Hepadnaviruses (HBV) – Unique replication: Hepatitis B virus is a DNA virus that replicates its genome using which key enzyme activity?

Introduction:
Hepadnaviruses, especially hepatitis B virus (HBV), possess a partially double-stranded DNA genome but replicate via an RNA intermediate. This replication strategy distinguishes them from typical DNA viruses and underlies the use of reverse-transcriptase inhibitors in therapy.

Given Data / Assumptions:

• HBV genome: relaxed circular, partially double-stranded DNA.
• Replication involves pregenomic RNA (pgRNA) as an intermediate.
• The key step resembles retroviral reverse transcription.

Concept / Approach:
HBV encodes a polymerase with multiple activities: reverse transcriptase to copy pgRNA into DNA, RNase H to degrade RNA in RNA–DNA hybrids, and DNA-dependent DNA polymerase to complete the second DNA strand. The signature, however, is reverse transcription of an RNA intermediate.

Step-by-Step Solution:
Step 1: Identify pgRNA as the template for DNA synthesis.Step 2: Recognize reverse transcriptase activity as essential for copying RNA to DNA.Step 3: Understand supporting roles of RNase H and DNA polymerase activities within the HBV polymerase.Step 4: Choose “Reverse transcriptase.”

Verification / Alternative check:
Antiviral nucleos(t)ide analogs (e.g., entecavir, tenofovir) target reverse transcription, clinically validating the mechanism.

Why Other Options Are Wrong:

• DNA-dependent DNA polymerase only: incomplete; ignores RNA intermediate.
• RNase H only/DNA ligase: accessory or unrelated to primary genome synthesis step.
• RNA-dependent RNA polymerase: used by RNA viruses, not HBV.

Common Pitfalls:
Assuming all DNA viruses replicate purely with DNA polymerases; overlooking HBV’s RNA intermediate.

Final Answer:
Reverse transcriptase.