What is telomerase in DNA replication?

Telomerase is an enzyme that adds short repeat DNA sequences to the ends of chromosomes so they do not shorten during DNA replication.

In every S phase, the normal DNA polymerase cannot fully copy the extreme 3′ ends of linear chromosomes (the “end-replication problem”). Without a fix, telomeres—repetitive caps such as TTAGGG in humans—would erode each cell cycle, risking gene loss and triggering senescence. Telomerase solves this. It is a ribonucleoprotein reverse transcriptase (TERT protein + an internal RNA template, TER/TERC). Using its RNA as a guide, telomerase extends the 3′ overhang with telomeric repeats. Conventional DNA polymerase and primase then fill the complementary strand, and ligase seals nicks.

• Type: Reverse transcriptase with built-in RNA template.
• Function: Extends telomeres to maintain chromosome stability.
• Where active: Germ cells, stem cells, activated lymphocytes; often reactivated in cancers; low/absent in most somatic cells.
• Clinical link: Telomerase upregulation supports tumor immortality; insufficiency leads to telomere syndromes (e.g., bone-marrow failure).

Process snapshot

1. Telomerase binds 3′ overhang at a telomere.
2. Extends the overhang by reverse-transcribing repeats.
3. Polymerase α/primase makes an RNA primer on the C-strand.
4. DNA polymerase δ/ε fills in; RNase removes primer; ligase seals.