SHAPE AND SIZE OF VIRUSES

Viruses are such smaller than the smallest bacterium .Their  size ranges  from 30to 300 nm  .That  is why their structure  was not known before discovery of the electron microscope .                                                                                             Viruses have assumed  different shapes .Some are helical or rod-shaped,e.g., tobacco  mosaic virus [TMV]  and influenza virus .Many  viruses are spherical   but on close examination ,their  shape is found to be many sided  or polyhedron.e.g., adeno virus ,herpes virus ,turnip yellow mosaic virus [TYMV] etc .Some viruses have complex shape .For example ,T4 bacteriophage  ,T4 bacteriophage ,or virus, vaccinia virus,etc.                                                                                                                                                                        
                       Structure of Viruses                                                                                                                              The electron  microscope  has shown  that all viruses exist as discrete particles ,each of which is called a virion .In other words ,a virion  is an individual of a virus .All  viruses have  a similar organization;avirion has acentral core of coiled strand of a nucleic acid,DNA or RNA  and a protein  coat called capsid  which surrounds the core .Some viruses  ,for example , herpes virus,have an outer  envelope ,composed  of polusacchearide or lipoprotein.                                                                                                                      The capsid consist of a number of  protein subunits  called capsomeres .The arrangement  of capsomers arround  the core  imparts  a particular  shape to virion of a  virus.The animal viruses , bacteriophages  and cyanophages  have DNA whereas plant viruses  other than  bacteriophages have RNA .