The control points that gate cell division
Dividing cells move through phases (G1, S, G2, and mitosis) and pause at checkpoints that verify conditions are right before continuing. The G1/S checkpoint, sometimes called the restriction point, checks for adequate size, nutrients, growth signals, and intact DNA before committing to copy the genome. The G2/M checkpoint confirms that DNA was fully and correctly replicated before mitosis begins.
During mitosis itself, the spindle assembly checkpoint ensures every chromosome is properly attached to the spindle so the two daughter cells each receive a complete set.
Why checkpoints help prevent cancer
Checkpoints give the cell a chance to repair damage or, if the damage is too severe, to halt or trigger self-destruction rather than pass on faults. Proteins such as p53 act at these decision points, which is part of why p53 is described as a guardian of the genome and is frequently mutated in cancers.
When checkpoint control fails, cells can divide with damaged or unequally distributed DNA, a step toward tumor formation. This is general educational information about cell biology, not medical advice.