Telomeres are repetitive nucleotide sequences located at the ends of chromosomes that protect the ends from deterioration or fusion with other chromosomes. They shorten each time a cell divides until a length is reached that triggers cell senescence or death. The enzyme telomerase maintains telomere length by adding telomeric repeats to chromosome ends using its own RNA component as a template. Telomerase is active in germ cells, stem cells, and cancer cells, allowing indefinite cell division, but not in most somatic cells, limiting their replicative lifespan.