Retrovirus and Viral Replication

Introduction to Retroviruses

• Retrovirus: A special case of viruses that don't fit neatly into lytic or lysogenic categories.
• Example of retrovirus: HIV.

Characteristics of Retroviruses

• Enveloped, single-stranded RNA viruses.
• Carry three special proteins: reverse transcriptase, integrase, and protease.

Entry into the Host Cell

• Enters via direct fusion (e.g., HIV).
• The nucleocapsid undergoes uncoating, releasing its contents.

Reverse Transcription

• Reverse transcriptase:
  • Converts RNA to complementary DNA (cDNA).
  • Produces a second cDNA strand, forming double-stranded DNA.
• This process is called reverse transcription because it reverses the typical flow of genetic information (RNA → DNA).

Integration into Host Genome

• Integrase enzyme:
  • Clips 3' ends of the cDNA creating sticky ends.
  • Integrates viral DNA into the host's DNA.
• Provirus Stage:
  • Viral DNA integrated into host DNA.
  • Actively transcribed along with host DNA.
  • Differentiates from lysogenic cycle by lacking repressor gene.

Viral mRNA and Protein Synthesis

• Viral mRNA produced from integrated DNA exits the nucleus.
• Translated into proteins including:
  • Capsid proteins.
  • Viral proteins: reverse transcriptase, integrase, and protease.

Assembly and Maturation of New Viruses

• Newly formed components self-assemble into new viruses.
• Immature viruses initially lack an envelope.
• Bud off from host cell to acquire an envelope.
• Protease cleaves other proteins to ensure functionality.

Summary

• Retrovirus replication is more complex with elements of both lysogenic and lytic cycles.
• Important aspects include reverse transcription, integration into host genome, and maturation via budding and protease action.