Chapter 18: The Genetics of Viruses and Bacteria

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Chapter 18The Genetics ofViruses andBacteria
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Important Point
If you are having trouble understanding lecture
material Try reading your text before
attending lectures. And take the time to read it
well!
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What is Microbiology?

• Microbiology is the science that studies
  microorganisms
• Microorganisms, roughly, are those living things
  that are too small to be seen with the naked eye
• Microorganisms cannot be distinguished
  phylogenetically from Macroorganisms, e.g.,
  includes fungi as well as bacteria, etc. (that
  is, they are not, as a whole, a closely related
  group of organisms)
• Microbiology is more a collection of techniques
  Aseptic technique, Pure culture technique,
  Microscopic observation of whole organisms, etc.
• A microbiologist usually first isolates a
  specific microorganism from a population and then
  cultures it

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Importance of Microbes

• Microbes are producesthey provide energy to
  ecosystems
• Microbes are fixersthey make nutrients available
  from inorganic sources, e.g., nitrogen
• Microbes are decomposersthey free up nutrients
  from no longer living sources
• Microbes form symbioses (such as mycorrhizal
  fungi associated with plant rootsthough these
  are somewhat macroscopic also the bacteria found
  in legume root nodules, etc.)
• Microbes serve as emdosymbionts (e.g.,
  chloroplasts and mitochondria)
• Microbes make fermentation products (ethanol!),
  food (beer! Cheese! Yogurt! Half-sour pickles!),
  Biotech products (e.g., recombinant insulin),
  etc.
• Germ theory of disease Normal flora

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Brueghel The Triumph of Death (1560)
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Relative Microbe Sizes
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Examples of Types of Viruses
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What is a Virus?

• Viruses consist of protein capsids and nucleic
  acid (DNA or RNA) plus some viruses (virions)
  have a lipid envelope (enveloped viruses)
• Viruses are ...infectious agents of small size
  and simple composition that can multiply only in
  living cells of animals, plants and bacteria
  plus fungi protozoa.
• Viruses are obligate parasites that are
  metabolically inert when they are outside their
  hosts. They all rely, to varying extents, on the
  metabolic processes of their hosts to reproduce
  themselves.
• The viral diseases we see are due to the effects
  of this interaction between the virus and its
  host cell (and/or the hosts response to this
  interaction). Encyclopedia Britannica

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Virus (Virion Particle)

• The Virion is what defines a virus as a virus
• A Virion is the extracellular state of a virus
• The job of Virions is to find new cells to infect
• As such, Virions are a durable state that is
  designed to attach to susceptible cells
• The Virion is then responsible for translocation
  of the virus genome into the cell
• The Virion consists of a DNA (or RNA) genome
  surrounded by Protein that, in turn, may be
  surrounded by a Lipid Bilayer
• The Protein layer is called a Capsid
• The Lipid Bilayer is called an Envelope

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Steps of Virus Replication

• Adsorption (attachment)
• Penetration (nucleic-acid release)
• Synthesis (of RNA and proteins, as well as DNA if
  DNA genome)
• Maturation (assembly of virion)
• Release (lysis or chronic release, e.g., budding,
  with the latter coinciding with release for
  various enveloped viruses)

Caveat It is important to realize that variation
among viruses is between virus strains/species
any one kind of virus cannot replicate in
multiple ways, have more than one virion
morphology, or vary in genome type, etc.
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DNA Virus Life Cycle
Lysis
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Bacteriophage Lytic Cycle
Lysis
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Lysogeny (Temperate Phage)
Only temperate phage are able to display lysogeny
Lysis
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Enveloped RNA Virus
An example of an animal virus
Budding
Acquisition of plasma membrane as envelope
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Human Immunodefiency Virus
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HIV Life Cycle
Budding
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Bacteria Sex

• Viruses move genetic material from cell to cell
• Mostly this material is their own genomes, i.e.,
  genes that collectively code for the production
  of new viruses
• Bacteria DNA also can move from cell to cell
• Once received by a cell, this DNA may be
  incorporated into the bacterial genome via
  recombination
• This idea of DNA sourced from different parents
  recombining into a single chromosome is
  equivalent to eukaryotic sex (i.e., fertilization
  followed by recombination)
• Transformation, Transduction, Conjugation

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Transformation
Transformation DNA picked up directly from the
medium and recombined into the genome
Competent cell capable of picking up DNA
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Generalized Transduction
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Plasmids
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Conjugation
Moves plasmid more so than chromosomal DNA
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Regulation of Metabolism
E.g., transcription
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Control of Gene Expression

• It makes energetic sense to make or use proteins
  responsible for certain metabolic processes only
  when those processes are needed
• Trp operon
• response to presence of tryptophan
• Repressible (Corepressed) operon
• Example of negative control of transcription
• Lac operon
• Response to presence of lactose
• Inducible operon
• Example of negative control of transcription
• Catabolite repressor protein
• Response to absence of glucose
• Example of positive control of transcription
• Protein binding to the operator controls RNA
  polymerase activity

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Trp Operon (low trp densities)
Recall that the promoter is the site of RNA
polymerase binding
Dont worry about the names of these genes and
products
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Trp Operon (higher trp densities)
Negative regulation
Corepression
Equilibrium Likelihood of being in bound state
depends on trp density
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?-Galactosidase
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Lac Operon (low Lac densities)
Negative regulation (binding blocks trasncription)
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Lac Operon (higher Lac densities)
Dont worry about the names of these genes and
products except for ?-Galactosidase
Equilibrium Likelihood of being in bound state
depends on Lac density
An inducible operon
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Catabolite Repressor Protein (CAP)
Positive control (binding increases transcription)
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The End