Telomerase is an enzyme that adds specific DNA sequence repeats (“TTAGGG” in all vertebrates) to the 3′ end of DNA strands in the telomere regions, which are found at the ends of chromosomes. This addition of repeated sequences replenishes the loss of DNA due to the end-replication problem, where complete replication of linear DNA molecules is impossible due to the directionality of DNA synthesis and the structure of DNA polymerase.

Sufficient activity of telomerase can maintain telomere length and prevent cellular senescence and apoptosis that would otherwise be caused by critically short telomeres. In this context, “sufficient” telomerase means the enzyme is present in enough quantity to preserve telomere length and allow for ongoing cell division.

However, it’s important to note that while sufficient telomerase can prevent replicative senescence, it can also contribute to uncontrolled cell growth and cancer. In humans and other mammals, telomerase activity is usually repressed in somatic (body) cells but is active in many cancer cells. This allows cancer cells to avoid senescence and apoptosis and to continue dividing indefinitely, one of the key characteristics of cancer.

On the other hand, in certain cells like stem cells, germ cells, and some white blood cells, telomerase is naturally active to maintain their self-renewal ability. The regulation of telomerase is thus a delicate balance: too little can lead to premature cellular aging and related diseases, while too much can lead to uncontrolled cell growth and cancer. Research is ongoing to better understand this balance and to develop therapies that can safely manipulate telomerase activity for the treatment of cancer and age-related diseases.