CHAPTER 19 MICROBIAL MODELS: THE GENETICS OF VIRUSES AND BACTERIA

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CHAPTER 19MICROBIAL MODELSTHE GENETICS OF
VIRUSES AND BACTERIA
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Objectives The Genetics of Viruses

• Describe the structural components of viruses
• Explain why viruses are obligate intracellular
  parasites
• Explain how a virus identifies its host cell
• Distinguish between the lytic and lysogenic
  reproductive cycles, using phage lambda as an
  example
• Describe the reproductive cycle of an enveloped
  virus. Explain the reproductive cycle of the
  herpesvirus
• Describe the reproductive cycle of retroviruses
• Explain how viral infections in animals cause
  disease
• Describe the best current medical defenses
  against viruses. Explain how AZT helps to fight
  HIV infections
• Describe the mechanisms by which new viral
  diseases emerge  

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Virus Structure

• Viral Genomes depending on the virus, viral
  genomes
• May be double stranded DNA, Double stranded RNA,
  or single stranded RNA
• Single nucleic acid molecules are linear or
  singular
• May have 4 to several hundred genes

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Capsids and Envelopes

• Capsid Protein coat that encloses viral genome
• May be rod shaped, polyhedral or complex
• Composed of many proteins subunits - capsomeres
• Envelope Membrane that cloaks some viral
  capsids
• Helps viruses infect their host
• Derived from host cell membrane and modified with
  viral proteins

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Viruses Can Only Reproduce With Host Cell

• Obligate Intracellular Parasites Can only
  express their genes and reproduce with a living
  cell

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Host Range

• Limited number of host cells a parasite can
  infect
• Viruses recognize host cells by a complimentary
  fit between external viral proteins and specific
  cell surface receptor sites
• Some have broad host ranges several species
• Some have narrow host ranges
• Infect only one species
• Infect only a single tissue type of one or more
  species (cold virus, HIV)

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• Many patterns of viral live cycles, but generally
  include
• Infect host with viral genome
• Co-opting host cell resources to
• Replicate viral genome
• Manufacture proteins
• Assembling newly produced viral nucleic acid and
  capsomeres into next generation of viruses
• Several Mechanisms used to infect host cells
• T-even phages use tailpiece to inject DNA

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Three patterns of viral genome replication

• DNA ? DNA. If viral DNA is double stranded, DNA
  replication resembles that of cellular DNA. Virus
  uses host DNA polymerase
• RNA ? DNA. Most RNA viruses contain a gene that
  codes for RNA replicase, an enzyme that uses
  viral RNA as a template to produce complimentary
  RNA
• RNA ? DNA ? RNA. Some RNA viruses encode reverse
  transcriptase, an enzyme that transcribes DNA
  from an RNA template

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Viral Genomic RNA
Reverse transcriptase
Viral DNA
transcribes
transcribes
genomic RNA for new virons
Mesenger RNA
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• All viruses use host cell resources for viral
  production enzymes, ribosomes, tRNAs, amino
  acids, ATP, etc
• Viral Nucleic Acid and capsid proteins assembles
  spontaneously into new virus particles

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Phages Exhibit Two Reproductive Cycles

• Lytic Cycle Virulent bacteriophages reproduce
  by lytic cycle
• Virulent Phages Phages that lyse their hosts
• Lytic Cycle A viral replication cycle that
  results in the death or lysis of the host cell

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Lytic cycle of phage T4

• Phage attaches to surface
• Phage contracts sheaths and injects DNA (ATP
  stored in tailpiece)
• Hydrolytic enzymes destroy host cell DNA
• E. coli host transcribes and translates viral
  genome
• Enzyme produced degrades host DNA phage DNA
  protected by modified cysteine not recognized by
  enzyme

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• Phage genome directs host cell to produce phage
  components DNA and capsid proteins
• Cell lyses and releases phage particles
• Entire cell cycle takes 20-30 minutes

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• Bacterial defenses against infection
• Bacterial mutations can change receptor sites
  no recognition site
• Bacterial restriction enzymes recognize and cut
  up foreign DNA
• Coevolution of bacterial hosts and viral
  parasites

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Lysogenic cycle Some viruses coexist with hosts
by incorporating their genome into hosts genome

• Temperate viruses Viruses that integrate genome
  into host and remain latent until lytic cycle
  two possible means of reproduction

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Phage Lambda life cycle

• Phage Lambda binds to surface of an E.coli cell
• Phage Lambda injects DNA into the bacterial host
  cell
• Lambda DNA forms a circle and wither begins a
  lytic cycle or a lysogenic cycle
• During a lysogenic cycle, Lambda DNA inserts by
  genetic recombination into a specific site on the
  bacterial chromosome and becomes a prophage

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Prophage A phage genome that is incorporated
into a specific site on the bacterial chromosome
(Example Varicella zoster virus)

• Most prophage genes are inactive
• One active prophage gene codes for a repressor
  protein which switches off other prophage genes
• Prophage genes are copied along with cellular DNA
  when the host cell reproduces
• Prophage may be carried in host for many
  generations

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• Excision process begins lytic cycle
• Lysogenic conversion change of a cells
  phenotype. Toxins may result from prophage genes
  diphtheria, botulism, scarlet fever

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Animal Viruses

• Reproductive Cycles of Animal Viruses
• Viruses with envelopes
• Attachment glycoprotein spikes protruding from
  viral envelope attach to receptor sites on hosts
  membrane
• Entry envelope fuses with membrane
  endocytosis
• Uncoating Cellular enzymes remove protein
  capsid from RNA
• Viral RNA and proteins synthesis
• Assembly and release during budding process
  virions envelop with host plasma membrane
• There is a variety in this process
• Provirus viral DNA that inserts into a host
  cell chromosome - herpes

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RNA viruses

• () RNA strand nucleotide sequence that codes
  for proteins
• (-) RNA strand template for synthesis of ()
  strand

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Retrovirus RNA virus that uses reverse
transcriptase to transcribe DNA from viral RNA
genome

• Reverse transcriptase DNA polymerase that
  transcribes DNA from an RNA template
• HIV is a retrovirus

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Viral Diseases

• Damage or kill cells
• Infected cells may produce toxins
• Respiratory cells recover (colds) nerve cells do
  not (polio)
• Indirectly responsible for disease symptoms
  fever, aches, inflammation result of immune
  system
• Vaccines harmless variants of pathogens

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Emerging Viruses

• Evolves and causes diseases in individuals with
  immunity to ancestral form
• Spreads from one host to another
• Disseminates from small populations to become
  more widespread

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Other Viruses transmitted from animals or insects
to humans West Nile, Avian Flu (?), Dengue, HIV,
Japanese Encephalitis, Hendra
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Viruses and Cancer

• Some tumor viruses cause cancer in animals (HPV
  cervical cancer)
• Oncogenes genes found in viruses, or as part of
  normal eukaryotic genome, that transform cell to
  a cancerous state (code for cellular growth
  factors)
• Carcinogens turn on cellular oncogenes

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Objectives The Genetics of Bacteria

• Describe the structure of a bacterial chromosome
• Compare the processes of transformation,
  transduction, and conjugation
• Define an episome. Explain why a plasmid can be
  an episome
• Explain how the F plasmid controls conjugation in
  bacteria
• Describe the significance of R plasmids. Explain
  how the widespread use of antibiotics contributes
  to R plasmid-related disease
• Explain how transposable elements may cause
  recombination of bacterial DNA
• Distinguish between an insertion sequence and a
  transposon
• Describe the role of transposase in the process
  of transposition
• Briefly describe two main strategies that cells
  use to control metabolism.29.
• Explain the adaptive advantage of genes grouped
  into an operon.30.
• Using the trp operon as an example, explain the
  concept of an operon and the function of the
  operator, repressor, and corepressor.31.
• Distinguish between structural and regulatory
  genes.32.
• Describe how the lac operon functions and explain
  the role of the inducer, allolactose.33.
• Explain how repressible and inducible enzymes
  differ and how those differences reflect
  differences in the pathways they control.
• Distinguish between positive and negative control
  and give examples of each from the lac operon.35.
• Explain how cyclic AMP and catabolite activator
  protein are affected by glucose concentration.

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Bacteria

• Bacterial Chromosomes (genophore)
• Double stranded DNA
• Found in nucleoid region not separated
  cytoplasm. Transcription, translation occurs
  simultaneously
• Plasmid A small double-stranded ring f DNA that
  carries extra chromosomal genes in some bacteria
• Binary fission

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Genetic Recombination Transposition Produce New
Bacterial Strains

• Transformation Process of gene transfer by
  which a bacteria assimilates foreign DNA from
  surroundings

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Transduction Gene transfer from one bacterium
to another by a bacteriophage (figure 18.13)

• Generalized Random pieces of host cell DNA are
  packaged within a phage capsid during the lytic
  cycle of a phage
• Specialized Prophage excises from bacterial
  chromosome and carries with it some host genes
  adjacent to the excision site

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Conjugation and Plasmids

• Conjugation Direct transfer of genes between
  two cells temporarily joined. Sex pili (F)
  attach to DNA receiving cell (F-). A cytoplasmic
  bridge forms DNA is transferred.
• Plasmid
• Carry only a few genes and are not required for
  survival or reproduction
• May be beneficial in stressful environments
• May carry antibiotic resistance

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Hfr high frequency of recombination, cell with
the F factor incorporated into its genome Allows
part of the bacterial genes to be transferred
during conjugation
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Transposons (Figs. 17.13-17.15) DNA sequences
that can move from one chromosomal site to another

• Conservative transposition Transposons genes
  not replicated before move
• Replicative transposition Replication
  occurs-genes are inserted into new site without
  being lost from old site

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Bacterial Control of Metabolism

• Regulation of enzyme activity (feedback
  inhibition)
• Regulation of gene expression Enzyme
  concentrations may rise and fall in response to
  cellular metabolic changes that switch genes on
  or off

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Operons (Fig 17.18) Regulated genes can be
switched on or off depending on the cells
metabolic needs. A regulated cluster of adjacent
structural genes (code for polypeptide) with
related functions

• Common in bacteria and phages
• Single promoter region mRNA will transcribe all
  genes
• Transcription single polycistronic mRNA with
  coding sequence for all enzymes in metabolic
  pathway

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• Polycistronic mRNA
• Translated into several polypeptides
• Contains stop/start codes for translation of each
  polypeptide
• Advantageous all genes in a metabolic pathway
  transcribed at one time and controlled by a
  single operator

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• Operator A DNA segment between operons
  promoter and structural genes which controls
  access of RNA polymerase to structural genes -
  on/off switch
• Repressor Protein that binds to an operator and
  blocks transcription of the operon
• Blocks transcription of RNA polymerase to
  promoter
• Similar to enzyme active site, operon specific,
  allosteric site
• Controlled by regulatory genes

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• Regulatory genes Code for repressors or
  regulators. TRANSCRIPTION OF REGULATORY GENE
  produces mRNA that is translated into REGULATORY
  PROTEIN that binds to OPERATOR that represses or
  activates TRANSCRIPTION OF OPERONS STRUCTURAL
  GENE.

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Repressor proteins have 2 forms active and
inactive. Example Tryptophan (Fig 17.17)

• Tryptophan (corepressor) present
• Repressor protein in active conformation
• Binds to operator
• Trp operon switched off
• Tryptophan absent
• Repressor protein in inactive conformation
• TRP operon turned on

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Repressible vs. Inducible Enzymes

• Repressible Synthesis inhibited by metabolite
  (tryptophan, Fig 17.17)
• Genes switched on until a specific metabolite
  activates repressor
• Anabolic pathways
• Pathway end product switches off its own
  production by repressing enzyme synthesis
• Inducible Synthesis stimulated by specific
  metabolites (lactose, Fig 17.18)
• Genes switched off until a specific metabolite
  actives repressor
• Catabolic pathways
• Synthesis switched on by nutrient pathway uses

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Positive Gene Regulation (Fig 17.19)

• CAP (catabolite activator protein) Protein that
  binds with an operons promoter region and
  enhances the promoters affinity for RNA
  polymerase
• Dual control of lac operon
• Negative control by repressor determines if
  operon transcribes structural genes
• Positive control by Cap determines rate of
  transcription
• E. coli prefers glucose over lactose for
  substrate for glycolysis

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