Size : Range 0.015 - 0.2 μm
Characteristics :
Viruses are acellular entities that must invade a host cell in order to replicate. They are the smallest of all microbes ( the smallest in 10,000 times smaller than a typical bacterium ). Viruses are very different from other microbial groups. They are so small that most can be seen only with an electron microscope and they are not cellular. Structurally very simple, a virus particle contains a core made of only on type of nucleic acid either DNA or RNA. This core is surrounded by a protein coat. Sometimes the coat is encased by an addition layer, a lipid membrane called an envelope. All living cells have RNA and DNA, can carry out chemical reactions, and can reproduce as self sufficient units. Viruses can reproduce only inside the cells of other organisms.
Thus all viruses are parasites of other forms of life ( obligate intracellular parasites ). Coliphages ( bacterial virus ), Poliovirus ( animal virus ) and Tobacco mosaic virus ( plant virus ) are the well known examples of viruses. Similarly insect viruses and phages of algae ( cyanophages ) as well as of fungi ( mycophages ) have also been recorded. In short the characteristics of viruses are as follows :
* Viruses are very small noncellular parasites
* Viruses do not grow on artificial laboratory media.
* They require living cells within which they are reproduced.
* They are obligate intracellular parasites.
* Electron microscope is required to see viruses.
Practical Significance :
* Viruses causes diseases in humans ( small pox, rabies, influenza, AIDS, the common cold and some cancers ) other animals and plants.
* They also infect microorganisms.