Which enzyme catalyzes the lengthening of telomeres in eukaryotic cells?

Which enzyme catalyzes the lengthening of telomeres in eukaryotic cells?

• A. Telomerase
• B. DNA ligase
• C. RNA polymerase
• D. Helicase

Answer: (A)

Telomeres are protected by a special enzyme that carries its own RNA template and acts as a reverse transcriptase. This enzyme lengthens the ends of chromosomes by adding telomeric repeats to the 3’ end, using its RNA component as the template. This activity counteracts the end-replication problem, which would otherwise shorten chromosome ends with each division. After telomerase extends the 3’ overhang, conventional DNA polymerases can fill in the rest, preserving chromosome integrity. This enzyme is especially active in germ cells and many stem cells, and is often reactivated in cancer cells, while most somatic cells have little activity, leading to gradual telomere shortening. The other enzymes don’t lengthen telomeres: DNA ligase seals nicks in DNA, RNA polymerase synthesizes RNA from a DNA template, and helicase unwinds DNA.

Telomeres are protected by a special enzyme that carries its own RNA template and acts as a reverse transcriptase. This enzyme lengthens the ends of chromosomes by adding telomeric repeats to the 3’ end, using its RNA component as the template. This activity counteracts the end-replication problem, which would otherwise shorten chromosome ends with each division. After telomerase extends the 3’ overhang, conventional DNA polymerases can fill in the rest, preserving chromosome integrity. This enzyme is especially active in germ cells and many stem cells, and is often reactivated in cancer cells, while most somatic cells have little activity, leading to gradual telomere shortening. The other enzymes don’t lengthen telomeres: DNA ligase seals nicks in DNA, RNA polymerase synthesizes RNA from a DNA template, and helicase unwinds DNA.