Viruses

• are obligatory intracellular parasite. They do not occur as free-living organism.
• Structurally, they have either type of nucleic acid DNA or RNA but never both.
• On the basis of DNA or RNA content virus are of two types:- DNA virus and RNA virus.
• Virus is single molecule. Either single stranded or double stranded nucleic acid.
• The outer shell – capsid is made up of only protein except a few animal viruses where additional polysaccharides are also present in the outer cell.
• They can be propagated or reproduce only in living culture media because they do not have self-protein synthesis machinery.
• They contain no metabolic enzyme protein synthetic machinery of their own. They used protein synthetic machinery of host for the synthesis of their protein.
• Their Nucleic acid directs the host cell to making various parts of the virus and assemble these parts into complete infectious particle called virions.
• Eg.: Tobacco mosaic virus (TMV), Gimini Virus, bacteriophages, retro virus etc.