Basic Components of Viruses

A virus particle or virion is consists of nucleic acid (DNA or RNA) that is covered by a

protein coat called capsid. The combined nucleic acid and capsid is called nucleocapsid.

The nucleocapsid can either be naked or enclosed by a membrane termed envelope. The

capsid itself is made up of subunits called capsomere.

The proteins that make up the virus particle are termed structural proteins. The viral

genome also codes for important enzymes called non-structural proteins required for

viral replication but are not incorporated in the virion.

Viral Nucleic Acid

 This can either be RNA or DNA

 It contains the information necessary for directing the infected cells to synthesis virusspecific

proteins

 It may be single stranded or double stranded

 It may be linear or circular

 It may be positive sense or negative sense ( a positive sense nucleic acid possesses the

same polarity as the mRNA and so can be translated directly into protein without first

being transcribed)

 It may be a single piece or segmented

 It is haploid except in retroviruses in which it is diploid

The Capsid

 It is made up of proteins arranged in multiple almost identical units called capsomere

 It offers protection for the nucleic acid against adverse conditions

 It facilitates attachment and entry of the virus into host cell

 It possesses antigens used for virus identification in serological tests

 It determines the symmetry of the virus

The Envelope

 Present only in some viruses

 It is made up of lipids

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 It is derived from the plasma membrane of the host cell during the release of the

virus from the cell by budding

 In enveloped viruses, capsomeres take the form of projections called spikes or

peplomers protruding out through the lipid bilayer of the envelope

 The spikes are glycoprotein in nature

 There may be a stabilizing protein membrane beneath the envelope lipid bilayer.

This is referred to as the membrane/matrix protein

 In some viral infections, the envelope is acquired from the endoplasmic reticulum,

the Golgi apparatus or the nuclear membrane

 Enveloped viruses are usually susceptible to detergent and are rendered noninfectiuos

following damage to the envelope

Fig 1 - Structure of a Virus (From: Encyclopedia of Molecular Biology, Blackwell Science Ltd,

1994)