How do the checkpoints of the cell cycle work?

A checkpoint is a stage in the eukaryotic cell cycle during which the cell analyzes internal and external cues and verifies whether or not to move forward with cell division. Checkpoints act as a type of quality control which ensures that the major events of the cell cycle occur without mistakes. Checkpoints act to either halt cell division or to “approve” to proceed to the next step of the cell cycle. If a checkpoint assesses that a cell is unable to continue with replication, it may also instead enter the G0 phase.