Structure of virus:

Virus is an obligate intracellular particle which is consider as borderline between living and non-living. Virus needs host cell for activation (metabolism) and outside the host cell it remains dormant or shows non-living properties.

Virus consists of a genome (DNA or RNA but not both) surrounded by a protein coat called capsid. The capsid together with the enclosed nucleic acid is called nucleocapsid. Some viruses have an additional protective layer, envelope. Virus without an envelope is called naked virus. Naked virus is more resistant to drying, heat, detergents, acids and proteases.

• Genome: this is either DNA or RNA. It can exists as single strand (ss) as in RNA viruses (except Reo virus) or as a double strand (ds) as with most DNA viruses (except Parvo virus). Genome can be linear or circular (super-coiled circular genome in case of PAPOVA virus).
• Capsid: protein coat enclosing the nucleic acid is capsid. It is made of small units called capsomeres. It posses antigenic properties (can evoke immune response) and contains special sites that allow to attached to the host cell (adsorption) and helps in penetration. Capsid protects the nucleic acid core from enzymatic digestion.
• Envelope: This is the outer covering of non-naked viruses acquired from the host cell membrane during viral maturation by a budding process. I t is a lipid bilayer. Virus encodes glycoproteins protruding from envelop and exposed on the external surface (peplomeres) (eg. Haemagluttinin).