Telomere shortening is proposed as a mechanism of cellular senescence. Telomerase uses an internal RNA template to add TTAGGG repeats. In which cells is telomerase constitutively active?
- A Germline cells, hematopoietic stem cells, and most carcinoma cells — not in fully differentiated somatic cells ✓
- B All actively dividing somatic cells (rapidly dividing crypts, skin basal layer)
- C Only neurons and cardiomyocytes, because they are post-mitotic and need telomere preservation
- D Only hepatocytes, due to high regenerative capacity of the liver
Explanation
Telomerase (TERT: telomerase reverse transcriptase + TERC: RNA component) is constitutively active in germline cells (to maintain heritable telomere length) and in hematopoietic stem cells (for life-long hematopoiesis). Most differentiated somatic cells have negligible telomerase, leading to progressive telomere shortening with each division and eventual replicative senescence (Hayflick limit). Carcinoma cells reactivate telomerase as a prerequisite for immortalization — ~85% of human cancers express telomerase. Dyskeratosis congenita results from TERT/TERC mutations causing premature telomere shortening.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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Written and medically reviewed by the StethoPrep medical team.