What are telomeres and what is the role of telomerase?

Telomeres are repetitive DNA sequences (TTAGGG in humans) at chromosome ends that protect against degradation and fusion. Due to the end-replication problem, telomeres shorten with each cell division, eventually triggering senescence or apoptosis - this limits cell lifespan (Hayflick limit). Telomerase is a ribonucleoprotein enzyme containing an RNA template and reverse transcriptase (TERT) that extends telomeres. It is active in stem cells, germ cells, and ~90% of cancers, enabling unlimited proliferation. Telomere length serves as a biological clock and potential therapeutic target for aging and cancer.