Telomere Shortening
Telomeres are protective caps of repetitive DNA sequences located at the ends of linear chromosomes. In humans, these sequences consist of thousands of repeats of the six-base pattern TTAGGG. Telomeres prevent chromosome ends from degrading and fusing with other chromosomes, thereby preserving the integrity of genetic information. However, telomeres cannot be fully replicated during cell division due to a fundamental limitation in DNA replication machinery, a phenomenon known as the end-replication problem.
The End-Replication Problem
DNA polymerase, the enzyme that copies DNA, can only synthesize new DNA strands in one direction and requires an RNA primer to begin synthesis. When DNA polymerase reaches the extreme end of a chromosome and the primer is removed, a small gap remains that cannot be filled. As a result, approximately 50–200 base pairs of telomeric sequence are lost with each cell division. Over successive generations, this progressive shortening gradually reduces the protective telomeric buffer.
Cellular Senescence
After approximately 50–70 cell divisions—a limit known as the Hayflick limit—telomeres become critically short. When telomeres reach this critical length, they trigger cellular senescence, a state in which cells cease dividing and eventually die. This mechanism acts as a built-in counter that limits the replicative potential of normal somatic cells. Telomere shortening is therefore considered a key driver of cellular aging and contributes to age-related decline in tissues that depend on frequent cell renewal, such as bone marrow and skin.
Certain cell types, including germ cells and most cancer cells, express the enzyme telomerase, which can rebuild telomeric sequences and extend cellular lifespan beyond the normal limit. In typical somatic cells, however, telomerase activity is absent or minimal, making telomere shortening an essentially irreversible process.