Viruses

Viruses are so small that they cannot be seen even with the highest magnification of the microscope using visible light.They are recognizable only by their biological behavior,such as, by th3e disease they cause.Their exact nature was a mystery for long.They were variously regarded as invisible form of bacteria,protozoa,enzymes,toxins or as unusual products of metabolism of the cells in which they were found.

An America biochemist,Stanley in 1935,however isolated by chemical means from the diseased tobacco leaves a material which appeared to be a protein of high molecular weight.He studied this material in detail, and found that it possessed all the properties of a tobacco mosaic virus,which has already be discovered.

There are a number of direct,indirect and circumstantial,evidences to show that the material itself is the virus.Stanley,on the basis of the chemical properties,identified the material as autocatalytic proteins which can multiply within the living cell only.Bawden purified the tobacco mosaic virus, and found it to be a crystalline nucleo-protein of very high molecular weight,retaining its infectivity even when diluted to a concentration of 1/1,000,000.

This virus when examined with the electron microscope using X-rays,is found to be in the form of bundles of rod-like protein.Such nucleo protein cannot be obtained from healthy plants and is the virus itself.In 1938 the study of potato virus confirmed this.Since then,proteins of high molecular weight possessing all the properties of the respective viruses have been isolated and studied.

Most of the viruses are crystalline and rod-like,but different in size.Tobacco mosaic virus is rod-like in shape,280 mmicro in length and 18mmicro in breadth.'Bushty stunt' virus of tomato is 274 mmicro in diameter and its molecular weight is 8,800,000-12,800,000.Stanley studied the nucleo proteins of the 'ring-spot' virus of tobacco.The virus is rounded with 19mmicro diameter and 3,400,000 molecular weight.Tobacco "necrosis' virus is smallest of all the plant viruses known so far.It is rounded with 13-20 mmicro diameter.

Before 1935 they were believed to be living on the basis of few following properties:

1. That they can live only in a living cell.
2. They can infect healthy plants just like bacteria and fungi.
3. They multiply in number and grow in size,as the living organisms reproduce and grow.
4. They have physiologic specialization in relation to the insect vectors and the plants.
5. They respond to stimuli,such as acids,alkalies,light and temperature.

The supporters of the non-living nature viruses based their views on the following few properties:

1. They are too small to be observed under visible light.
2. They retain infectivity even in very low concentrations.
3. They can be crystallized like a chemical substance.
4. They can be sedimented like proteins.
5. They can be precipitated by a number of chemical substances.
6. They retain the power of infection even after 31 years in non-living tobacco leaves.

Some smokers did not agree with any of the above extreme views and adopted a middle course.They regarded viruses,representing a stage in between living and non-living with the acquired property of multiplication.

At present most of the virologists have agreed that viruses are nucleo-proteins of high molecular weight and have the power of multiplication.