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  1. Viruses definition
    Obligate intracellular parasites

    Viral structure:



  2. Viral shapes
    –Helical, Polyhedral, Enveloped, Complex

    –Most <0.2mm; requires electron microscope

    • –Virion – fully formed virus able to
    • establish infection
  3. Viral Structure
    Viruses are made of:

    • 1.Nucleic acid – genome
    • •May be DNA or RNA; single stranded or
    • double stranded
  4. Viral Structure 2
    Capsid (protein coat)

    •Capsomeres – individual protein unit
  5. Viral Structure 3
    Envelope – lipid membrane

    •Spike proteins

    •Not found in all viruses
  6. Capsids
    • –All viruses have capsids - protein coats that enclose and protect their nucleic
    • acid.
  7. nucleoscapsid
    The capsid together with the nucleic acid
  8. Capsids 2
    –Some viruses have an external covering called envelope; those lacking an envelope are naked
  9. Viruses lack
    –Any cellular components


  10. Viruses have
    –Genetic information

    –Genes for replication and infection

    –Some have a few enzymes
  11. cell (tissue) cultures
    –cultured cells grow in sheets that support viral replication and permit observation for cytopathic effect
  12. Bacteriophage
    viruses that infect bacteria

    • •Host cell “lawn”
    • –Infection visible as clear plaques (where cells were infected then lysed)
  13. Cytopathic Effect (CPE)
    Infection of Cell Culture is obvious due to Cytopathic Effect (CPE)
  14. Bacteriophages
    •Multiplication goes through similar stages as animal viruses.

    •Only the nucleic acid enters the cytoplasm -  uncoating is not necessary.

    • •Release is a result of cell lysis induced
    • by viral enzymes and accumulation of viruses - lytic cycle.
  15. 6 Steps in Phage Replication
    1. Attachment – binding of virus to specific molecule on host cell

    2. Entry  –genome enters host cell

    3-4. Synthesis – bacterial DNA degraded.  Viral components produced

    5. Assembly - viral components assembled

    6. Release –viruses leave cell to infect other cells
  16. Lytic life cycle

    (part 1 of lysogenic life cycle)
    •Attachment, Entry, DNA circularizes

    •Transcription & Translation, Replication of DNA

    •Assembly & Release via lysis
  17. Latent life cycle (part 2 of lysogenic life cycle)
    doesn’t kill host cell

    •Attachment & Entry

    •DNA integrates into the host chromosome = prophage

    •Bacteria replicates normally
  18. Induction
    •can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis.
  19. Prophage
    • •The viral genome inserts into bacterial genome and becomes an inactive prophage - the
    • cell is not lysed.

    • Prophage is retained and copied during normal cell division resulting in the transfer of
    • temperate phage genome to all host cell progeny – lysogeny
  20. temperate phages
    •Not all phages complete the lytic cycle.

    • •Some DNA phages, called temperate
    • phages, undergo adsorption and penetration but don’t replicate.
  21. Phage conversion
    • •When the phage integrates into the
    • bacterial chromosome it brings new genes to the bacteria!

    •May encode toxins
  22. Adsorption
    –binding of virus to specific molecule on host cell
  23. Penetration
    –genome enters host cell

    Pinocytosis or Fusion
  24. Uncoating
    –the viral nucleic acid is released from the capsid
  25. Synthesis
    –viral components are produced
  26. Assembly
    –new viral particles are constructed
  27. Release
    • –assembled viruses are released by budding
    • (exocytosis) or cell lysis
  28. DNA viruses
    are replicated and assembled in the nucleus.
  29. RNA viruses
    •are replicated and assembled in the cytoplasm.

    • –Positive-sense RNA contain the message
    • for translation.

    • –Negative-sense RNA must be converted into
    • positive-sense message.
  30. Viral Release
    by Budding
  31. Types of Infection
    • 1.Acute – short duration
    • •Common cold

    • 2.Persistent – virus remains in tissues
    • •Complication of measles infection, SSPE (subacute sclerosing panencephalitis)
  32. Types of Infection 2
    • 1.Chronic – virus continually released from
    • host
    • –Hepatitis B virus

    • 2.Latent – virus remains dormant in host
    • and can reactivate later (replicate and release virus)

    • –measles virus – may remain hidden in
    • brain cells for many years

    • –herpes simplex virus – cold sores and
    • genital herpes

    • –herpes zoster virus – chickenpox and
    • shingles
  33. Prions
    misfolded proteins, contain no nucleic acid

    –cause transmissible spongiform encephalopathies – fatal neurodegenerative diseases

    •Extremely resistant to usual                 sterilization techniques
  34. Antigenic shift
    • occurs when there is genetic recombination
    • between 2 different influenza strains
Card Set:
2015-03-30 07:55:56

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