Chapter 12: Viruses, Viroids, and Prions

Acellular Infectious Particles

• Types: Viruses, viroids, prions
• Infect a range of hosts, including humans, animals, plants, and bacteria

Viruses

• Size: Measured in nanometers; much smaller than bacteria
  • Example sizes: Influenza virus (~150nm), Rhinovirus (~50nm)
• Visualisation: Requires electron microscopes
• Genetic Material: DNA or RNA (never both)
• Metabolism: None outside of a host cell

Characteristics

• Do not grow or respond to the environment
• Cannot reproduce independently; rely on host cell’s metabolic pathways
• States:
  • Extracellular: Has protein coat (capsid) or phospholipid envelope surrounding the capsid
  • Intracellular: Capsid is removed, virus only as nucleic acid
  • Capsid: Made of capsomeres
  • Envelope: Contains protein spikes for host cell recognition

Classification

• Naked vs Enveloped: Determined by outer layer structure
• Genetic Material:
  • DNA or RNA
  • Double-stranded or single-stranded
  • Linear, segmented, or circular
• Host Specificity:
  • Species-specific (e.g., HIV infects human T cells)
  • Generalists (e.g., Rabies infects humans and animals)

Replication

• Animal Viruses:

  • Recognition and attachment
  • Entry via fusion or endocytosis
  • Synthesis of viral parts using host enzymes
  • Assembly and release (budding, exocytosis, or lysis)

• Bacteriophage:

  • Lytic Cycle: Results in cell lysis
  • Lysogenic Cycle: Viral DNA integrates with host DNA as prophage
    • Induction by DNA damage transitions to lytic cycle

Role in Human Cancers

• 20-25% of human cancers linked to viral infections
• Examples: Kaposi's sarcoma, cervical cancer (HPV)

Viroids

• Structure: Small, circular RNA without capsid
• Infection: Known only in plants
• Significance: Can cause reduced yield in crops

Prions

• Composition: Infectious protein, no nucleic acid
• Diseases: Spongiform encephalopathies (e.g., mad cow disease)
• Resistant: To heat, UV light, proteases
• Destruction: Only by incineration or autoclaving in sodium hydroxide
• Effects: Fatal neurological degeneration, vacuoles in brain tissue

Conclusion

• Viruses, viroids, and prions are non-living but have certain living characteristics.
• They are significant due to their role in diseases and infection across various hosts.