In normal cells, the cell cycle is controlled by a complex series of signaling pathways by which a cell grows, replicates its DNA and divide. There are various checkpoints which control all the stages of the cell cycle. These are called as checkpoints. If there is any thing wrong or missing then these checkpoints stops the cell cycle and first in normal c ells first the abnormality is corrected and there the cell cycle is proceeded whereas in cancer cells these checkpoint do not correspond
to abnormality. The checkpoints does not halt the cell cycle in any case which results in the abnormal cell division and growth resulting in cancer.
So in short it can be said normal checkpoints do not work properly to stop the cell division at the time of any abnormality.