RECOMBINATION IN VIRUSES AND BACTERIA 



sni-iv 



sni-1 



Sm-la or-16 I 



sm-ic I — ^ — fr^i I sui-N 



sni-2 1 ■■ — V-^i I siii-A^ 



] Slll-iV 



FIGURE 5.12. A scheme for the structure of the DNA in the strains listed in Table 

 5.4, which will explain the results obtained. Only a few interactions are indicated 

 as an example of how the model works. 



amounts of capsule; each one yields DNA which will transform R cells 

 only to its own type. But when these DNA's are used to transform one 

 or another of the intermediate types, results of the sort shown in Table 

 5.4 are obtained. At first glance these seem indeed to be complex re- 

 sults, but there is a very simple explanation. You will note that most 

 transformations are to the same type as that of the DNA employed; these 

 are called autogenic. But some are to the wild-type condition and are 

 called allogenic. The wild type was not involved in the experiment, 

 however, either as the treated cell or as the source of the DNA, and 

 mutations to that condition are not frequent enough to account for the 

 allogenic transformations. This suggests that recombination is at work. 

 If we assume that the different types have DNA's of the structure shown 

 in Figure 5.12, allogenic transformation can be explained on the basis of 

 a copy-choice mechanism of DNA duplication. The validity of this 

 model is yet to be attested, but it is clearly established that all trans- 

 formants contain DNA characteristic of their phenotypes. Therefore 

 new types of DNA are produced in transformation by some mechanism of 

 recombination. 



These new SIII-N types do not subsequently break down into their 

 component parts in pneumococcus but in another bacterium, Hemophilus 

 influenzae, where the situation is formally the same, they do so. DNA 

 from an Hemophilus strain with an antigenic capsule of type a will trans- 



