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A virus is a non-living obligate intracellular parasite. They can only replicate inside of a living cell as they lack the necessary enzymes and molecular building blocks to be self sufficient. Viruses can be classified by structure (icosahedral, enveloped, complex etc), genome (retroviruses have an RNA genome) or by the route through transcription (Baltimore classification).

They have a large protein coat, called a capsid, which is formed from many repeating protein subunits which join non-covalently to produce a large icosahedron sphere. Inside this sphere is where the virus' free nucleic acids are kept. The structure of this sphere is such that it protects the nucleic acid but also allows the nucleic acid to exit so it can go on to infect other cells[1].

Some viruses' capsids are surrounded by an envelope. The envelope is a lipid bilayer which contains glycoproteins, these glycoproteins are used to help the virus to detect and bind onto receptors on the surface of the host cell[2].

Although biology has shown proof that there is a linkage between the viruses infecting the trinity of life domains- archaea, bacteria and eukarya to its historical root of origin, as of today, viruses are not considered to be part of the “universal tree of life , which is thus only a tree of cellular life”[3].


  1. Alberts, B. Johnson, A. Lewis, J. Raff, M. Roberts, K. Walter, P. (2008) Molecular Biology Of The Cell. Fifth Edition, New York:Garland Science(148-149)
  2. Lucas, William(Apr 2010) Viral Capsids and Envelopes: Structure and Function. In: eLS. John Wiley and Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001091.pub2]
  3. Brussow, H. (2009). The not so universal tree of life or the place of viruses in the living world. Philosophical Transactions of the Royal Society B: Biological Sciences, 364(1527), pp.2263-2274.
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