David Sadava H. Craig Heller Gordon H. Orians William K. Purves David M. Hillis Biologia.blu B

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David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.bluB Le basi molecolari della vita
e dellevoluzioneThe Genetics of Viruses and
Prokaryotes
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The Genetics of Viruses and Prokaryotes

• How do viruses reproduce and transmit genes?
• How do prokaryotes exchange genes?
• How is gene expression regulated in prokaryotes?

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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?

• Classification
• genome of DNA or RNA
• nucleic acid is single- or double-stranded
• simple or complex shape
• whether virion is surrounded by a membrane or
  not
• type of organism it infects
• manner of the infection.

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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?
Virions come in various shapes
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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?

• Viruses that infect bacteria are called
  bacteriophage or phage.
• Phage binds to a receptor on the host cell wall,
  injects the nucleic acid, then one of two things
  happens
• 1) phage reproduces immediately and kills the
  host cell - lytic cycle - cell bursts and
  releases progeny viruses
• 2) postpones reproduction by integrating into the
  host cells genome - lysogenic cycle.

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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?
The lytic and lysogenic cycles of bacteriophage
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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?

• Animal viruses enter cells in several ways
• a naked virion is taken up by endocytosis
• the enveloped virus has glycoproteins that bind
  to receptors on host cell, also taken in by
  endocytosis (e.g., influenza)
• the membrane of the host cell and enveloped virus
  fuse (e.g., HIV).

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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?
The reproductive cycle of the influenza virus
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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?

• HIV is a retrovirus, it has reverse
  transcriptase, which facilitates RNA-directed DNA
  synthesis.
• A DNA provirus is produced that is integrated
  permanently into the hosts genome.
• When proviral DNA is activated, new virions are
  produced.

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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?
The reproductive cycle of the HIV
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The Genetics of Viruses and Prokaryotes - How do
viruses reproduce and transmit genes?

• Prokaryotes have several ways of recombining
  genes
• conjugation
• transformation
• transduction
• plasmids
• transposable elements.

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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?
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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?
Bacterial conjugation
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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?
Recombination following conjugation
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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?

• Plasmids exist independently of the main
  chromosome.
• They can be transferred during conjugation.
• Plasmids dont need to recombine with the main
  chromosome.

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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?
Gene transfer by plasmids
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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?

• Many plasmids have special genes.
• Metabolic factors are plasmids with genes for
  unusual metabolic functions such as breaking down
  hydrocarbons.
• Fertility factors (F factors) have genes needed
  for conjugation F factor can be transferred
  during conjugation.

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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?

• Resistance factors (R factors or R plasmids) code
  for proteins that modify or destroy antibiotics.
• Some provide resistance to heavy metals.
• R factors have become more abundant in modern
  times, possibly because of the heavy use of
  antibiotics.
• Antibiotic resistance is a serious threat to
  human health.

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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?

• Transposable elements DNA sequences that are
  inserted into new locations
• Transposon is a longer transposable element
  (5,000 base pairs) that carries one or more
  additional genes.
• Transposable elements have contributed to the
  evolution of plasmids.

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The Genetics of Viruses and Prokaryotes - How do
prokaryotes exchange genes?
Transposable elements and transposons
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The Genetics of Viruses and Prokaryotes - How is
gene expression regulated in prokaryotes?

• Prokaryotes shut down transcription by placing an
  obstacle - the operator - between the promoter
  and the structural gene. The operator binds to a
  protein called a repressor-blocks transcription
  of mRNA.
• Operon the whole unit - promoter, operator, and
  one or more structural genes. Operon containing
  genes for lactose metabolism lac operon.

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The Genetics of Viruses and Prokaryotes - How is
gene expression regulated in prokaryotes?
The lac Operon of E. coli
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The Genetics of Viruses and Prokaryotes - How is
gene expression regulated in prokaryotes?
The lac Operon an inducible system (part 1)
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The Genetics of Viruses and Prokaryotes - How is
gene expression regulated in prokaryotes?
The lac Operon an inducible system (part 2)
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The Genetics of Viruses and Prokaryotes - How is
gene expression regulated in prokaryotes?
The trp Operon a repressible system (part 1)
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The Genetics of Viruses and Prokaryotes - How is
gene expression regulated in prokaryotes?
The trp Operon a repressible system (part 2)