According to Annenberg Learner, when the synthesis of certain growth factors is inhibited, cells stop dividing. Normal cells display contact inhibition as they respond to forms of contact from other cells by causing cell division to end.
Cells age, die and are then replaced by newer cells. Apoptosis is the programmed and normal death of a cell. Normal cells are able to divide approximately 50 times before they are replaced. This process is related to the replication of DNA, which is limited. Every time the chromosomes in the cell replicate, the telomeres shorten. In the growing of new cells, telomerase replaces the lost telomeres.