72 



Inside the Living Cell 



not only of reproducing itself but also of forming the proteins present 

 in the complete bacteriophage particles. 



The bacteriophages offer a very favourable opportunity for the 

 study of the elementary processes of reproduction as there is no 

 doubt that replication occurs within the bacterium. M. Delbriick 

 and W. T. Bailey discovered that bacteriophage particles are made 

 up of still smaller units. They found that two different related strains 

 of bacteriophage can be crossed with each other so as to produce 



D N A 



Tail protein 

 l,000A long 



:LSite of 

 attachment 



FIG. 16. A bacteriophage particle 

 (slightly diagrammatic) (from E. A. 

 Evans, Texas Reports Biol. Medicine, 

 15, 783 [1957]) 



a kind of hybrid. For example, one strain of virus known as T2 

 produces a small colony and can destroy a strain of the bacterium 

 called A; while another strain known as T4r produces a large colony 

 and can destroy a mutant strain called C. When both types are added 

 together to a culture, not only are both of these original types released 

 when the bacteria burst, but also two intermediate types, in which 

 the characteristics are combined in a different way. One produces 

 a large colony and destroys bacterium A. The other produces a small 

 colony and destroys bacterium C. It appears that the characteristics 

 of T2 and T4r have got mixed up in the new types, each of which 

 has features drawn from both of the two parent types. The inference 



