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In recent years, biologists have discovered certain living units called viruses. They are so small that they cannot be recognised even with the help of an ordinary microscope. But they exhibit certain properties, for example, the ability to reproduce their kind, which are normally associated with life.
The viruses can now be seen with the modern electron microscope. They resemble large molecules of matter to some extent and cause certain specific diseases such as ‘small-pox’ and ‘measles’ in animals. Plant diseases such as ‘blue stem’ in potatoes or ‘curly top’ in cabbages and other vegetables are also associated with virus infection.
Viruses are so small that they cannot be seen as individual particles under ordinary microscopes. That means most viruses are smaller than 210 millimicron (mµ). Viruses range in size from about 17 to 450 millimicron. The shape of all viruses are not same. They may be spherical (Polio virus), rod shaped (Tobacco mosaic virus or TMV), or tadpole like (Bacteriophage).
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The structure of virus is rather simple. It has an outer coat of protein and an inner core of DNA or RNA. Most of the viruses that parasitise animal cells have a core of DNA. In some animal virus and in all viruses that parasitise the cells of higher plants the core is made up of RNA. The core in case of influenza- and polio virus contains both DNA and RNA.
The viruses are completely inactive outside the host’s cell. They require for their life processes the enzyme system present in the cells of their hosts. Once inside the host’s cell the DNA or RNA, as the case may be, of the virus starts taking care of the host cell’s biochemical activities.
The host cell, instead of making its own characteristic nucleic acid and protein, begins to make different ones specifically suitable for the virus. Within the host cell the virus replicates producing hundreds of new viruses all exactly like the original virus that invaded the host cell (Fig. 8A).
Most plant viruses are spread by insects. The aphids and leaf-hoppers head the list. Plant viruses are transmitted by the mechanical rubbing of one infected leaf of a plant against another. In a few cases, seeds of plants are known to carry virus particles and thus to transmit the virus from one generation to the next.
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Animal viruses are transmitted in a variety of ways. Viruses that cause pneumonia, the common cold, influenza, mumps, measles are expelled in droplets by coughing, sneezing or talking of the infected person and infect susceptible hosts. Some animal and human viruses like polio, infective hepatitis are excreted in feces. Flies carry the virus to food or drinking water and spread the disease.
All attempts to make the viruses to grow and reproduce outside the living cells have failed. Viruses are strict parasitise and do not have any enzyme system of their own. The true nature of the viruses still remains a debatable issue.
Are they primitive living particles that have not yet evolved the enzyme systems required for an independent existence? Or are they the descendent of some complex living organisms which have lost their capacity for an independent existence during the passage of time?
The answer we may never know. However, it is suggested that the viruses most probably represent an intermediate stage between living and non-living forms of matter.
