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.