Telomeric

Telomeric is an adjective relating to telomeres, the protective, repetitive nucleotide sequences at the ends of linear chromosomes. In humans the canonical telomeric repeat is TTAGGG, present in many thousands of copies at chromosome ends, where they help preserve genetic information during replication.

Telomeres are bound by the shelterin protein complex, which includes TRF1, TRF2, POT1, TIN2, TPP1, and RAP1, and they form a t-loop structure that protects chromosome ends from being recognized as DNA damage. With each cell division, the end-replication problem causes telomeres to shorten unless ends are lengthened by a maintenance mechanism; critically short telomeres can trigger cellular senescence or apoptosis.

Most somatic human cells lack active telomerase, the ribonucleoprotein reverse transcriptase that extends telomeres by adding TTAGGG repeats using an RNA template (TERC). In contrast, germ cells, certain stem cells, and many cancers express telomerase, enabling longer telomeres and continued cell proliferation. Some organisms and cancers also use alternative lengthening of telomeres (ALT), a telomerase-independent pathway based on homologous recombination to maintain telomere length.

Telomere length and integrity are studied as biomarkers of aging, stress, and carcinogenesis, though measurement can vary by tissue type and assay. Telomeres can form secondary structures such as G-quadruplexes, which can influence replication dynamics and nearby gene expression through effects like telomere position effect.