How many checkpoints are there in the cell cycle?

There are three major checkpoints in the cell cycle: the G1/S checkpoint, the G2/M checkpoint, and the spindle assembly checkpoint (SAC). The G1/S checkpoint occurs at the end of the G1 phase before the S phase. It checks whether the cell is ready to move forward with DNA replication. Cell size, nutrient availability, growth factor cues, and DNA damage are all analyzed as part of the process. If the conditions are deemed to be favorable, the cell moves forward to the S phase. If the conditions are unfavorable, the cell may enter the G0 phase or undergo apoptosis. The G2/M checkpoint occurs at the end of the G2 phase before mitosis. This checkpoint ensures DNA replication in the S phase has properly occurred and that the cell is ready for cell division. The checkpoint verifies DNA integrity and repairs any remaining errors. If the conditions are favorable, the cell will progress into mitosis. If conditions are not favorable, the cell cycle may be halted to allow for DNA repair. The SAC occurs during mitosis and monitors the attachment of chromosomes to the mitotic spindle. The checkpoint verifies that all chromosomes are properly aligned at the metaphase plate. If errors are detected, the checkpoint delays anaphase onset to allow for chromosome realignment.