A repetitive DNA region at the end of a chromosome that protects it from degradation.A telomere is the stretch of repetitive DNA that sits at the very tip of a linear chromosome, like the plastic cap on a shoelace. In humans it consists of the same short sequence — TTAGGG — repeated thousands of times, bound by a dedicated set of proteins. Because the machinery that copies DNA cannot finish the very end of a linear strand, each round of cell division would otherwise nibble away at the chromosome's coding regions. The telomere absorbs that loss instead, acting as a disposable buffer. When it becomes critically short, the cell interprets this as a signal to stop dividing and enter a permanently arrested state, or to self-destruct.
This matters because telomere length sets a kind of counting mechanism on how many times most body cells can divide. It links a purely structural problem — copying the ends of a line — to ageing, tissue renewal, and cancer. Cells that must divide indefinitely, such as stem cells and cancer cells, reactivate an enzyme that rebuilds telomeres, while ordinary cells do not. The telomere therefore sits at the crossroads of two of biology's biggest questions: why organisms age, and how tumours escape the normal limits on growth.
What the telomere explains is why cells have a finite division budget and why chromosomes stay stable. Without it, chromosome ends would be mistaken for broken DNA and fused together, producing the kind of genomic chaos seen in many cancers. It also explains why a single fertilised egg can produce a whole body yet most of that body's cells cannot keep going forever.
Telomeres were first described in the 1930s by Barbara McClintock and Hermann Muller, who independently noticed that chromosome ends behave differently from broken ends. Their molecular structure and the enzyme that maintains them were worked out in the 1970s and 1980s by Elizabeth Blackburn, Carol Greider and Jack Szostak, who shared the 2009 Nobel Prize in Physiology or Medicine for the work.
The enzyme that duplicates DNA, a polymerase, can only extend a strand by adding new units onto an existing starting point. At the extreme end of a linear chromosome there is no place for that starting point to sit, so the final stretch is left uncopied. The telomere, being a long run of meaningless repeats, takes the hit. A specialised enzyme called telomerase can add repeats back onto the end, using a small RNA molecule as a template, but in most adult human cells it is switched off. When repeats run low, the exposed end triggers a DNA-damage response that halts division.
Telomere length is measured in research on ageing and age-related disease, and telomerase is a major target in oncology, since most cancers depend on it to keep dividing.
The record for the longest natural telomeres belongs to some strains of laboratory mice, whose telomeres can be many times longer than a human's — yet mice age far faster, showing that telomere length alone does not set lifespan.