For more than four decades, viruses
were not considered as organisms, because they don’t show many of the
characters associated with living organisms’ viz. cellular structures, its own
metabolism and replication, etc. But, at the same time, they were found to
cause many plant and animal diseases and hence were considered to be important
biologically. They were known as filterable
agents. M.W. Beijerinck, a Dutch microbiologist coined the term, virus (Latin “poison”) for them. They
are also known as viruses or bacteriophage
(Gr. “bacteria”, phagein” meaning to eat bacteria).
Viruses have very simple structure with a nucleic acid genome encased in a protein coat. The genome can be RNA (e.g’, TMV, Influenza, virus,
etc) or DNA (Adenovirus), which may in turn be single-or double –stranded. Most
DNA viruses are double-stranded.
Viruses replicate by taking over a host’s
cell system and are thus obligate intracellular parasistes.
In 1953 W.M. Stanley,
an American Chemist, isolated the Tobacco Mosaic Virus (TMV) in pure
crystalline form, which could be kept for long periods of time.
