Normal cells have checkpoints or inhibitory mechanisms that allow for assessment as to whether to proceed through the cell cycle. For example, if cells accumulate too much DNA damage, then the cell cycle can be arrested until the DNA can be repaired.
If there is too much damage, the cell may be forced into undergoing apoptosis, or cell death. Cancer cells have lost some of these checkpoints and proceed prematurely through the cell cycle, passing on mutations to daughter cells.
What cell cycle transitions do the tumor suppressors pRB and p53 help to regulate?