Question

Select the wrong statement.
(a) Viroids were discovered by D.J. Iwanowski.
(b) W.M. Stanley showed that viruses could be crystallized.
(c) The term ‘contagium vivum fluidum’ was coined by M.W. Beijerinck.
(d) Mosaic disease of tobacco and AIDS in human beings are caused by viruses.

Answer 1

Virology as a field of study began with the discovery of the tobacco mosaic virus, but many scientists before the discovery had predicted the presence of a parasitic submicroscopic organism and they were defined by various terms.

Complete answer:
- Viroids were discovered by T.O. Diener. D.J. Iwanowski is known as the father of virology and is credited to discovering the Tobacco Mosaic Virus.
- Viroids are the smallest infectious pathogens, composed of a short strand of circular, single-stranded RNA with no protein coating.
- All viroids are known to infect only higher plants.
- The first viroid was discovered by T.O. Diener which was the potato spindle tuber disease viroid, also known as PSTVd.
- Viroids contain nucleic acid but do not code for protein; they use the RNA polymerase II enzyme from the host cell for their replication.
So, the wrong statement is ‘Viroids were discovered by D.J. Iwanowski’.

Additional Information:
- W.M Stanley was responsible for discovering that viruses could be crystallized and he demonstrated it in the Tobacco Mosaic Virus. He also discovered that crystallized viruses could still infect actively.
- The ‘contagium vivum fluidum’ was coined by Beijerinck which meant that the virus was somewhat fluid-like in nature. It literally means ‘contagious living fluid’. Years after this statement it was discovered (from finding that viruses can be crystallized) that viruses were particulate in nature.
- The mosaic disease of tobacco is caused by the Tobacco Mosaic Virus and AIDS in humans is caused by the Human Immunodeficiency Virus.

Note:
- The Hepatitis D virus which affects human beings is similar to a viroid.
- The two families of viroids are Pospiviroidae and Avsunviroidae.
- Viroids can be transmitted by aphids, my mechanical damage to parts of the plant, and by leaf to leaf contact.