Viruses are very minute particles that they can be seen only under electron microscope. They are measured in millimicrons ( 1 millimicron = 1/1000micron). (1micron - 1/1000 millimeter). Generally they vary from 2.0 mm to 300 mm in size.
The following data is used for reference.
a. Staphylococcus has a diameter of 1000 mm.
b. Bacteriophage varies in size from 10-100 nm.
Broadly speaking viruses occur in three main shapes:
1. Cubic symmetry: polyhedral or spherical - eg. Adeno virus, HIV
2. Helical symmetry: e g . Tobacco Mosaic virus (TMV), Influenza virus.
3. Complex or atypical eg. Bacteriophage, Pox virus.
A virus is composed of two major parts
1. Capsid (the protein coat)
2. Nucleic acid.
The capsid is the outer protein coat. It is protective in function.
It is often composed of many identical subunits called capsomeres. Some of the viruses have an outer covering called envelope eg. HIV. They are called enveloped viruses.
Others are called naked viruses or non- enveloped viruses. The capsid is in close contact with the nucleic acid and hence known as nucleocapsid. The nucleic acid forms the central core. Unlike any living cell a virus contains either DNA or RNA, but never both. The infective nature of the virus is attributed to the nucleic acid while host specificity is attributed to the protein coat.