Lecture Notes on Viruses

Overview of Viruses

• Unique group of pathogens
• Simple acellular organization
• Major cause of disease

Structure of Viruses

• No cytoplasmic membrane, cytosol, or functional organelles
• Capable of infecting all cell types, including bacteria (bacteriophages)
• Do not possess metabolic activity on their own
• Rely on host cells for replication and production of viral components

Viral States

Extracellular State

• Known as a virion
• Inactive and incapable of reproduction
• Consists of:
  • A capsid (protein coat)
  • Nucleic acid core (genetic material)
  • May have an envelope (phospholipid membrane from host cell)

Intracellular State

• Capsid removed; virus becomes active
• Exists solely as nucleic acids
• Induces host to synthesize viral components
• Baryons are assembled and released

Capsid and Viral Shapes

• Capsid protects viral genome and aids in transfer
• Three main shapes:
  1. Helical
     • Central cavity or hollow tube made of proteins
     • Example: Tobacco mosaic virus
  2. Icosahedral
     • Made of equilateral triangles forming a spherical shape
     • Examples: Poliovirus, Rhinovirus, Adenovirus
  3. Complex
     • Combination of icosahedral and helical shapes
     • Includes head-tail morphology (specific to bacteriophages)
     • Head: icosahedral shape, Tail: helical shape

Viral Genetic Material

• Contains viral genome (genetic material)
• Can be:
  • DNA or RNA
  • Single-stranded or double-stranded
  • Linear or circular
• Total genetic content is usually small
• Encodes proteins not present in host cell

Replication Processes

• DNA Viruses:
  • Enter the nucleus
  • Use host's DNA polymerase to replicate
• RNA Viruses:
  • Often remain in the cytoplasm
  • Use host's ribosomes to produce viral RNA polymerase
• Higher mutation rates in RNA viruses compared to DNA viruses due to replication errors

Summary

• Viruses are unique infectious agents with a simple structure and distinct replication patterns
• They exist in two states: extracellular (inactive) and intracellular (active)
• Virions consist of a capsid and nucleic acid core, with some having an envelope
• Viruses exhibit diverse shapes based on capsid symmetry, including helical, icosahedral, and complex forms.