The role of telomeres in ageing and cancer.

Telomeres are regions of DNA that cap the ends of linear chromosomes. In somatic cells the telomeres shorten progressively with every cell division, reducing the number of tandem repeat sequences. Eventually the chromosomes become unstable and the cell is no longer able to replicate. This represents an inherent biological clock in which the somatic cell has only a finite capacity for division. In contrast, germ cells do not undergo telomeric shortening and have relatively unlimited capacities for cell division. The difference is that germ cells retain the enzyme telomerase which is able to restore the telomere ends that are lost during cell division. Although telomerase activity is absent in most somatic cells, cancer cells acquire the ability to activate the enzyme, ensuring their immortal growth characteristics and selective advantage over normal somatic cells.