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2012-11-01 12:20:31

Show Answers:

  1. Define virion, prion, and viroid, and phage
    • Virion: complete, infectious virus
    • Prion: Infectious protein
    • Viroid: Infection RNA (plant pathogen)
    • Phage: virus that infects bacteria
  2. Describe morphological classes of viruses and give an example of each
    • Helical viruses: long rods with hollow cylindrical capside (Ebolavirus, rabiesvirus)
    • Polyhedral viruses: many-sided capsids (adenovirus, poliovirus)
    • Complex viruses: has additional structure attached to capsid (bacteriophage T4)
  3. Define plaque forming unit, Explain the plaque method (its uses and procedures)
    • Viruses must be grown in living cells
    • Plaque: clearing in plate caused by lysing of bacteria as viruses multiply
    • PFU: theory that each plaque initially came from a single virus
    • Plaque method: PFUs counted and viruses determined in that manner
  4. Define lysogenic conversion and give examples of diseases that result from this
    • Lysogenic (phage) conversion: host cell exhibits new properties (production of toxins by prophage)
    • Scarlet fever, diptheria, and botulism can result
  5. List and describe the 5 steps of viral multiplication (lytic cycle)
    • Attachment: phage attaches to host cell
    • Penetration: phage penetrates host cell and injects DNA
    • Biosynthesis: Phage DNA directs synthesis of viral components by the host cell
    • Maturation: viral components are assembled into virions
    • Release: host cell lyses, and new virions are released
  6. Compare and contrast lytic and lysogenic cycles
    • Lytic cycle: phage DNA not incorporated into host DNA, causes lysis and death of host cell
    • Lysogenic: phage DNA incorporated in host DNA (prophage), host is not killed, phage conversion can occur, specialized transduction can occur upon excisement.
  7. Differentiate between specialized and generalized transduction
    • Specialized transduction: Prophage is within the chromosome.  Excises innapropriately from the chromosome and takes a gene with it.
    • Generalized transduction: Bacterial chromosome broken to pieces during viral construction, accidentally packaged into virus and released.
  8. Explain the mechanism by which some viruses (retroviruses) induce tumors (cancer) give examples
    • Lysogenic cyle integrates DNA into host's DNA, which can alter the nucleotide sequence forming infections and cancers.
    • The cell recieves new properties which lead to increased growth and loss of contact inhibition.
    • DNA viruses: adenoviridae, herpesviridae, poxviridae, palpvaviridae, hepadnaviridae
    • RNA viruses: retroviridae, HTLV-1, HTLV-2
  9. Explain the importance of reverse transcriptase and describe what occurs in the biosynthesis of a virus with reverse transcriptase
    • Reverse transcriptase is necessary for retroviridae to infect the host cell.  RNA is converted to DNA, and the DNA is inserted into the host cell's chromosome.
    • The reverse transcriptase enzyme is packaged within the viral capsid during biosynthesis.
  10. Describe how the following are cultured in the laboratory: bacterial viruses (bacteriophages), animal viruses
    • Bacteriophages: plaque method (clearing zone is 1 PFU)
    • Animal viruses: living animals, embryonated eggs, or cell culture (plants or animals)
  11. List the major methods used for viral identification
    • Cytopathic effects: destruction of cells
    • Serological tests: detect antibodies against viruses in a patient (neutralization tests, viral hemagglutination, Western blot)
    • Nucleic acids: RFLPs (similar to DNA fingerprinting), or PCR (specific viral sequences)