VIRUSES,, VIROIDS, PRIONS

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.

 

VIROIDS

Recently, some sub-viral particles have been discovered viz. viroids and prions. Viroids are tiny, naked molecules of circular RNA, with only a few hundred nucleotides. There are important infectious agents of some plant diseases viz. potato spindle tuber diseases in U.S.A, (Incidentally, the term viroid was coined by T.O. Diener for the causative agent of this disease), Cadang-cadang disease of coconut palm in Philippines and Chrysanthemum stunt (CS), etc. they use host proteins to replicate.

 PRIONS

For decades, scientists had been baffled by a peculiar group of fatal brain diseases. These diseases showed a peculiar property of showing their symptoms sometimes as late as 30-40 years after the infection. These diseases include scrapie disease in sheep, Creutzfeldt-Jacob disease (CJD (in humans, bovine spongiform encephalopathy (BSE) or “mad cow” disease in cattle, “Kuru” disease in villagers of New Guinea, etc. These diseases are collectively known as transmissible spongiform encephalopathies (TSEs). 

 In the early 1970, physician Stanley Psrusiner began to study TSEs and despite repeated trials, he the infectious TSE preparations. He, then concluded, as Alper and Griffith had earlier suggested, that the infectious agent was a protein. He named it as a prion, - a “proteinaceous infectious particle”. Prions consist of misfolded proteins that cause related cellular proteins to also misfold.