What lengthens telomeres?

Telomeres can be lengthened by the enzyme telomerase. Telomerase is responsible for adding repetitive DNA sequences to the ends of chromosomes (telomeres), counteracting the natural shortening of telomeres that occurs during cell division. In certain cells, such as stem cells, germ cells, and many cancer cells, telomerase is active and maintains telomere length. Telomerase uses an RNA template to extend the telomeres by adding nucleotide repeats. However, in most somatic cells, telomerase is inactive, leading to progressive telomere shortening.