5. Viruses/Viroids/Prions

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  1. viruses
    obligate intracellular parasites
  2. capsid
    outer protein coat surrounding nuceleic acid
  3. purpose of capsid
    • protects nucleic acid from attack
    • attaches to a specific receptor site of a cell membrane
  4. structure of the virion
    • capsomeres
    • nucleocapsid: capsid and nucleic acid
  5. capsid designs
    • polyhedral/ icosohedral: 20 faces, each is an equilateral triangle, nucelic acid within capsid
    • helical: nucleic acid attached directly to the capsid
    • complex structure: ex. binal form in bacteriophages
  6. envelope
    • cover capsids
    • derived from portions of host cell membranes
    • help viruses enter host cells
    • viruses without envelopes (naked viruses) - have better survival outside of cell
  7. polymerase
    enzyme viruses use for replication of nucleic acid
  8. four classes of polymerases
    • RNA dependent RNA polymerase (replicase)
    • RNA dependent DNA polymerase (reverse transcriptase)
    • DNA dependent DNA polymerase
    • DNA dependent RNA polymerase
  9. viral attachment proteins of non-envelope viruses
    it is one of the surface proteins that forms the capsid
  10. viral attachment proteins of envelope viruses
    protrudes from viral envelope
  11. purpose of penetration and uncoating
    • must penetrate so virus can replicate within cells
    • uncoating so that genome is exposed
  12. methods of penetration
    • injection of nucleic acids by nonenveloped bacteriophages
    • translocation- transit of nucleic acid across cell membrane by nonenvelope viruses
    • membrane fusion by fusion factors by enveloped viruses
    • endocytosis/phagocytosis - virus is engulfed by cell by enveloped and nonenveloped viruses
  13. replication of viral proteins
    • dna -> mRNA-> proteins
    • transcription translation
  14. replication of genome
    replication strategy depends on type of genomic material
  15. Group I: (±) dsDNA
    use (-) DNA strand as template for mRNA
  16. Group II: (+) ssDNA
    • makes dsDNA
    • use (-) DNA strand as template for mRNA
  17. Group III: (±) dsRNA
    use (-) DNA strand as template for mRNA
  18. Group IV: (+) RNA
    • make (-) RNA
    • use (-) RNA strand as template for mRNA
  19. Group V: (-) RNA
    use (-) RNA strand as template for mRNA
  20. Group VI: (+) RNA
    • make (-) DNA copy using reverse transcriptase
    • makes dsDNA
    • use (-) DNA strand as template for mRNA
  21. Group VII: (±) dsDNA
    • make RNA use (-) DNA strand as template for mRNA
    • make (-) DNA copy using reverse transcriptase
    • make dsDNA
    • use (-) DNA strand as template for mRNA
  22. assembly
    • replicated viral nucleic acid and viral capsid find each other within the host cell and combine, facilitated by - localization, chemical attraction
    • location of assembly of nucleocapsids
    • insertion of peplomers
    • enzymes
  23. release
    • budding through plasma membrane or some other membrane - does not kill host cell
    • lysis of cell (lytic infection) - kills host cell
  24. virus can't infect cell or non-productive infection
    • no virus receptor on cell surface
    • no suitable replication enzymes and protein synthesizing machinery - abortive infection, non-permissive infection
    • latent infection - virus incorporates itself into host genome and becomes provirus/prophage and enters lysogenic state which lies dormant until lytic state
  25. host range
    • kingdom level (animal, plant, bacteria)
    • species level
    • tissue level
    • viruses that infect other microorganisms
  26. classification of viruses based on
    • type of nucleic acid
    • mode of mRNA production (baltimore scheme)
    • type of capsid
    • enveloped or naked
    • type of enzymes
    • replication pattern
    • nucleic acid sequence
  27. viroids
    • free, short single stranded RNA lacking protein coat
    • causes numerous plant diseases
    • causes hepatitis in humans
  28. prion
    • protein in a misfolded form
    • causes scrapie, kuru, Creuztfeldt-Jacod disease (CJD), transmissible (bovine) spongiform encephalopathy
Card Set
5. Viruses/Viroids/Prions
public health microbiology lecture 5
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