Viruses are minute parasitic, ultramicroscopic disease producing organism with dimensions of less than 200 mili microns. Each particle of virus is composed of many similar sub units arranged in regular manner in a space to give a fixity of structure that contrast vividity with continuously varying arrangements of the varied components inside living cells. Some plant viruses have isomeric particles.
Tobacco mosaic virus is rod shaped, vaccinia virus is rectangular like a brick, and influenza virus is spherical in shape. Smallest virus is 3 to 5 m μ.
Internal structure of virus: Nearly each virus particle ahs wrapping of protein within which lies the nucleic acid
(DNA). Bawden in 1959 by x-ray studies of rod like tobacco mosaic told that this rigid rod like structure is revealed to be a hollow tube with a deeply grooved exterior. Hollow tube is limed with a thin layer of protein and the nucleic acid has its phosphate sugar
Back bone at a radial distance of about mili micron from the centre of the tube where it is deeply embedded in protein. All the viruses contain protein subunits which are arranged in a regular protein. The ratio of nucleic acid and protein varies in different viruses. Nucleic acid in case of most plant viruses consists of RNA in bacteriophage and animal viruses it consist of DNA or sometimes both RNA and DNA.
Bacteriophage: The viruses which infect bacteria and kill them are called bacteriophage. Such viruses contain a substance called bacteriophage which is fatal to bacteria. It may be said that the bateriophages are the casual agents of the disease of bacteria. These bacterial viruses are tadpole like in appearance and consist of a tail and head. Chemically these contain deoxyribose nucleic acid (DNA) and Protein.
Multiplication of Bacterial Viruses:
Reproduction in bacterial viruses has been mainly studied by Stent in 1955. The bacteriophages involve their host i.e. bacteria by attaching themselves by tail, at first loosely, but then irreversibly. The tail then seems to make hole in the cell wall of host through which passes the nuclei acid of viruses into the host and thus protein coat of phage remains as a empty inert shell on the outside of bacterial cell. Inside the bacterial cell the phase DNA takes over the metabolic machinery and begins to make phase protein and new phase DNA. Eventually the protein combines with DNA to form a complete phase and all this occurs at the metabolic expense of the host cell where newly developed bacteriophage accumulate, the bacterial died and bursts there within a few minutes 100 to 200 new phage particles are formed. In this process only DNA is necessary for multiplication. This also indicates that a foreign DNA is capable of influencing the synthetic capacities of the cell. This fact is largely responsible for our present belief that an altered DNA or a virus DNA may be the cause number diseases particularly cancer
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