What does telomerase do to the length of your telomeres?

Telomerase is an enzyme that replenishes the ends of chromosomes, the telomeres, with each cell division. It continuously attaches new pieces of DNA (the sequence TTAGGG) to those ends. Without this enzyme, telomeres become slightly shorter with every cell division, until the cell stops dividing. That process is regarded as a biological ageing clock.

In most ordinary body cells, telomerase is barely active after the early embryonic phase. As a result, those cells become exhausted with repeated division: the telomeres grow too short, the cell enters a kind of dormant state (senescence) and no longer divides. Stem cells and germ cells are an exception; these keep telomerase active for longer.

Telomerase does not operate without regulation either. Specific proteins on the telomeres determine when the enzyme gains access. The shortest telomeres take priority: they are replenished first. In this way the system maintains a balance and the telomere does not grow indefinitely.

There is an important downside. In most cancer cells, telomerase becomes active again, allowing those cells to keep dividing without limit. Telomere shortening in healthy cells acts precisely as a brake: a cell that has divided too many times simply stops. That brake also restrains tumour formation.

When telomerase does not function properly due to an inherited mutation, so-called telomeropathies arise. These are conditions in which telomeres wear down structurally too quickly, with serious consequences for tissues that require a great deal of cell division, such as bone marrow and lungs. This shows how essential a properly functioning telomerase system is.