Viruses differ greatly in size. They range in length from 0.01 to over 0.3 micrometers; yet over 500 of them can fit on the point of the pin.

A disease of tobacco plants, known as tobacco mosaic first led to the identification of the virus as the pathogen. This disease causes the leaves of tobacco plants to become mottled with a yellow and green mosaic pattern. In 1892 the Russian biologist Dmytro Ivanowskyi squeezed the fluid from a diseased plant containing some factor which caused disease. He examined the fluid and the remainder left on the filter under the light microscope. In both instances he found nothing. Yet rubbing the fluid on a healthy plant caused the plant to become diseased. In 1898 the Dutch botanist Martinus Beijerinck repeated Ivanowski’s work. He concluded that the fluid contains the unknown factor, smaller than bacteria. He called this factor a virus from Latin “poison”. An American biologist, Wendell Stanley, finally isolated the tobacco mosaic virus in 1935. He extracted and crystallized the virus from the fluid of thousands of diseased plants. From a ton of diseased tobacco leaves Stanley produced a teaspoonful of crystals. In their crystallized form, the viruses did not appear to be alive. However, when the virus crystals were put back into solution and rubbed on a healthy plant, the plant contracted the disease.

In 1935 Stanley isolated and crystallized the virus of tobacco mosaic. Since that time many other viruses were examined in their crystalline forms. In 1956 Stanley declared, that he lucked to decompose a virus particle the principle components-protein and nucleic acid and then again reunite them with the formation of an active virus. In the further experimental study Frencel – Conrad had found that nucleic acid possessed infection by itself but less significant then in the whole virus particle. Nowadays it is finally established that many diseases of plants and animals have virus character. The most harmful of them for agriculture are: pig cholera and influenza, hen cancer. Among the man’s virus diseases are wide-spread such as: hydrophobia, children paralysis, smallpox, measles, yellow fever, infectious cold.

Date: 2014-12-22; view: 1198