Chapter 14: Virions, Prions, and Viroids Infectious Agents of Animals and Plants

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Chapter 14Virions, Prions, and
ViroidsInfectious Agents ofAnimals and Plants
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Important Point
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material Try reading your text before
attending lectures. And take the time to read it
well!
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Vertebrate RNA Viruses
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More RNA Viruses
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Vertebrate DNA Viruses
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Routes of Transmission
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Routes of Transmission
Some viruses are acquired via a respiratory route
but cause symptoms elsewhere so arent considered
respiratory viruses example measles virus
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Animal Virus Life Cycle

• Attachment to susceptible cell
• Uptake of nucleocapsid into cell
• Targeting of nucleocapsid to site of reproduction
• Uncoating of nucleic acid from capsid
• Synthesis of viral proteins
• Replication of nucleic acid
• Maturation of virion particles
• Virion release from cell
• Virus spread within host
• Exit of virus from host
• Transmission of virus to new host

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Steps not seen with Phage

• Attachment to susceptible cell
• Uptake of nucleocapsid into cell
• Targeting of nucleocapsid to site of reproduction
• Uncoating of nucleic acid from capsid
• Synthesis of viral proteins
• Replication of nucleic acid
• Maturation of virion particles
• Virion release from cell
• Virus spread within host
• Exit of virus from host
• Transmission of virus to new host

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Entry of Enveloped Viruses
Note that viral proteins remain on outside of
cell and therefore exposed to host immunity.
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Entry of Enveloped Viruses
Fusion occurs with vesicle membrane rather than
plasma membrane
Note that viral proteins dont remain on outside
of cell.
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Many Transcription Strategies
Dont worry about virus names (in red).
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Dependence on Host Enzymes
In other words, the greater the size of a virus
genome, the less dependent that virus is on the
host metabolism.
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Budding of Enveloped Viruses
Not all enveloped viruses bud through the plasma
membrane.
Note though that naked animal viruses generally
lyse their host cell to effect virion release.
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Cytopathic Effects
Uninfected cells.
Cytopathic Effects are changes virus infection
makes on host-cell morphologies.
Cells infected with adenovirus.
Cells infected with HSV.
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Various Impacts of Animal Viruses on Cells
Impact is virus-type and also host-cell type
specific.
Concentrate on the non-tumorgenic aspects of
viral infection.
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HIV Life Cycle(through reverse transcription)
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HIV Life Cycle(through virion maturation)
Note that maturation actually takes place via
virion budding!!!
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HIV Life Cycle(budding through plasma membrane)
Reverse transcriptase enzyme already incorporated
into capsid.
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Time Course of Infections
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How the Flu Changes its Spots
E.g., swapping of coat proteins between avian and
human strains of influenza virus.
Particularly important is the protein virus
protein, hemagglutinin.
But note that the diversity of influenza
sequences worldwide in any given year appears to
be roughly comparable to the diversity of HIV
sequences found within a single individual at one
time point. (Bette Korder as cited in Microbe,
2006 1(3)111-112)
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Prion-Associated Disease
These are all Transmissible Spongiform
Encephalopathies and all appear to be caused by
proteinaceous infectious agents called Prions.
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Hypothesis for Prion Action
Accumulated abnormal proteins kill neuron, with
prions spreading to adjacent neurons.
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