Know about virus

The Russian Botanist D.I Ivanowsky prepared an infectious extracts from tobacco plants that were suffering from mosaic disease. When the extract was allowed to pass through a filter which is able to prevent the passage of bacteria, the filtered fluid was still infectious. In 1898 , Beijerink coined the name 'virus' (Latin-poison) due to infectious nature. Mayer (1886), Beijirink (1898) established the existence of viruses by discovering many viral diseases in plants and animals. In 1900, Walter Reed discovered the virus of yellow fever , the first known viral disease of man. In 1915, F.W Twort and F.D Herelle independently noticed that bacteria were also infected by virus and those viruses that infect only bacterias were named as bacteriophase. Schelsinger (1933) was first to determine the chemical composition of viruses.Wendell Stanley, in 1935 isolated a virus in a crystalline form and reported that viruses were made exclusively of proteins. Pirie and Bawden (1936 ) give the concept that viruses are made up of nucleic acid and protein. Likewise, Harshey and Chase (1952) showed that only he nucleic acid is responsible for infection but protein is non infectious.

In 1981, Matthews defined virus as the set of one or more nucleic acid molecules enclosed in a protective coat of protein and able to organize its own replication within a suitable host cell.
Viruses are the causative agents of many infectious diseases like mumps, common cold, flu, small pox etc. It is an obligatory parasitic pathogen with at least one dimension of less than 200 millimicrons; on average they are about 50 times smaller than bacteria. They do not have a cellular structure or any metabolic activity as they do not have enzyme systems and protein synthesis machinery and cannot reproduce outside living cells . They can be recognised only by their biological behaviour such as, by the disease they cause. Their exact nature was a mystery for long. Most of the virus have only one nucleic acid DNA or RNA. Rous sacroma virus, producing certain cancer is only virus having both DNA and RNA . Some small viruses have been crystallised and behave as chemical like TMV and poliomyelitis.

Viruses are found in
1. Helical shape eg. TMV, influenza virus.
2. Polyhedral shape eg. polio virus
3. Tadpole shape eg. bacteriophases

Viruses are broadly classified into following types.
1. Plant virus: Virus which infects only plant is known as plant virus. Their genetic material is RNA. The first plant virus to discover was Tobacco Mosaic Virus . eg of plant virus are bean mosaic virus, potato virus.
2. Animal virus: Virus which infects only animal is known as animal virus. Their genetic material is DNA. eg. rabies virus
3. Bacteriophase: Virus infecting bacterial cell is known as bacteriophase. They contain DNA as genetic material.

They are also classified based on the type of their nucleic acid which are as follows:
1. DNA virus (Deoxyvira): Virus containing DNA is known as DNA virus.
2. RNA virus (Ribovira): Virus containing RNA is known as RNA virus.