Microbial Models: The Genetics of Viruses and Bacteria

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Microbial Models The Genetics of Viruses and
Bacteria

• A virus is an infectious particle and
• the simplest forms of life are bacteria.

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Outline

• 1. Key Concepts
• 2. The Structure of Viruses
• 3. Viral Reproduction
• 4. Infectious Agents Tinier Than Viruses
• 5. Key terms
• 6. Conclusions
• _____________________________________
• 7. Genetic recombination of Bacteria
• 8. Key Terms
• 9. Conclusions

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Key Concepts

• A virus is an infectious particle
• Viruses are non-cellular
• Almost all viral multiplication cycles proceed
  through the same 5 steps
• Attachment
• Penetration
• Replication
• Assembly
• Release

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Key Concepts

• The simplest forms of life are bacteria
• Bacteria are the only prokaryotes
• Bacteria reproduce by binary fission
• The first living organisms on earth were bacteria
  Microorganisms have been adapting, are diverse,
  and are surviving and replicating like us
• A bacterium can transfer its genes to another
  bacterium by transformation, transduction, or
  conjugation.

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The Structure of Viruses

• 1. Non-cellular infectious agent
• 2. Protein coat surrounding a nucleic acids core
• 3. Genomes DNA or RNA (four to hundreds genes)
• 4. Capsids and envelopes
• Capsid Protein coat that encloses the viral
  genome
• rod-shaped, polyhedral, or complex
• composed of many capsomeres
• Envelope Membrane that cloaks some viral
  capsids
• Helps viruses infect their host
• Derived from host cell membrane and
  virus-modified
• 5. Reproduce inside a host cell

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Comparing the sizes of a virus, a bacterium, and
a eukaryotic cell
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The Structure of Viruses
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viral RNA
protein subunits of coat
80-nm diameter
18-nm diameter, 250-nm length
lipid envelope proteins span the envelope, line
its inner surface, and spike out above it
DNA
protein coat
sheath
viral RNA
base plate
tail fiber
65-nm diameter head, 225-nm total length
reverse transcriptase
viral coat (proteins)
100-120 nm diameter
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Viral Multiplication Cycles

• 5 Steps
• Attachment
• Penetration
• Replication
• Assembly
• Release

• Lytic pathway
• Host cell lysis
• Virulent virus
• Lysogenic pathway
• Viral DNA integrates into bacterial chromosome
  (prophage)
• Temperate virus

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Viral Multiplication Cycles
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Viral Multiplication Cycles
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Viral Multiplication Cycles
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Classes of Animal Viruses
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Smallpox
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Herpes simplex lesion of lower lip, second day
after onset
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Measles
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HIV, a retrovirus
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Couple at AIDS quilt
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Infectious Agents Tinier Than Viruses

• Prions
• Proteins
• Altered products of a gene
• Diseases
• Mad cow disease
• Creutzfeldt-Jakob disease

• Viroids
• Tight folds of RNA
• Plant diseases

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Key Terms
Provirus Retrovirus Reverse transcriptase Reverse
transcription Viroid Prion
Capsid Viral envelope Bacteriophage (phage) Lytic
cycle Virulent virus Lysogenic cycle Temperate
virus Prophage HIV (human immunodeficiency
virus) AIDS (acquired immunodificiency syndrome)
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In Conclusion

• 1. Viruses are obligate intracellular parasites,
  needing the biochemical machinery of living cells
  to reproduce.
• 2. Bacteriophages are viruses that infect
  bacteria.
• 3. In the lytic cycle, the host cell breaks open,
  releasing new phage particles.

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In Conclusion

• 4. Some phages can also undergo a lysogenic
  cycle, in which their DNA is inserted into the
  host chromosome, where it replicates for
  generations. When conditions are appropriate, the
  lysogenic DNA exits the host chromosome and
  enters a lytic cycle.
• 5. Most of the many types of RNA and DNA viruses
  that infect animals cause diseases.
• 6. A reverse transcriptase can use RNA as
  template for DNA synthesis.