cyclin/CDK start the cell cycle at different checkpoints and monitor the cycle for quality control. In G1 stage, the cell is growing and preparing to replicate DNA. DNA replication occurs in the S stage . In G2 stage , the cell takes inventory to ensure it is ready to proceed to mitosis . If the spindle apparatus malfunctions, the cycle won’t proceed to cytokinesis.
Each stage of the cell cycle has related cyclins that help the progression of that phase. Cyclins bind to cyclin-dependent kinases and activate them. The kinases are therefore able to phosphorylate particular protein and activate or deactivate them appropriately to progress the particular cell cycle stage. P53 tumor suppressor protein proteins help regulate this progress at each checkpoint. They do so by activating CDK inhibitors when required (such as when there is damage to the cells DNA).
