VIRUSES

Viruses are the smallest objects  that can  pass through  the filter  capable  of trapping even the smallest bacteria .These  are very minute particles  of  nucleoproteins   which can be usually viewed  only under  the electron microscope ;however, some are so minute that they can  not be viewed even under  the electron  microscope .Inspite  of this  ,scientists , better  say ,virologists  [specialists of viruses] ,have evolved special  techniques  to study  their structure ,behaviour and reproduction .These deserve special  consideration  in our survey of the  distinction between  the living and non living objects because they show  of the characteristics of both  and thus  ,they form a  connecting link between  the two types of objects .In order to evacuate  them more clearly as alink  between the two types of objects ,information about their structure  and lifestyle is essential .Such information is also important  due to this fact  that the viruses  are pathogenic ,causing harmful diseases  in man ,animal and plants .                                                                                                                                       Discovery of virus  .People  were not familiar with viruses till the end  of the 18th  century  but they definitely knew them to cause diseases .First of all ,Edward Jenner [1796] tried to  know about small pox and then Louis Pasteur [1882] tried to know  about the virus , causing  rabies .However  ,in 1892 Ivanovsky discovered that  the bacteria -free  extract of the tobacco plant ,already  affected by the  tobacco-mosaic disease,was successful in causing these diseases in healthy  tobacco plants  .this  fact was confirmed  by Beijernick [1898] and he named this extract  or filtrate  as contagium  vivum  fluidum  which  ,later , was named  filterable virus  ,and  finally ,virus literally meaning poison .The first animal  virus  responsible  for the foot-mouth  disease of cattle  was discovered by Reeds  in 1902 .The bacteria attacking viruses were discovered  for the first time by twort [1916]  in england and then  by d'Herelle [1917] in France .Later on ,W.M.Stanley [1935]  was able to isolate  Tobacco Mosaic  Virus [TMV] from the diseased  tobacco leaves .Since then many more viruses  have been discovered  and the diseases  caused by the m  have been recognized                                                                                                                                                                          CHARACTERISTICS OF VIRUSES                                                                                                Viruses are  curious and interesting .These exhibit  the following  characteristics :                                         1.Virus is a very minute particle  of nucleoproteins which can be seen  only with the  help of the electron microscope .                                                                                                                                                        2.  These  are very simple in organization  and lack cellular structure .                                                            3. These are very simple in organization  and lack cellular structure .                                                                 4. These   are composed of  DNA or RNA ,single or double stranded  and proteins as a sheath ,called  capsid .                                                                                                                                                              5. These donot have cytoplasm and plasmalemma [plasma membrane ].