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Virus Structure and Classification

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Visualization is usually with an electron microscope ... icosahedron regular, solid shape that enfolds viral genome. Viral Caspid Structures ... – PowerPoint PPT presentation

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Title: Virus Structure and Classification


1
Chapter 5
  • Virus Structure and Classification

2
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

3
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

4
Virus Structure
5
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

6
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7
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

8
Viral Caspid Structures
9
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

10
Symmetry
  • Icosahedral capsid has 3 distinct symmetry
    patterns interactions of one plane to another
  • 2 fold
  • 3 fold
  • 5 fold

11
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

12
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

13
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

14
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
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