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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Telomerase

Telomerase lengthens chromosome ends (telomeres) using its own RNA as a template; it is active in stem cells and in most cancers.

Updated 2026-11-01

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## Key facts

- It is a reverse transcriptase: the TERT protein + the TERC (hTR) template RNA, which holds the sequence complementary to the repeat.
- It adds TTAGGG repeats to the 3' end of the G-rich strand.
- It solves the 'end-replication problem': without it telomeres shorten with each division.
- Active in germ cells, stem cells and about 85–90% of cancers; almost off in adult somatic cells.
- Very short telomeres → senescence or apoptosis (Hayflick limit).

**Also called**
TERT, Telomere reverse transcriptase

**EC number**
[2.7.7.49](https://www.enzyme-database.org/query.php?ec=2.7.7.49)

**Genes**
TERT, TERC

**Structure (PDB)**
[9SHZ](https://www.rcsb.org/structure/9SHZ)

## Regulation

- TERT transcription is the limiting step; TERT promoter mutations switch it on in melanoma, glioblastoma and bladder cancer.
- The protective shelterin complex (TRF1, TRF2, POT1, TPP1) recruits telomerase and limits its access to the telomere.
- Dyskerin (DKC1) stabilises telomerase RNA.

## Inhibitors and toxins

- Imetelstat (an oligonucleotide against the RNA template; used in some myeloid neoplasms)

## Clinical notes

- Short-telomere syndromes (dyskeratosis congenita: dystrophic nails, leukoplakia, reticular pigmentation), with bone marrow failure and pulmonary fibrosis.
- TERT and TERC variants are found in part of familial idiopathic pulmonary fibrosis.
- Its reactivation lets tumour cells divide without limit: a hallmark of cancer.

## Related conditions

- [Interstitial lung disease](https://durgical.com/en/conditions/interstitial-lung-disease/)

## Related

- [DNA replication: the replication fork](https://durgical.com/en/biochemistry/topics/dna-replication-the-replication-fork/)
- [DNA polymerases](https://durgical.com/en/biochemistry/enzymes/dna-polymerases/)
- [Central dogma of molecular biology](https://durgical.com/en/biochemistry/topics/central-dogma-of-molecular-biology/)
- [DNA structure and chromatin](https://durgical.com/en/biochemistry/topics/dna-structure-and-chromatin/)

## Test yourself

Think of the answer, then tap the question to see it.

## Why is telomerase a reverse transcriptase?

It uses its own RNA (TERC) as a template to make DNA: it copies RNA into DNA.

## Which sequence does human telomerase add?

TTAGGG repeats at the 3' end of the telomere's G-rich strand.

## Why do most cancer cells reactivate telomerase?

To stop their telomeres shortening into senescence, so they can divide without limit.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [TERT: Telomerase reverse transcriptase (human)](https://www.uniprot.org/uniprotkb/O14746/entry) (UniProt, 2026); [TERT gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/tert/) (MedlinePlus, US National Library of Medicine, 2026); [9SHZ: Cryo-EM structure of the catalytic core of human telomerase at the elongation state of the repeat addition cycle](https://www.rcsb.org/structure/9SHZ) (RCSB Protein Data Bank, 2026); [Telomere Maintenance](https://reactome.org/PathwayBrowser/#/R-HSA-157579) (Reactome, 2026); [Telomere](https://www.genome.gov/genetics-glossary/Telomere) (National Human Genome Research Institute (NHGRI), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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