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An Introduction to the Viruses. Viral Components. All viruses have capsids- protein coats that enclose & protect their nucleic acid Viruses may have a membrane envelope Nucleic acid genome: may be dsDNA, ssDNA, ssRNA, dsRNA. Size Comparison of Viruses. Viral Taxonomy.

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viral components
Viral Components
  • All viruses have capsids- protein coats that enclose & protect their nucleic acid
  • Viruses may have a membrane envelope
  • Nucleic acid genome: may be dsDNA, ssDNA, ssRNA, dsRNA
viral taxonomy
Viral Taxonomy
  • Main criteria presently used are structure, chemical composition, and genetic makeup
  • Currently recognized: 3 orders, 63 families, and 263 genera of viruses
  • Family name ends in -viridae, i.e.,Herpesviridae
  • Genus name ends in -virus, Simplexvirus
  • Herpes simplex virus I (HSV-I)
host range
Host range
  • Spectrum of cells a virus can infect
    • cell must have a specific structure (receptor) on its surface for viral attachment
    • cell must contain all of the enzymes and materials needed to produce new virions
  • May be one species or many
  • May be one tissue or many within a host
animal virus replication
Animal Virus Replication
  • Adsorption
  • Penetration/uncoating of genome
  • Duplication/synthesis
  • Assembly
  • Release
medical importance of viruses
Medical Importance of Viruses
  • Viruses are the most common cause of acute infections
  • Several billion viral infections per year
  • Some viruses have high mortality rates
  • Possible connection of viruses to chronic afflictions of unknown cause
cytopathic effects virus induced damage to cells
Cytopathic effects: virus-induced damage to cells
  • Changes in size & shape
  • Cytoplasmic inclusion bodies
  • Nuclear inclusion bodies
  • Cells fuse to form multinucleated cells
  • Cell lysis
  • Alter DNA
  • Transform cells into cancerous cells
other noncellular infectious agents
Other noncellular infectious agents
  • Prions - misfolded proteins, contain no nucleic acid
    • cause spongiform encephalopathies – holes in the brain
    • common in animals
      • scrapie in sheep & goats
      • bovine spongiform encephalopathies (BSE), aka “mad cow disease”
      • humans – Creutzfeldt-Jakob Disease
  • Viroids - short pieces of RNA, no protein coat
    • only identified in plants, so far
diagnosis of viral diseases
Diagnosis of viral diseases
  • More difficult than other agents
  • Consider overall clinical picture
  • Take appropriate sample
    • Infect cell culture- look for characteristic cytopathic effects
    • Screen for parts of the virus
    • Screen for immune response to virus (antibodies)
    • PCR to detect presence of viral genome