Virus Structure and Classification

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Chapter 5

• Virus Structure and Classification

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VirusStructure

• Visualization is usually with an electron
  microscope
• Cant see the virus in cells by light microscopy
  but can see the effect of virus in cells
• enlarged nuclei in tissues
• cytopathic effect in tissue culture

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Composition of Viruses

• Genome or core RNA or DNA
• Capsid protects genome and is virally encoded
  structural proteins
• also have other virally encoded proteins that
  help in replication
• Some viruses have envelopes on the outer virion
  surface that have proteins that interact with the
  surface of cell that the virus infects

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Virus Structure
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Viral Genomes

• RNA or DNA single-stranded or double-stranded
  circular or linear
• Used to classify viruses
• DNA viruses replicate in host nucleus, share
  common patterns as host cell
• RNA viruses must have specific enzymes that are
  not in cells so can make RNA for new viruses
• Viral and cellular proteins are not identical
• Virus encodes enzymes and proteins that modify
  the cell in which virus replication

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Viral Capsids

• Complex, many identicakl subunits of viral
  protein subunit capsomere
• Protects viral genome in 2 stable shapes for a
  particle of nucleic acid and protein
  nucleoprotein
• helix capsomere associates with helicalk genome
• icosahedron regular, solid shape that enfolds
  viral genome

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Viral Caspid Structures
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Capsids have Symmetry

• Capside around the genome is unique for each type
  virus
• They have symmetry which is a criteria for
  classification
• Capsid structure is defied by X-ray
  crystallography
• Capsids have various complexity of structures
  based on the viral genome

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Symmetry

• Icosahedral capsid has 3 distinct symmetry
  patterns interactions of one plane to another
• 2 fold
• 3 fold
• 5 fold

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Viral Membrane Envelopes

• Some viruses have a lipid envelope that surrounds
  the capsid
• Lipid bilayer is from the cell in which the virus
  replicates and virally encoded membrane
  associated proteins also used to characterize
  viruses
• Usually is amorphous because shape is defined by
  the capsid

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Classification Scheme

• Based on basic properties of the virus and
  specific features of the replication cycle
• What type of genome?
• RNA/DNA, ss/ds, circular/linear, single
  piece/segmented
• How is capsid arranged
• symmetry and dimension
• What other components?
• envelope
• proteins in virion responsible for infection or
  maturation

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Baltimore Scheme

• Used mainly for RNA viruses and based on how mRNA
  is synthesized
• limited by type of genome in the virus
• sense RNA virus RNA serves as mRNA template
• - sense RNA virus RNA serves as complimentary
  sense of mRNA
• DNA has both senses
• Retrovirus - sense RNA but use DNA in
  replication cycle are not classified
• Table 5.1 combination of Baltimore scheme and
  criteria listed above

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Classification by Disease

• Value to medical and public health professionals
• Arthropod-borne arboviruses
• Some viruses can cause similar disease so must do
  diagnostic tests to determine etiology (causative
  agent) of the disease