Viruses which infect bacterial cells are known as bacteriophage or bacteria eaters. They contain DNA as genetic material. There are many varieties of bacteriophages. Usually, each kind of bacteriophage will attack only one species or only one strain of bacteria.
fig:structure of bacteriophage
The shape and size of bacteriophage may vary from species to species. Some are spherical, comma- shaped while some are tadpole-shaped with a polyhedral head, a short neck, collar and straight tail. The head is bipyramidal hexagonal in shape. The contents of the head are enclosed by a capsid which is about 35ºA thick. The head enclosed a double stranded greatly folded DNA which contains more than 75 genes. The tail is a hollow structure that is enclosed by a spring-like a contractile sheath.
Chemically viruses are nucleoprotein entities, which are composed of nucleic acid and proteins. Nucleic acid forms the central core and is surrounded by a protein coat. The capsid is composed of a number of subunits called capsomeres. The capsomeres are closely packed and are arranged in the helical or polyhedral in shapes. The nucleic acid occupies about 1-4 % of the whole amount of the virus. It is tightly fitting by a protein sheath. The nucleic acid is of either DNA or RNA.
Mode of transmission
- Some viruses like tobacco mosaic virus, potato mosaic virus, the wheat mosaic virus lies in the soil with the plant debris.
- A large number of viruses are transmitted mechanically from diseased plants by contact of infected and healthy leaves by the wind, grafting infected buds on to healthy plants, agricultural tools like knife play quite an important part to spread the diseases.
- A number of viruses and legumes are transmitted through the infected seeds.
- Many soil-borne viruses like tobacco necrosis virus, wheat mosaic virus etc are transmitted by nematodes and fungi present in the soil.
- Viruses can be transmitted by the slight rubbing contact of the infected and healthy plant organs by the action air or other means.
- Some viruses are transmitted through the vectors like insects, aphids, etc.
- The viruses are transmitted through one to other by the means of grafting.
Economic Importance of viruses
- Viruses may destroy the bacterial cell and make water free from bacteria.
- Bacteriophages can be used in the treatment of various diseases like cholera, dysentery etc.
- Some of the viral infected plants are beautifully variegated. They are used in a curio. Eg; Tulip flower
- Viruses cause numerous diseases in human beings like AIDS, cancer, Anthrax, polio, rabies, mumps, small pox etc.
- Some of the common viral plant diseases are tobacco mosaic, tomato mosaic, potato mosaic, yellow diseases of peach etc.