Viruses are the most primitive cellular and non-cytoplasmic infectious agents. Russian botanist D.J. Iwanowski (1892) first discovered virus in an infected tobacco plant. However, M.W. Beijerink (1898) coined the term virus. Then American chemist W.M. Stanley (1935) isolated pure crystal of Tobacco Mosaic Viruses (TMV) and concluded that viruses are made of nucleoproteins.
General Characteristics of Viruses:
(i) Viruses are a cellular, non-cytoplasmic infectious agents.
(ii) They are smaller than bacteria, and this can pass through bacteriological filter.
(iii) Viruses are transmissible from disease to healthy organisms.
(iv) All viruses are obligate parasites and can multiply only within the living host cells.
(v) Viruses contain only a single type of nucleic acid either DNA or RNA.
(vi) Viruses are host specific that they infect only a single species and definite cells of the host organisms.
(vii) Viruses are effective in very small doses. They are highly resistant to germicides and extremes of physical conditions.
Generalised Structure of Viruses:
(i) Shape and size:
The shape varies considerable. They may be spherical or golf ball-like, rod-shaped, tadpole-like, helical or polyhedral. Plant viruses are smaller than bacteria.
(ii) Chemical structure and function:
Viruses have a very simple structure. The core of the viruses is made upon of nucleic acid, which is surrounded by a protein coat called capsid. The nucleic acid always contains only a single kind of nucleic acid i.e. either DNA or RNA. The infectious property of a virus is due to its nucleic acid.
Capsid or the protein coats:
It is made up of many identical protein sub-units called capsomeres. The capsomeres are composed of either one or several type of proteins. Capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus. The host specificity of virus is due to proteins of the capsid.
Biological position of viruses:
Viruses lack a cytoplasmic membrane and they do not have the basic component of a cell. They can only replicate inside the host cell. Outside the host cell, they are non-living. Thus, viruses show characters of both living and non-living.
(I) Non-living Characters of Viruses:
Following characters of viruses assign them as non-living:
(a) They can be crystallized.
(b) Outside the cell, they behave like inert chemicals.
(c) They do not show growth, development, nutrition, reproduction, etc.
(d) They can be precipitated.
(II) Living characters of viruses:
(a) They multiply within host cells.
(b) They possess genetic material, either DNA or RNA.
(c) There are definite races or strains.
(d) They exhibit mutations.
Because of the above reasons, viruses form unique bridge between living and non-living things.