Chapter 16.3: Viruses

Overview

• Focus on viruses, not considered truly living organisms.
• Viruses are infectious particles, not cells.
• Basic structure: protein coat (capsid) containing genetic material (DNA or RNA).

Virus Structure

• Capsid: Protein shell housing genetic material.
  • Helical Capsid: Simple shape.
  • Complex Capsid: Multiple components (e.g., helical, head, tail).
  • Capsomerin: Protein subunits making up the capsid.
• Viral Envelope: Membranous cover containing host cell molecules.
  • Found mostly in animal viruses.
  • Contains glycoproteins for host cell recognition and binding.

Virus Types

• DNA Viruses and RNA Viruses: Named based on the type of genetic material.
• Bacteriophages: Viruses that infect bacteria.
• Retroviruses: Use reverse transcriptase to convert RNA to DNA.
  • Example: HIV.

Virus Life Cycles

• Lytic Cycle:
  • Virus infects, replicates, and causes host cell to burst (lysis).
  • Produces new viruses, known as virulent.
• Lysogenic Cycle:
  • Viral DNA integrates into host genome as a prophage.
  • Host cell reproduces with viral DNA inside.
  • Can switch to lytic cycle under certain conditions.

Host Range and Specificity

• Viruses have a specific host range determined by their structure and surface proteins.
• Cross-species transmission:
  • Humans and pigs share many genetic markers, enabling viral transfer.
  • Pigs can exchange viruses with domesticated and wild fowl, leading to potential new strains affecting humans (e.g., swine and bird flu).

Mechanisms of Viral Mutation

• Viral genetic material often mutates, leading to new virus strains.
• Eukaryotic cells have mechanisms to prevent rapid mutation, unlike viruses.

Applications in Biotechnology

• Restriction Enzymes: Derived from studies of bacteriophages.
  • Cut DNA at specific sequences, used for gene cloning.
• Vaccines: Derived from viral components to stimulate immune defense.

Additional Infectious Agents

• Viroids: Infectious RNA molecules affecting plants.
• Prions: Misfolded proteins causing diseases like mad cow disease and Creutzfeldt-Jakob disease.
  • Convert normal proteins into misfolded prion state.

Conclusion

• Understanding viruses is essential for fields like medicine, biotechnology, and epidemiology.
• Viruses play a significant role in gene therapy, vaccine development, and understanding cellular processes.