Notes on Viruses

Introduction to Viruses

• Viruses are not considered alive.
• They are smaller and simpler than unicellular organisms (e.g., bacteria).
• Do not meet biological criteria for life:
  • No metabolism (cannot make energy from food).
  • Cannot reproduce on their own.
• Viruses exist in a gray area between simple molecules and living organisms.

Structure of Viruses

• Basic structure: genetic material (DNA or RNA) encased in a protein shell (capsid).
  • No membrane or organelles present.
• Types of viruses:
  • Rod-shaped (e.g., Tobacco Mosaic Virus)
  • Icosahedral (e.g., Adenovirus)
  • Membranous envelope (e.g., Influenza Virus)
  • Bacteriophages (combination of rod-shaped and icosahedral with fiber tails).
• Genetic material can be:
  • Double-stranded or single-stranded
  • Circular or linear in form.
• Capsid made of subunits called Capsomeres.

Viral Reproduction

• Viruses reproduce by hijacking host cell machinery.
• Recognition between virus and host cell is crucial.
  • Specificity for surface receptors on host cells.
• Infection process:
  • Viruses inject genetic material or are taken into the cell via endocytosis.
  • Viral DNA is transcribed and translated by the host cell's machinery.
  • New viral particles are assembled and exit the host cell, often causing damage.

Viral Replication Mechanisms

1. Lytic Cycle:
   • Host cell is destroyed at the end of the cycle.
   • Cell bursts (lysis) releasing viruses to infect others.
2. Lysogenic Cycle:
   • Host cell survives; viral DNA integrates into the host genome (prophage).
   • Prophage can remain dormant until triggered to enter the lytic cycle.

Other Types of Infectious Agents

• Retroviruses:
  • Contain reverse transcriptase, transcribing RNA to DNA.
• Viroids:
  • Naked circular RNA molecules that disrupt regulatory processes in plants.
• Prions:
  • Infectious proteins causing aggregation of brain cell proteins, linked to diseases like Alzheimer's and Parkinson's.

Evolutionary Dynamics

• Bacteria and viruses are in a constant evolutionary race.
• Chance mutations can lead to resistance in bacteria, while viruses can mutate to exploit new receptors.
• The origin of viruses is debated, likely emerging shortly after unicellular life.
• Some anomalous viruses contain a significant number of genes, related to cellular genomes.

Conclusion

• Discussion of specific diseases caused by viruses and combat strategies is reserved for pathology courses.
• Next topic will cover the biological structure of the simplest organisms.