is the normal number of divisions a cell (or a group of cells) is capable of before it halts proliferation. This is thought to be a result of telomeres reaching a critical length. During DNA replication in cell divisions, the telomeres (a region at the end of DNA sequence of a chromosome) are not able to be duplicated with the rest of the DNA sequence, but instead, those are lost. Hence, after a series of cell divisions, the telomeres are depleted. As a result, the cell undergoes apoptosis to prevent developing replication errors which can result in mutations.