CHAPTER 3 



VIRUSES, RICKETTSIAE AND BACTERIA 



If one assumes that the origin of life was a unique occurrence 

 then it follows that all the present-day living things must be 

 derived from this original source. This then poses the problem, 

 " What is the relationship between the present-day forms? ' In 

 many cases it is difficult to form any definite conclusion 

 regarding these relationships and this certainly seems to 

 hold for the relationship between Viruses, Rickettsiae and 

 Bacteria. 



The viruses 



The viruses are of interest since they show many of the 

 properties of living material. At first they were described as 

 material that would pass through a bacterial filter and which w T as 

 capable of reproducing in the living cell. But later on consider- 

 able confusion arose over the chemical nature of viruses, the main 

 trouble being one of over-simplification. Many people thought 

 that the viruses were necessarily simple because they had been 

 prepared in a crystalline condition. This concept was furthered 

 when chemical analysis showed that the virus was composed 

 of a " simple chemical substance " — nucleoprotein. With more 

 advanced techniques it became clear that there was considerable 

 variability in virus structure. Markham, Smith and Lea (1942) 

 showed that when the tobacco mosaic virus was irradiated, only 

 a small part of the virus proved sensitive to radiation. This part 

 was some 5%-6% of the virus area and in effect it behaved like 

 the nucleus of the virus. In 1951 Markham and Smith presented 

 evidence that the turnip mosaic virus contained at least two 

 distinct components, a nucleic acid component (38%) and a 

 structural protein component (62%). 



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