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Medical Microbiology

Medical Microbiology. Chapter 6 Viral Classification, Structure, and Replication. Introduction To Viruses. Viruses are obligate intracellular parasites they cannot reproduce outside a living cell Viruses do not contain an ATP-generating system

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Medical Microbiology

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  1. Medical Microbiology Chapter 6 Viral Classification, Structure, and Replication

  2. Introduction To Viruses • Viruses are obligate intracellular parasites • they cannot reproduce outside a living cell • Viruses do not contain an ATP-generating system • Viruses do not contain any protein synthesizing machinery • What is the structure of a virus?

  3. Introduction To Viruses (cont.) • Classification: • Genetic material • DNA viruses (double- and single-stranded) • RNA viruses (double- and single-stranded) • Presence of an envelope • Means of reproduction

  4. Virion Structure • The virion or virus particle consists of: • nucleic acid (RNA or DNA) • surrounded by a protein coat - Capsid • envelope (if present) • Originates from the host cell’s membrane • The capsid (for animal viruses) comes in 2 basic shapes: • icosahedral - many-faced, spherical shape • helical - tube of protein • Figures 6-5, 6-6, and 6-7

  5. Viral Structure (cont.) • Some virions also contain essential enzymes or other proteins • Reverse transcriptase in HIV • Figure 6-5 • The capsid or the envelope has special attachment proteins or glycoproteins that mediate the attachment of the virus to the target cell • Virus Attachment Proteins (VAP) • Figures 6-5 and 6-7

  6. Viral Structure (cont.) • Naked viruses – very resistant to drying, acids, detergents, etc. • Enveloped viruses – must remain in aqueous solution • They are damaged by drying, acids, detergents, etc. • Importance in Transmission • Different Types of Viruses – Figure 6-4 and Tables 6-1, 6-2, 6-3, and 6-4

  7. Viral Replication • How viruses affect the cells they infect depends on the virus and the host cell • Some viruses will immediately begin producing new virions • if the new virions are released by the cell breaking open, the cell dies (most naked viruses) • some virions can leave the cell without killing it (most enveloped viruses) • Other viruses will insert their DNA into the host chromosome and cause a latent infection

  8. Viral Replication • Contains six steps: • Attachment – VAPs attach to cell receptors • This determines the virus host range and tissue tropism • Epstein-Barr virus~C3d on human B-cells • rhinovirus~ICAM-1 of epithelial tissue • HIV~CD4 on helper T-cells • Influenza virus~sialic acid on epithelial cells • rabies virus~acetylcholine receptors on neurons

  9. Viral Replication • Penetration - this occurs by: • Receptor-mediated endocytosis (most naked viruses) • Viropexis – naked viruses slip directly through the membrane • Membrane fusion – enveloped viruses • Uncoating - the capsid is lost, exposing the DNA or RNA

  10. Viral Replication • Macromolecular synthesis of Viral Proteins and Nucleic Acids • How would this work for DNA viruses? • In what ways would the process be different for RNA viruses • Positive sense (+) RNA • Negative sense (-) RNA

  11. Viral Replication • Retroviruses are especially complex • these are RNA viruses that convert their RNA into DNA • the enzyme reverse transcriptase is responsible for this conversion • the DNA is then incorporated into the host cell’s genome • the viral DNA is then transcribed and translated to produce more virions

  12. Viral Replication • Assembly – once all the necessary proteins and nucleic acids are synthesized, • They automatically assemble into new virions • This happens in the cytoplasm or nucleus in naked viruses • It happens at the membrane for enveloped viruses

  13. Viral Replication • Release – the virions exit the cell • Cell lysis or exocytosis for naked viruses • Budding for enveloped viruses

  14. Disease of the Day • West Nile Virus – transmitted from birds to humans by mosquitoes • Details

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