Intro to Virology

• Viruses


  Obligate intracellular parasites that contain either DNA or RNA which depends on the synthetic machinery of the cell for replication of specialized elements that can transfer the viral genome to other elements
• Characteristics of Viruses


  • Small size (10-300 nm)
  • Contains only a single type of nucleic acid as their genome
  • Replicate from their own genetic material utilizing different replication strategies, not by the process of binary fission
  • Obligate intracellular parasites that are dependent on cellular energy production and cellular machinery for protein synthesis
  • Lack the capacity to make energy or substrates, cannot make their own proteins
  • Synthesize their parts separately and then rely on spontaneous macromolecular interactions for their maturation
  • Not sensitive to the usual antibiotics
• Nucleic acid (viral) genome

  Contains the information necessary for virus multiplication
• Types of viral nucleic acid

  • DNA
  • RNA
  • Double-stranded
  • Single-stranded
  • Linear
  • Circular
  • Segmented
  • Unsegmented
• Capsid


  Composed of structural units called capsomers which are clusters of polypeptide
• Functions of capsid


  • Protection of viral genome from both physical destruction and enzymatic hydrolysis of host cell nucleases
  • Receptor site necessary for naked viruses to initiate infection
  • Stimulates antibody production
  • Site of antigenic determinants important in some serologic tests
• Nucleocapsid


  Nucleic acid + capsid
• Types of nucleocapsid symmetry

  • Helical
  • Icosahedral
  • Complex
• Envelope


  Surrounds the nucleocapsid of enveloped viruses, membrane-like lipid bilayer with viral coded glycoprotein (spikes or peplomers) & cell derived lipids
• Importance of envelope


  • Contains molecules necessary for enveloped viruses to initiate infection
  • Acts as stimulus for antibody production
  • Serves as antigen in serologic tests
• Virion or viral particle

  The entire virus, including nucleic acid, capsid, envelope & glycoprotein spikes
• Viral features important for classification

  • Nucleic acid type
  • Nucleic acid strandedness
  • Physical nature of viral nucleic acid
  • Size of viral genome
  • Location of nucleic acid replication & assembly
  • Symmetry
  • Replication strategy
  • Virion diameter & capsomer number
  • Site of nucleocapsid involvement
  • Enzymes
  • Ether sensitivity
  • Antigenic cross reactivity
  • Symptomatology
• Virus Multiplication Cycle


  1. Eclipse period

  2. Adsorption or attachment

  3. Penetration

  4. Uncoating

  5. Synthetic phase

  6. Assembly & maturation

  7. Release of progeny virus
• Pathogenesis of Viral Infections

  • Virus entry, dissemination, excretion & transmission
  • Replication of virus & damage to cells
  • Spread to other cells & organs
• Nonspecific Defenses


  • Anatomic Barrier
  • Non-specific inhibitor
  • Phagocytosis
  • Fever & inflammation
  • Interferon
• Specific Defenses


  • Humoral Immunity
  • Cell-mediated immunity