Nature of Virus

Nature of Virus

  • The virus is generally considered as a living organism as it contains genetic material and reproduces inside the host cell.
  • Virus are known to be host-specific.
  • Invisible in ordinary microscopic methods.
  • They are not capable to propagate themselves outside a living cell.
  • They are the cellular parasites.
  • Viruses are found to be smaller than bacteria.
  • Most animal and plant viruses cannot be seen under a light microscope
  • The diameter of the smaller viruses is 200 Å
  • Viruses don’t possess enzyme systems and protein synthesis machinery.
  • They are the obligatory intracellular parasites.
  • A nucleic acid core is surrounded by a protein coat.
  • Some viruses have a membranous envelope.
  • The envelope consists of proteins, carbohydrates and lipid outside the protein coat.
  • The envelope is derived from the host cell.
  • Cytoplasmic organelles are absent due to absence of cytoplasm.
  • They also lack the cell membrane.
  • One nucleic acid either RNA or DNA is present.
  • Some viruses have both DNA and RNA. Ex Rous Sarcoma virus.
  • Viruses can be crystallised. They behave like chemicals.
  • The virus multiplies its genetic material in the host cell.
  • Size of viruses ranges in between 100A to 250mu.
  • Psittacosis is the virus larger than bacteria. They measure 0.75mu in diameter.
  • Viruses have 3 main shapes.
    • Spherical / cubical/ polyhedral
    • Helical / cylindrical / rod-shaped
    • Complex
  • Cubical shapes may have 4 faces/ 12 faces / 20 faces.
  • Example of :
    • Cubical shape – Herpesvirus
    • Helical shape – TMV
    • Complex shape – Bacteriophage

Nature of Virus

  • The virion is the intact virus unit.
  • Capsomeres are the subunits of the capsid. The capsid is its protein coat.
  • The nucleic acid is protected by capsid against the action of nuclease.
  • Some proteins in capsid help in binding the virus to the surface of host cells.
  • Surface proteins also act as an enzyme.
  • They dissolve the surface layer of the host cell.
  • This helps in penetration of nucleic acid into the host cell.