Structure of viruses

• Contain either DNA or RNA
• Capsid made up of repeated subunits of a virally encoded protein
• Icosahedral symmetry: virus consists of repeated subunits that make up equilateral triangles arranged in a symmetrical fashion
• Helical symmetry: made up of a single repeated unit

Viruses and disease

1. Viral replication
2. Attachment - interact with specific receptors in target cell
3. Entry - endocytosis
4. Uncoating - viral nucleic acid released from capsid
5. Nucleic acid and protein synthesis – host ribosomes used (host polymerases may also be used) to produce new viral proteins
6. Assembly - nucleic acids and proteins packaged together
7. Release
   • Budding - virus released with envelope derived from host cell membrane, doesn’t kill cell
   • Lysis - viruses accumulates until cell bursts, killing cell

Pathogenesis

• Cell death due to lysis
• Cell death due to immune system
• Cell proliferation - cancer

Immune response

• Cytotoxic T lymphocytes - recognise foreign cell surface proteins, signal to infected cell to commit suicide
• Neutralising antibodies - IgG, IgM