Viruses

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Viruses

• Structure
• Not cells, not alive. genome, capsid, envelope
• Function
• entry, replication, gene expression,
  self-assembly
• Some assimilate into host genome
• Origin as runaway genes

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Some representative viruses

• TMV tobacco mosaic virus
• Adenoviruses
• Influenza (flu) viruses
• Retroviruses
• Bacteriophages

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Life cycles of bacteriophages
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The lytic cycle of phage T4
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Lysogenic and lytic cycles of phage ?, a
temperate phage
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Retroviruses

• RNA genome
• Reverse transcriptase makes DNA using RNA as a
  pattern
• Includes Human Immunodeficiency Virus (HIV) which
  causes Acute Immunodeficiency Syndrome (AIDS)

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HIV, a retrovirus
See Figure 13.6 Sadava
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Viral diseases of man

• herpes, influenza, colds, polio, mumps, measles,
  smallpox, AIDS, SARS, ebola, hantavirus, others
• Severity depends on the cells affected
• cold virus nasal epithelia
• polio virus- motor neurons
• HIV virus - helper T-cells

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• Viral genes can make bacteria toxic (e.g.
  diptheria, scarlet fever, botulism)
• Some viruses cause cancer
• Cant use antibiotics against virus (why)?
• Vaccination- exposure to inactivated virus to
  sensitize immune system.

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Where did they come from?

• Many viruses can become part of host chromosome-
  prophage or provirus
• may have originated from mobile genetic elements
  basically, genes that can move between cells or
  between chromosomes
• These elements may have evolved because they
  facilitate genetic recombination

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Genetic recombination

• Creates new combinations of alleles
• Eukaryotes use meiotic sex
• Prokaryotes have other ways to exchange and
  recombine genes
• plasmids, transformation, transduction,
  conjugation, transposons

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Bacterium releasing DNA with plasmids
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Bacterium releasing DNA with plasmids
Plasmid
Plasmids
Plasmids
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Bacterial conjugation
Conjugation tube
Plasmids, or chromosomal DNA can be passed from
donor cell to recipient. Genes from donor can
become part of recipient cell chromosome
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Detecting genetic recombination in bacteria
(compare with Sadava fig. 13.10)
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R-plasmids

• Antibiotic resistance plasmids carry from 1-10
  different antibiotic resistance genes
• Evolution caused by use of antibiotics in
  medicine, livestock
• How could several resistance genes end up
  together in one plasmid?

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Transposons

• Genetic elements that can move
• Occur in both prokaryotes and eukaryotes
• Simplest form is insertion sequence that inserts
  randomly, causes mutation
• Complex transposon two insertion sequences
  bracket move other genes
• alters positions (linkage) of genes

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Insertion sequences (transposable elements) the
simplest transposons
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Insertion of a transposon and creation of direct
repeats
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A composite transposon with an antibiotic
resistance gene
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Control of gene expression

• Prokaryotes have operons
• Operon functionally related genes grouped
  together on chromosome, switched on or off
  together.
• control region structural genes

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• Eukaryotes dont have operons
• functionally related genes are not necessarily
  grouped spatially
• coordinated expression is achieved by multiple
  similar control regions associated with
  functionally related genes

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Example Trp operon

• Genes for enzymes that synthesize the amino acid
  tryptophan
• Regulatory gene makes repressor protein
• Repressor is activated by binding tryptophan, and
  blocks transcription by binding operator
• Negative feedback- shuts down operon if there is
  plenty of tryptophan present

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The trp operon regulated synthesis of
repressible enzymes
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The trp operon part 1
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The trp operon part 2