Genome and Cell Entry
Hepadnaviruses possess a unique genetic apparatus that fundamentally distinguishes them from other viral families. Their genome is represented by a circular double-stranded DNA molecule, but with a characteristic structural anomaly: one of the strands is incomplete and contains a physical nick.
When the virion (such as the hepatitis B virus) enters a susceptible cell, its core is initially released into the cytoplasm. To initiate the infectious process, the viral genome must be purposefully transported into the cell nucleus, where the early stages of replication unfold.
Nuclear Phase: Repair and Transcription
Once inside the nucleus, the viral DNA undergoes an obligatory repair phase. The incomplete plus-strand is extended by cellular machinery, resulting in the formation of a stable, full-length covalently closed circular DNA (cccDNA).
This repaired molecule serves as an ideal template. Cellular DNA-dependent RNA polymerase initiates transcription, generating two critically important types of viral RNA:
- Messenger RNA (mRNA): Required for the subsequent synthesis of all viral proteins.
- Pregenomic RNA: Functions as a full-length template for future viral genome replication.
Cytoplasmic Phase: Translation and Assembly
The synthesized viral RNAs exit the nucleus and move back into the cytoplasm. Here, translation is initiated using the mRNA template. The cell actively produces viral components: capsid proteins, viral polymerase, and surface antigens.
Concurrently with protein synthesis, new nucleocapsids are assembled. The core proteins of the virus densely package around the pregenomic RNA molecule and newly synthesized viral polymerase molecules. This establishes the platform for the final stage of genome maturation.
Reverse Transcription and Exocytosis
The defining feature of hepadnaviruses is that the synthesis of new DNA occurs inside the forming capsid. The viral RNA-dependent DNA polymerase (reverse transcriptase) utilizes the pregenomic RNA as a template.
The process proceeds in a strict sequence:
- First, a full-length minus-strand DNA is synthesized on the pregenomic template.
- Then, based on the completed minus-strand, a plus-strand is synthesized, which remains incomplete, mimicking the original viral genome structure.
Once the nucleocapsid is formed, it is transported to the membranes of the endoplasmic reticulum (ER) or the Golgi apparatus. There, interacting with regions containing the surface antigen (HBsAg), the virion acquires an envelope (supercapsid). Fully formed viral particles exit the host cell via exocytosis.