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An Introduction to the Viruses

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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An Introduction to the Viruses

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  1. An Introduction to the Viruses

  2. 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

  3. Size Comparison of Viruses

  4. 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)

  5. 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

  6. Animal Virus Replication • Adsorption • Penetration/uncoating of genome • Duplication/synthesis • Assembly • Release

  7. 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

  8. 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

  9. Cytopathic Changes in Cells

  10. 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

  11. 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

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