telomerase
Meaning of telomerase: The enzyme that rebuilds chromosome ends, switched off in most adult tissue and switched back on in the great majority of cancers.
Definition of telomerase
Telomerase is an enzyme that lengthens telomeres, the repeated stretches of DNA capping each end of a chromosome. It carries its own short piece of RNA as a template and copies that template into DNA, rebuilding sequence lost every time a cell divides. Most mature body cells make little or none of it, while germ cells, stem cells and most cancer cells keep it working.
Why do chromosome ends need rebuilding?
Because of a flaw in how DNA is copied. The machinery that duplicates a strand cannot finish the very last stretch at one end, so every round of division leaves the chromosome slightly shorter. Caps of repeated, meaningless sequence sit there to be sacrificed, which keeps the loss away from working genes. The caps do a second job as well: they hide the chromosome end, which would otherwise look like a snapped strand and invite repair machinery to fuse it to another chromosome.
What does the enzyme actually do?
It is unusual among enzymes because it brings its own instructions. A short RNA sequence is built into the enzyme, and the protein part reads that RNA and lays down matching DNA on the chromosome end, then shuffles along and repeats. Working from RNA back to DNA is the reverse of the usual direction of information flow inside a cell, so telomerase belongs to the small group described as reverse transcriptases.
How does this connect to aging and cancer?
| Cell type | Telomerase | Consequence |
|---|---|---|
| Egg and sperm precursors | Active | Full-length caps passed to the next generation |
| Stem cells | Partly active | Renewal continues, though slowly declining |
| Most mature body cells | Little or none | Caps shorten with every division |
| Most cancer cells | Switched back on | Division without a built-in limit |
A cell whose caps run down usually does not die on the spot. It stops dividing permanently, a stalled state that limits tissue repair with age and also acts as a brake on tumors. Reawakening telomerase is one of the steps by which a cancer escapes that brake, which makes the enzyme an attractive drug target, though a minority of tumors lengthen their caps by a recombination route instead and would be untouched by blocking it. Inherited faults that leave telomerase underpowered are also recognized, and they show up as marrow failure, scarred lung and early graying rather than as anything resembling longevity.
Also known as
Used in a sentence
Telomerase activity was detectable in the tumor sample but not in adjacent normal tissue.
Good to know
Telomerase carries its own RNA template rather than reading the chromosome it extends, and a cell that exhausts its caps generally stops dividing permanently rather than dying, a state that restricts tissue repair as much as it restrains tumors.
Word origin
Where “telomerase” comes from
The name joins telomere, itself from the Greek telos, meaning end, and meros, meaning part, to the ending -ase, which marks an enzyme.