VIRUSES, RICKETTSIAE AND BACTERIA 19 



Takahashi and Ishii (1953) showed that it was possible to find 

 the structural protein in the sap of the plants infected with the 

 mosaic virus and that it differed from the normal plant proteins. 

 This protein had no power of reproduction but required the 

 presence of nucleic acid. If the nucleic acid was added to the 

 structural protein, then it became capable of reproduction inside 

 the cell (Fraenkel-Conrat and Williams 1955). 



The chemical analyses of virus structure have been paralleled 

 by studies using the electron microscope. These show that the 

 tobacco mosaic virus is often found in rod-like forms, the rods 

 being made up of a series of discs each with a hole in the centre. 

 The hole is apparently filled with the nucleic acid whilst the disc 

 itself is probably the structural protein (Fig. 2). 



Other viruses such as bacteriophage which attacks bacteria 

 have an even more complex structure. The bacteriophage has a 

 tadpole-shaped head and a small tail (Fig. 3). The head consists 

 of a shell of structural protein inside which is the nucleic acid. 

 Hershey (1956) described how it was possible to remove the 

 nucleic acid from the bacteriophage and leave the tail and the 

 shell. This skeleton was still capable of attacking a bacterium 

 and killing it, but it was not capable of self-reproduction. 



Detailed chemical analysis and electron microscope studies 

 have therefore shown that viruses are not simple single chemical 

 substances. There is a considerable range of structural and 

 chemical complexity within the group of viruses and it is possible 

 to draw up a table showing the differences in their chemical 

 composition. 



Material Present Virus 



RNA 

 DNA 



Protein S- Animal virus 



Fats 



Carbohydrates J 



i 



RNA 



DNA ? 



Protein Y Bacteriophage 



Fats 



3— IOE 



