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Viral Classification

  • Viruses can be classified in a number of different ways in order to highlight the nature of their genome, the geometry of their capsid, whether they possess an lipid envelope, or their tropism for a particular tissue type.
Genome Classification
  • Overview
    • Viral genomes are first categorized by their chemical composition as either DNA or RNA genomes. The headings of the sections below are interlinks which will list viruses within that category.
  • DNA Viruses
    • dsDNA Viruses: dsDNA genomes possess two anti-parallel strands in a double-helix structure
    • ssDNA Viruses: ssDNA genomes possess only a single strand and do not form a double-helix structure
  • RNA Viruses
    • dsRNA Viruses: dsRNA genomes possess two anti-parallel strands much like DNA in a double-helix-like structure
    • ssRNA Viruses: ssRNA genomes possess only a single strands which can be organized in different orientations
    • Positive ssRNA Viruses: Orientation of genes in the RNA genome is such that they can be directly translated to proteins much like a host cell's mRNA
    • Negative ssRNA Viruses: Orientation of genes in the RNA genome is reversed such that they must be first replicated to the positive RNA strand prior to protein translation
    • Retroviruses: These are RNA viruses whose genome must first be "Reverse-transcribed" to DNA and only then transcribed and translated.
Capsid Geometry Classification
  • As discussed in Viral Structure, viral capsids are highly ordered protein shells which possess specific geometries that can be used for classification
  • Icosahedral Viruses: Capsid is an Icosahedron which possesses twenty faces of equilateral triangles
  • Helical Viruses: Capsid is arranged as a long helix
  • Complex Viruses: Capsid has a complex shape that does not fall into the above categories
Possession of an Envelope
  • Nonenveloped Viruses: Do not possess an outer lipid envelope
  • Enveloped Viruses: Possess an outer lipid envelope
Tissue Tropism