TTAGGG caps shrink ~50-100 bp per division; below critical length cells senesce or undergo apoptosis.
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Telomere Shortening Across Cell Divisions
Telomeres are protective caps on the ends of your chromosomes, like the plastic tips on shoelaces. They prevent your genetic code from unraveling or fusing with neighboring chromosomes during cell division.
Telomere cap
Chromosome
Telomerase
Damage signal
TTAGGG repeat
Each time a somatic cell divides, the DNA replication machinery cannot fully copy the very ends of chromosomes. The result is a loss of roughly 50-100 base pairs from the TTAGGG repeat caps on each end, every generation. The diagram shows five stages of this process from newborn to senescent cell, with a dashed line marking the critical length (~5 kb) below which the cell interprets the short cap as DNA damage and halts division (senescence via p16/Rb) or self-destructs (apoptosis via p53).
Telomeres form G-quadruplex structures and recruit the shelterin complex, which hides the chromosome end from DNA-damage sensors. As caps shorten, shelterin protection weakens. ATM/ATR kinases then detect exposed single-stranded DNA, phosphorylate H2AX, and trigger p53 -> p21 -> Rb dephosphorylation, locking the cell in G1 arrest permanently.
Telomerase (TERT + TERC) reverse-transcribes the TTAGGG repeat back onto the 3-prime end, extending the cap. Telomerase is active in germline, embryonic, and adult stem cells, as well as ~85% of cancer cells. Most somatic cells have low or absent telomerase activity, which sets the Hayflick limit at roughly 40-70 divisions.
Blackburn, Greider, and Szostak shared the 2009 Nobel Prize in Physiology for telomere and telomerase discovery. Epidemiological data (Cawthon et al., Lancet 2003) links shorter leukocyte telomere length to higher all-cause mortality. Interventions shown to slow shortening in humans include exercise, stress reduction, and adequate sleep; none have reversed it reliably in large RCTs.
Telomeres and telomerase in ageing, disease, and cancer , Blackburn EH, Greider CW, Szostak JW (2006)
Hallmarks of aging: an expanding universe , Lopez-Otin C et al. (2023)
Good for
Longevity and aging articles
Telomere supplement explainers
Biology education
Source & accuracy
Blackburn EH et al. Science 2006; Lopez-Otin et al. Cell 2013 (hallmarks of aging)
How telomeres shorten with each division
Telomeres are repetitive DNA sequences, the hexamer TTAGGG in humans, that cap the ends of every chromosome and protect the coding DNA from erosion and from being mistaken for a broken strand. Each time a somatic cell divides, the DNA replication machinery cannot fully copy the very end of the lagging strand, a limitation known as the end-replication problem. As a result the telomere loses roughly 50 to 100 base pairs per division.
Newborn human telomeres measure on the order of 8,000 to 10,000 base pairs. Because the loss is incremental, telomere length acts as a rough mitotic clock: the more times a cell lineage has divided, the shorter its telomeres tend to be.
The Hayflick limit and senescence
When a telomere erodes below a critical length, the cell can no longer protect its chromosome ends and exits the cell cycle. It either enters replicative senescence, a stable non-dividing state, or undergoes apoptosis. This ceiling on the number of divisions a normal cell can complete is the Hayflick limit, typically around 40 to 60 divisions for cultured human fibroblasts.
The enzyme telomerase can extend telomeres by adding TTAGGG repeats, but it is largely switched off in most adult somatic tissue. It stays active in germ cells and stem cells, and is reactivated in the majority of cancers, which is one way tumour cells escape the normal division limit.
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