BACTERIOPHAGE GENETICS 313 



marker to the unmodified phage which were produced in a mixed infection. 

 They made a mixed infection of S. typhimurium cells, using particles which 

 had been grown on S. typhimurium of one genetic type and particles of 

 another genetic type which had been grown on 8. gallinarum. About half of 

 the cells liberated at least one phage which had the genetic marker intro- 

 duced by the modified parent. However, the contribution to the burst of the 

 genetic marker from the modified phage was very much less than that of the 

 marker from the unmodified phage. Under these conditions of mixed infec- 

 tion one can conclude that many of the cells incorporated the genetic material 

 of the modified phage. However, this is not proof of normal DNA injection 

 by the modified phage, since it is possible that the addition of the unmodified 

 particles to the same cell altered the membrane in such a way that some 

 injection could now take place. 



Luria and Smith (1958), in further studies of the T*2 system, have shown 

 that although the modified phage adsorbs normally to E. coli B, two-thirds 

 of the particles adsorbed do not kill the bacterium. Nor does the modified 

 phage with the /+ marker contribute to the growth of T2rII in K12 (A). 

 These authors conclude that the most likely explanation of the phenomenon 

 is that there is genetically normal DNA in a faulty injecting system. If so, 

 the modification is another example of nongenetic, phenotypic variation. 



IX. Conclusion 



Much of the work on phage genetics has as its aim t he understanding of the 

 elementary processes in terms of the molecular interactions occurring in the 

 infected cells. Any genetic system must be such that it can be duplicated, 

 that it can mutate, and that it can form genetic recombinants. In addition 

 the genetic material must be able to control the formation of the various 

 enzymes, structural proteins, and other components required for the de- 

 velopment of the organism. During the last few years there have been great 

 advances in understanding the first of these properties. The discovery by 

 Avery and associates (1944) that DNA can transfer genetic information from 

 one bacterium to another, and the work of Hershey and Chase (1952), which 

 demonstrated that the phage DNA and protein are separated at the time of 

 infection, show that in these organisms for at least a part of their life cycle 

 DNA does contain the genetic information. The elucidation of the structure 

 of this molecule by Watson and Crick and the attendant suggestions which 

 it gave as to how this molecule could be duplicated have been followed by 

 intensive isotopic studies aimed at testing the hypothesis which developed 

 from the molecular model. All the experiments performed on phage (Levin- 

 thai, 1955, 1956), on Viciafaba (Taylor et ah, 1957), and most definately on 



