Cell cycle checkpoints are times during the cell cycle in which the cell checks to see whether it is ready to proceed with mitosis or cell division. Checkpoints occur at three different times during the cell cycle: G1, G2 and M.
The G1, or first gap, checkpoint makes sure that the cell is big enough and contains all necessary ingredients to begin mitosis. The cell must receive a signal before it continues with the cell cycle; without the signal, the cell goes into the G0 state, where the cell has been taken out of the cell cycle and is not actively dividing. With a "go" signal from G1, the cell moves into the S phase, where DNA begins to replicate. After DNA replication, the cell goes into the G2, or second gap, stage, a checkpoint that makes sure all DNA has been accurately replicated. The final checkpoint, the M, or metaphase, checkpoint, ensures that the chromosomes have lined up at the center of the cell and that protein fibers called kinetochores have correctly attached to the chromosomes.