What controls the cell cycle at key checkpoints?

The cell cycle is controlled at key checkpoints by a group of molecular proteins which have the ability to switch on and off during different stages of the cell cycle. These proteins are known as cyclin-dependent kinases (CDK). Cyclin-CDK complexes play a key role in controlling the cell cycle by phosphorylating target proteins which stimulate cell cycle progression. Different cyclin-CDK complexes are active at specific checkpoints and control the transition between cell cycle phases. Checkpoint proteins also play a role in regulating the cell cycle at key checkpoints. For example, tumor suppressor proteins p53 and retinoblastoma protein (Rb) monitor the integrity of the cell cycle and generate responses to DNA damage. If required, these proteins also have the capability of halting the cell cycle and inducing cell death. Mitotic checkpoint proteins also Mad1, Mad2, and Bub1 monitor the attachment of chromosomes to the spindle apparatus and prevent premature separation until all chromosomes are successfully aligned.