Some cells can prevent telomere shortening by providing telomerase, an enzyme that lengthens chromosome telomeres. Telomerase is an RNA-dependent DNA polymerase, that is, an enzyme that synthesizes DNA on a matrix of RNA.

How does telomerase work? The enzyme binds to a special RNA molecule that contains a sequence complementary to the telomere repeat. It adds nucleotides to the end “tail” of the telomere DNA strand, using this complementary RNA as a matrix. When the tail becomes long enough, the complementary strand is synthesized using the normal DNA replication mechanism (that is, using the RNA primer and DNA polymerase), thus producing double-stranded DNA.
The primer is rarely exactly at the end of the chromosome and cannot be replaced with DNA, so the “tail” will still remain. Nevertheless, the telomere length will still increase.

Telomerase is generally not active in most somatic cells (body cells), but it is active in sex cells (sperm- and egg-producing cells) and some adult stem cells. Such cells must divide very many times, and in the case of germ cells, they give rise to new organisms in which the telomere “clock” must be “reset”.

Interestingly, many cancer cells have shortened telomeres and telomerase is active in them. If telomerase is inhibited by drugs used for anti-tumor therapy, their progressive division and thus the growth of the cancerous tumor can potentially be stopped.