They also knew that there was a string of dna at the end of each chromosome called a telomere, which shortened a tiny bit each time a cell divided, like time ticking off a clock

Telomeres are regions that make up the end of each chromosome. These 5 nucleotide repeats help preserve the integrity of the DNA during replication. However, in each division, the telomeres shrink. Some scientists have seen in this mechanism a biological clock governing the aging of cells. Until now no one has shown a link between the two phenomena.

The size of the telomeres (the number of repetitions) varies from one individual to another at birth. At each replication cycle, the chromosomes shorten. This is because the enzyme responsible for its replication cannot copy the end of the chromosomes. It is estimated that 100 nucleotides decrease the size of telomeres by division in humans. When the telomeres become too short and before the genes are affected, or the chromosomes merge with each other, the cells stop their division and enter into senescence.

A single enzyme is able to extend the telomeres, telomerase. This enzyme is not active in somatic cells. However, there are somatic cells that are immortal, these are the cancer cells. They have lost control of their cell cycle and can divide indefinitely. This ability to multiply without limit is accompanied by the activation of telomerase.