162 I The Process of Evolution 



with a particular trait can transform a strain lacking this trait into 

 one that possesses it. Virulence in pneumococcus depends on 

 the polysaccharide envelope of the bacterial cell. If a nonencapsu- 

 lated, nonvirulent strain is grown with purified DNA extract from a 

 virulent strain, virulent cells with a capsule will develop in the 

 culture. Thus the DNA determines the polysaccharide coat. Trans- 

 formation has been achieved for a large number of traits, and there 

 is an equal probability of transformation in either direction. Linkage 

 of traits in the DNA material also has been found, since in experi- 

 ments involving differing traits double transformations occur more 

 frequently than would be expected for independent events. 



Recombination in Viruses 



A rather complex relationship exists between certain bacteria and 

 bacterial viruses known as bacteriophages. A single phage consists 

 of two major structural elements, a tail and an enlarged ( often hexag- 

 onal) head. A protein coat surrounds a DNA core. When a bac- 

 terial cell is infected, the phage attaches itself by the tail to a 

 specific receptor site on the cell wall. During the course of the next 

 few minutes the DNA leaves the phage and enters the bacterium. 

 Within the bacterium, the phage multiplies and the cell eventually 

 breaks open (undergoes lysis), releasing the new phages. During 

 the period of multiplication, new phage DNA is synthesized and 

 new phage protein formed, the result being a hundred or so new 

 phages of the same type as the infector. The original infector has, 

 with its DNA, managed to preempt the synthetic processes of the 

 bacterium and turn them to its own use, that of duplicating the 

 phage. 



Since the beginnings of their scientific study, phages with many 

 different traits have been found. It is possible to infect a culture of 

 bacteria with a mixture of phages with different traits. When this is 

 done, recombination of traits occurs in the phage progeny. With 

 some characteristics, recombinants appear with equal frequency; 

 with others there is a reduction in the number of recombinants, in- 

 dicating that linkage exists between the genetic factors affecting the 

 traits. What has happened then is recombination of the genetic 

 material of the phage in the host cell of the bacterium. Radioactive- 

 tracer studies have shown this genetic material to be the DNA. 

 Such evidence, taken with that from experiments with bacterial 

 transformation, in which the transforming agent is DNA, shows that 

 in these forms the genetic material is DNA. It also shows that, in the 



