Lecture Notes: Subviral Particles

Introduction to Subviral Particles

• Definition: Smaller than viruses; categorized as nonliving infectious agents.
• Characteristics:
  • Nonliving: Acellular, lack organelles, can't make ATP, can't reproduce/divide independently.
  • Infectious agents: Require a host to perform necessary functions.

Subviral Particles

• Two main types:
  • Viroids
  • Prions

Viroids

• Structure: Single strand of circular RNA.
• Infection:
  • Historically infected plants (illustrated with green color).
  • Recently found in humans (e.g., Hepatitis D).
• Replication: Thought to be catalytic RNA (self-cleaving to create more viroids).
• Important Distinction:
  • Viroids vs. Virions:
    • Viroids: Single strand of RNA without protein coat.
    • Virions: Whole viruses (RNA/DNA + protein coat and possibly an envelope).

Prions

• Origin: Recently discovered, debated if proteins alone can be infectious.
• Structure: Proteinaceous infectious particles, made only of proteins.
  • Prion Proteins (PrP): Typically in beta-sheet conformation.
  • Normal Proteins: Generally in alpha-helix form.
• Infectious Mechanism:
  • Prion proteins (beta-sheets) convert normal proteins (alpha-helix) into beta-sheets.
  • Results in protein deposits.
  • Consequences: If occurring in the brain, cleanup processes create holes causing diseases.
• Comparison with Other Types:
  • No genetic material (DNA/RNA), unlike viruses and viroids.

Summary

• Subviral particles are distinct from traditional viruses due to their size and lack of genetic material or complete viral structures.
• Important to distinguish between viroids and virions.
• Prions represent a unique category due to their protein-only composition and mechanism of infection.