Lecture Notes: Viruses

Introduction to Viruses

• Definition: Viruses are obligate intracellular parasites and considered non-living as they require a host for replication.
• Structure: Consists of a capsid, genome, and possibly an envelope.
• Size Comparison: Smaller than 0.2 microns, not visible under a compound microscope, require electron microscopes.
• Variety of Shapes: Helical, polyhedral, enveloped, complex.

Viral Structure

• Genetic Material: Can be either DNA or RNA, not both.
  • DNA/RNA can be single or double-stranded.
• Capsid: Protein coat made of individual units called capsomers. Protects the nucleic acid.
• Envelope: Some viruses have a lipid envelope with possible spike proteins.
  • Examples: COVID-19 (enveloped virus), Influenza.

Virus Types

• Enveloped Viruses: Have a lipid envelope and spike proteins.
• Naked Viruses: Only have a capsid without an envelope.
• Examples of Specific Viruses:
  • Ebola: Causes hemorrhagic fever.
  • Bacteriophage: Infects bacteria, has unique structures.

Coronavirus (COVID-19)

• Structure: Single-stranded positive-sense RNA, enveloped with spike proteins for attachment.
• Protein Components:
  • S Protein (Spike): Used for cell entry.
  • M Protein: Maintains viral shape.
  • E Protein: Assists viral assembly.
  • N Protein: Binds to RNA within the nucleocapsid.

Virus Replication

• Lytic Cycle:
  • Steps: Attachment, entry, synthesis (using host machinery), assembly, and release via cell lysis.
• Lysogenic Cycle:
  • Steps: Attachment, entry, integration into host genome as a prophage, replication with host cell.
  • Can eventually enter the lytic cycle if excised from the host genome.
• Benefits to Host:
  • Phage conversion provides new genes/virulence factors.
  • Specialized transduction can occur, transferring genetic material.

Laboratory Growth of Viruses

• Challenges: Require a host, do not grow on standard agar plates.
• Animal Viruses: Grown in tissue culture or bird embryos.
• Bacteriophages: Grown by infecting bacterial lawns on agar plates.
  • PFUs (plaque-forming units) indicate virus presence as clear areas.

Summary

• Viruses vary in complexity and structure but share common pathways for replication and infection.
• Understanding their structure and replication cycles is crucial for developing treatments and vaccines.
• Specific examples such as COVID-19 and bacteriophages highlight the diversity and impact of viral infections.