What are cell cycle checkpoints and how do they function?

Cell cycle checkpoints are surveillance mechanisms ensuring proper cell division. The G1/S checkpoint (restriction point) verifies cell size, nutrients, and DNA integrity before S phase commitment - p53 and Rb are key regulators. The G2/M checkpoint confirms DNA replication completion and repairs damage before mitosis. The spindle assembly checkpoint (SAC) ensures proper chromosome attachment before anaphase. Checkpoints work through cyclin-dependent kinases (CDKs) regulated by cyclins, CDK inhibitors (p21, p27), and phosphatases. Checkpoint failure leads to genomic instability and cancer; DNA damage activates ATM/ATR kinases that halt the cycle.