Q.52 Which of the following statements is/are true about telomerase?
(A) Telomerase has 5′-3′ DNA-dependent DNA polymerisation activity
(B) Telomerase has 5′-3′ RNA-dependent DNA polymerisation activity
(C) Telomerase contains an RNA subunit
(D) Telomerase has 3′-5′ DNA-dependent DNA polymerisation activity

Telomerase is a specialized ribonucleoprotein enzyme critical for maintaining telomere length at chromosome ends. This article breaks down a common exam question on its properties, identifying correct statements among the options.

Correct Answer

Both (B) and (C) are true. Telomerase exhibits 5′-3′ RNA-dependent DNA polymerization activity and contains an essential RNA subunit.​

Option Breakdown

• (A) Telomerase has 5′-3′ DNA-dependent DNA polymerisation activity: False. Standard DNA polymerases (like Pol δ/ε) are DNA-dependent, using DNA templates; telomerase instead uses its internal RNA template, functioning as a reverse transcriptase.​

• (B) Telomerase has 5′-3′ RNA-dependent DNA polymerisation activity: True. The catalytic subunit (TERT) adds telomeric repeats (e.g., TTAGGG in humans) in the 5′-3′ direction, templated by the RNA component (TERC).​

• (C) Telomerase contains an RNA subunit: True. TERC provides the template sequence for DNA synthesis and aids in processivity.​

• (D) Telomerase has 3′-5′ DNA-dependent DNA polymerisation activity: False. No polymerase, including telomerase, synthesizes in the 3′-5′ direction; proofreading exonucleases act 3′-5′, but telomerase lacks this.​

Telomerase Mechanism

Telomerase extends the 3′ overhang of telomeres via repeated cycles: RNA template annealing, 5′-3′ DNA synthesis, translocation, and repeat addition. This counters the end-replication problem, active mainly in stem, germ, and cancer cells.​