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Viral proteins

  1. Proteins, which are a part of the virion, are called as structural proteins (capsid, surface glycoprotein, enzymes (in some viruses))
  2. Functions of structural proteins
    1. Facilitate transfer of viral nucleic acid from one host cell to another
    2. Protect the viral genome from nucleases
    3. Promote attachment of the virus particle to susceptible cell
    4. Provide structural symmetry to the virus particle

3. Capsid

  1. Composed of repeating protein units called as capsomeres
  2. Capsid with enclosed nucleic acid is called as nucleocapsid
  3. Capsid shows three types of symmetry
    1. Icosahedral/ cubic symmetry: capsomeres are arranged in such a way that the capsid appears as an icosahedron having 20 faces and 12 vertices e.g. adenoviruses. There is a possibility of formation of some empty particles devoid of nucleic acid.
    2. Helical symmetry: the nucleic acid and capsomeres are wound together in the form of a helix e.g. orthomyxoviruses. It is not possible for empty helical particles to form.
    3. Complex symmetry: viruses, which do not show either icosahedral or helical symmetry but are more complicated in structure e.g. poxviruses.
      Some viruses carry enzymes, which are essential for the initiation of the viral replicative cycle when a virion enters the host cell. For e.g. RNA polymerase carried by the viruses with negative sense RNA genomes (orthomyxoviruses, rhabdoviruses)

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