Viral replication cycles

Viral Replication - The Host Hijack

• Goal: Seize host cell machinery to create viral progeny.
• Phases: Follows a specific sequence to successfully propagate.

• Synthesis is Key:
  • DNA viruses: Replicate in the nucleus (most).
  • RNA viruses: Replicate in the cytoplasm (most).
  • Uses host ribosomes, enzymes, and nutrients.
• Release Mechanisms:
  • Lysis: Host cell ruptures (e.g., adenoviruses).
  • Budding: Acquires envelope from host membrane (e.g., influenza, HIV).

⭐ Poxvirus (a DNA virus) replicates in the cytoplasm. Influenza virus and retroviruses (RNA viruses) replicate in the nucleus.

Lytic vs. Lysogenic - Boom or Doom

Two primary replication strategies for bacteriophages. The choice determines the immediate fate of the host cell: rapid destruction or a period of dormancy.

• Lytic Cycle (Virulent Phages): Rapidly hijacks host machinery to produce new virions, culminating in host cell lysis and release. "Boom!"
• Lysogenic Cycle (Temperate Phages): Viral DNA integrates into the host genome, becoming a prophage. It replicates silently with the host cell. "Delayed Doom."

📌 Mnemonic: Lysogenic = Latent/Latent; Lytic = Lysis.

⭐ Lysogenic Conversion: The host bacterium acquires new properties from the integrated prophage. This is a major mechanism for virulence factor acquisition, such as the production of toxins in Corynebacterium diphtheriae (Diphtheria toxin) and Vibrio cholerae (Cholera toxin).

Viral Blueprints - DNA, RNA & Retro

• DNA Viruses: Most replicate in the nucleus, using host cell polymerases.
  • All are dsDNA, except Parvoviridae (ssDNA).
  • 📌 Mnemonic: "HHAPPPy" viruses (Hepadna, Herpes, Adeno, Pox, Parvo, Papilloma, Polyoma).
  • ⚠️ Exception: Poxvirus replicates in the cytoplasm.
• RNA Viruses: Most replicate in the cytoplasm, using their own viral RNA-dependent RNA polymerase.
  • All are ssRNA, except Reoviridae (dsRNA).
  • ⚠️ Exceptions: Influenza virus and Retroviruses replicate in the nucleus.
• Retroviruses (e.g., HIV):
  • Use reverse transcriptase to create dsDNA from their ssRNA(+) genome. This dsDNA integrates into the host genome as a provirus.

⭐ Poxviruses, being DNA viruses that replicate in the cytoplasm, must bring their own machinery, including a DNA-dependent RNA polymerase. They are unique in this regard.

High‑Yield Points - ⚡ Biggest Takeaways

• Viral entry hinges on receptor binding, followed by endocytosis or membrane fusion.
• Most DNA viruses replicate in the nucleus; most RNA viruses replicate in the cytoplasm.
• Key exceptions: Poxvirus (DNA in cytoplasm) and Influenza/Retroviruses (RNA in nucleus).
• (+) sense RNA is directly translated; (-) sense RNA needs its own RNA-dependent RNA polymerase.
• Retroviruses uniquely use reverse transcriptase to integrate into the host genome.
• Enveloped viruses exit via budding; naked viruses cause cell lysis.