Telomeres and ageing, explained in plain English

At the end of every chromosome sits a stretch of repetitive DNA that does no coding work of its own. It exists to protect the genetic material it caps. These structures are called telomeres, and they shorten a little each time a cell divides. That slow erosion has made telomeres one of the most discussed ideas in the biology of ageing, often presented as a kind of countdown clock for the body. The real picture is more interesting, and more cautious, than the headline suggests. Here is what telomeres are, why they shorten, and what the evidence on their link to ageing actually shows.

What telomeres actually are

A telomere is a region of repeated DNA sequence, together with the proteins bound to it, that sits at each end of a chromosome. In humans the repeated unit is a short six-letter sequence, copied thousands of times. Telomeres do not carry instructions for building proteins. Their job is structural. They stop the natural ends of chromosomes from being mistaken by the cell for broken DNA, which would otherwise trigger repair machinery and risk chromosomes being fused or rearranged. A 2021 review in Cell describes telomeres as essential guardians of genome stability, and frames their gradual loss as a thread running through many of the processes we associate with growing older.

Why telomeres shorten

The shortening has a precise mechanical cause. When a cell copies its DNA before dividing, the enzymes that carry out the copying cannot quite finish the very end of each strand. A small amount is left uncopied and is lost. This is known as the end-replication problem, and it means that with each division a typical body cell ends up with slightly shorter telomeres than before. Over many divisions the telomere is whittled down. A 2024 review in Subcellular Biochemistry sets out this mechanism clearly and explains how it places a natural limit on how many times most cells can divide.

There is an enzyme that can counteract this loss. It is called telomerase, and it adds telomeric repeats back onto the ends of chromosomes. The catch is that most adult body cells switch telomerase largely off. It stays active in germ cells, which produce eggs and sperm, and in certain stem cell populations, allowing those cells to keep dividing. In the ordinary cells that make up most of our tissues, telomerase is quiet, so the erosion continues. Damage from other sources, including oxidative stress and chronic psychological stress, can accelerate the process beyond what division alone would cause, as reviewed in Ageing Research Reviews in 2021.

Telomeres and ageing: the biomarker question

Because telomeres shorten with cell division and with age, researchers have asked whether their length might serve as a readout of biological ageing, a measure of how worn a body is rather than how many birthdays it has counted. On average, telomere length measured in blood cells does decline across the population as people get older, and shorter telomeres have been associated with several age-related conditions. This is why telomere length is sometimes offered as a biomarker of ageing.

The association is real but loose. Telomere length varies enormously between individuals of the same age, it differs between tissues within the same person, and it is shaped by inheritance, early-life conditions and measurement method. The 2021 Ageing Research Reviews analysis stresses that the length a person starts life with may matter more than the rate at which it later declines, and that a single measurement is a noisy snapshot rather than a precise clock. Telomere length is best read as one signal among many, not a verdict on how long or how well someone will live.

Why longer is not simply better

The countdown-clock framing invites an obvious conclusion: if short telomeres accompany ageing, longer telomeres must be better, and reactivating telomerase to lengthen them must be desirable. The evidence does not support that leap, and this is the most important nuance in the whole field.

The reason is cancer. A cancer cell must divide without limit, and to do so it has to overcome the telomere erosion that would normally stop it. Most cancers achieve this by switching telomerase back on. Telomere shortening, then, is not only a feature of ageing. It is also a brake on uncontrolled division, a barrier that a would-be tumour has to break through. Lengthening telomeres across the body, or switching telomerase on broadly, risks lowering that barrier. The 2024 Subcellular Biochemistry review is explicit that any strategy aimed at maintaining telomere length has to be weighed against this cancer risk.

This is not only a theoretical concern. Genetic studies that use inherited variants to estimate a person’s natural telomere length, an approach called Mendelian randomisation that helps separate cause from coincidence, have found that a predisposition to longer telomeres is associated with a higher risk of several cancers. A large analysis published in European Urology in 2017 reported that genetically inferred longer telomere length was associated with more than double the risk of renal cell carcinoma, the most common form of kidney cancer. Similar patterns have been described for other tumour types. Longer telomeres, in other words, are not an unalloyed good. The relationship between telomere length, ageing and disease points in different directions depending on the outcome you measure.

How telomeres connect to the wider biology of ageing

Telomeres do not act in isolation. When a telomere becomes critically short or damaged, the cell recognises it as a problem and can respond by entering a permanent halt to division, the state known as cellular senescence. A 2022 review in Nature Cell Biology sets out how telomere dysfunction is a recognised trigger for senescence, and how the accumulation of such cells is thought to contribute to the decline of tissues over a lifetime. In this way telomere attrition feeds into other processes researchers track when they study ageing, which is why it appears as one of the established hallmarks of ageing in the influential 2023 framework published in Cell.

What the evidence does and does not show

The mechanics are well established. Telomeres shorten with division, telomerase can replenish them, and critically short telomeres can drive cells into senescence. The link to human ageing and disease is genuine but more tangled than popular accounts allow. Telomere length is an average-level signal with wide individual variation, not a precise personal clock, and the assumption that longer is always better runs directly into the evidence on cancer risk. There is no established intervention shown to slow human ageing by altering telomere length, and any approach that lengthens telomeres has to reckon with the protective role that telomere shortening plays. The honest summary is that telomeres are a real and important part of the ageing story, that measuring or modifying them is harder and more double-edged than it first appears, and that this is a field to follow with interest and caution rather than treat as settled.

Further reading

Continue reading from the journal: The hallmarks of cellular ageing, in plain English and Cellular senescence, explained in plain English.

Sources

• Chakravarti D, LaBella KA, DePinho RA. Telomeres: history, health, and hallmarks of aging. Cell, 2021. doi:10.1016/j.cell.2020.12.028
• Rossiello F, Jurk D, Passos JF, d’Adda di Fagagna F. Telomere dysfunction in ageing and age-related diseases. Nature Cell Biology, 2022. doi:10.1038/s41556-022-00842-x
• Dunn PL, Logeswaran D, Chen JJ. Telomerase-Mediated Anti-Ageing Interventions. Subcellular Biochemistry, 2024. doi:10.1007/978-3-031-66768-8_1
• López-Otín C, Blasco MA, Partridge L, Serrano M, Kroemer G. Hallmarks of aging: an expanding universe. Cell, 2023. doi:10.1016/j.cell.2022.11.001
• Machiela MJ, Hofmann JN, Carreras-Torres R, et al. Genetic variants related to longer telomere length are associated with increased risk of renal cell carcinoma. European Urology, 2017. doi:10.1016/j.eururo.2017.07.015
• Lin J, Epel E. Stress and telomere shortening: insights from cellular mechanisms. Ageing Research Reviews, 2021. doi:10.1016/j.arr.2021.101507