70 Telomerase, a RNA-protein complex which completes the replication of telomeres during DNA synthesis, is a specialized

(1) RNA dependent DNA polymerase

(2) DNA dependent DNA polymerase

(3) DNA dependent RNA polymerase

(4) RNA dependent RNA polymerase

 

---

Introduction

Telomerase is a unique ribonucleoprotein enzyme complex that plays a vital role in eukaryotic chromosome replication by extending telomeres—the repetitive DNA sequences at chromosome ends. Unlike conventional DNA polymerases that use DNA as a template, telomerase uses an RNA template to synthesize DNA, classifying it as an RNA-dependent DNA polymerase or a specialized reverse transcriptase. This activity is crucial for maintaining chromosome integrity and cellular longevity.

---

What Is Telomerase?

Telomerase consists of two main components:

• TERT (Telomerase Reverse Transcriptase):
  The catalytic protein subunit that synthesizes DNA using reverse transcriptase activity.

• TERC (Telomerase RNA Component):
  An intrinsic RNA molecule that provides the template for adding telomeric repeats.

Together, these components enable telomerase to add repetitive DNA sequences (e.g., TTAGGG in humans) to the 3′ ends of chromosomes, compensating for the inability of regular DNA polymerases to fully replicate chromosome termini.

---

Why Is Telomerase an RNA-Dependent DNA Polymerase?

• RNA Template Usage:
  Telomerase carries its own RNA template (TERC) within the enzyme complex, which it reverse transcribes into DNA. This is distinct from DNA polymerases that rely on a DNA template.

• Reverse Transcriptase Activity:
  The catalytic subunit TERT shares structural and functional similarities with viral reverse transcriptases, enabling DNA synthesis from an RNA template.

• Telomere Extension:
  By using the RNA template, telomerase adds telomeric repeats to chromosome ends, preventing progressive shortening during DNA replication.

---

Role of Telomerase in DNA Replication

During DNA replication, the end-replication problem arises because DNA polymerases cannot fully replicate the 3′ ends of linear chromosomes. This results in gradual telomere shortening, which limits cellular lifespan. Telomerase solves this by:

1. Binding to the 3′ Overhang of Telomeres:
   Recognizing chromosome ends requiring extension.

2. Using RNA Template to Synthesize DNA:
   Adding telomeric repeats to lengthen the chromosome end.

3. Allowing Complete Replication:
   Providing a substrate for DNA polymerases to fill in complementary strands, maintaining telomere length.

---

Distinction from Other Polymerases

---

Clinical and Biological Significance

• Cellular Aging:
  Telomerase activity is low or absent in most somatic cells, leading to telomere shortening and cellular senescence.

• Stem Cells and Germ Cells:
  High telomerase activity maintains telomere length, supporting continuous cell division.

• Cancer:
  Many cancer cells reactivate telomerase, enabling unlimited proliferation.

---

Final Answer

(1) RNA dependent DNA polymerase

---

Keywords

telomerase, RNA-dependent DNA polymerase, reverse transcriptase, telomere replication, TERT, TERC, chromosome ends, end-replication problem, DNA synthesis, genomic stability, cellular aging, cancer

---

Conclusion

Telomerase is a specialized RNA-dependent DNA polymerase that extends telomeres by reverse transcribing its RNA template into DNA repeats at chromosome ends. This unique enzymatic activity is essential for overcoming the end-replication problem, maintaining chromosome integrity, and regulating cellular lifespan. Its distinct mechanism sets it apart from other polymerases involved in DNA and RNA synthesis.

---

Correct option:
(1) RNA dependent DNA polymerase