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

• 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.