Viruses are the smallest obligate intracellular infective agents containing only one type of nucleic acid (DNA or RNA) as their genome. They have no metabolic activity outside the living cells. They do not possess a cellular organization and lack the enzymes necessary for protein and nucleic acid synthesis. Viral genome (nucleic acid) diverts the host’s metabolism to synthesis a number of virus specific macromolecules required for the production of virus progeny.

Viruses occupy the twilight zone that separates the ‘living’ from the ‘non-living’. The demonstration by Stanley (1935) that viruses could be crystallized like chemicals, and the extraction by Geirer and Schramm (1959) of ‘infectious nucleic acid’ from a virus that could infect host cells and yield complete virus progeny made it appear that viruses were only ‘living chemicals’.



viruses are much smaller than other organisms. The extracellular infectious virus particle is called the virion. The size of viruses ranges from 20 to 300nm in diameter. The largest virus is the smallpox virus (300 nm) and the smallest is the parvovirus (20nm).



The virion consists of a nucleic acid core (genome) surrounded by a protein coat, the capsid. The capsid together with the enclosed nucleic acid is known as the nucleocapsid. The capsid is composed of a large number of protein subunits (polypeptides) which are known as capsomers. Two major functions of capsid are, forming an impenetrable shell around the nucleic acid core and to introduce viral genome into the host cells by absorbing readily to cell surfaces. Certain viruses also contain envelope that surrounds the nucleic acid. The envelope is acquired by the virus during release by release by budding through the host cell membrane. It is lipoprotein in nature. The lipid is largely of host cell origin while the protein is virus coded. Protein subunits are exposed as projectile spikes on the surface of the envelope. These structures are called polymers.

2. Symmetry

Icosahedral (most common)Polygonal with 12 vertices/ corner, 20 facets/ sides
HelicalSome RNA virus contain this type of shape
ComplexPox virus virus contain this type of shape


A . Nucleic Acid

Viruses contain only one kind of nucleic acid, either single or double stranded DNA or RNA.

B. Viral Protein and Lipids

Viruses contain protein which makes up the capsid. Viral protein, besides protecting the nucleic acid, also determines the antigenic specificity of the virus. In case of enveloped viruses, they contain lipids (present in the envelope) derived from the host cell membrane.


Viral genome directs, the biosynthesis machinery of the host to synthesize components of “new progeny virus”. Viral proteins are synthesized mostly in cytoplasm. Most of the DNA viruses replicate in the nucleus except Pox virus. Most of the RNA viruses replicate in the cytoplasm except Influenza and retro virus. Most of DNA viruses are double stranded except parvo virus and most of the RNA viruses are single stranded except Reo virus.

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