230 BACTERIOPHAGES 



most all these experiments have been done with the T series of 

 coli phages, and most of them with T2 and its relatives. First of 

 all the transfer experiments confirm the differentiation of the 

 phage particle into a DNA core and a protein membrane. The 

 components are structurally different and functionally distinct. 

 Upon adsorption to a host cell, most of the protein membrane 

 remains on the bacterial surface while most if not all the nucleic 

 acid core penetrates to the interior of the bacterium. Accordingly 

 little phage protein, unlike the injected DNA, is incorporated 

 into progeny particles. The Blendor and transfer experiments 

 thus establish the physical basis for the disappearance of the in- 

 fective virus particle upon infection. 



In contrast to the rather static role of phage protein, the 

 nucleic acid is functionally and materially active. The Blendor 

 experiment of Hershey and Chase suggested a primary genetic 

 role for phage DNA and a major aim of present day transfer ex- 

 periments is to establish a connection between genetic function 

 and material behavior of DNA. Early experiments showed that 

 approximately 50 per cent of the parental DNA reappeared 

 within progeny particles but they left open the question whether 

 the phage DNA remains intact during replication or whether it 

 is dispersed and fragmented. After much early confusion and 

 ambiguity this problem now seems capable of experimental reso- 

 lution. The entire DNA content of the phage particle certainly 

 does not remain intact during replication. This was shown by 

 the initial P^^-suicide experiments. It was not apparent, how- 

 ever, whether this dispersal of DNA involved fragmentation of a 

 single molecule, the distribution of several intact molecules 

 among several progeny particles, or possibly both processes. 

 This question can now be attacked by methods capable of 

 analyzing the isotopic composition of single progeny particles. 

 Independently, Levinthal and Stent and Jerne have achieved 

 this objective, the former by the development of an elegant auto- 

 radiographic technique, the latter by subtle manipulation of the 

 P^2-suicide experiment. Both agree that the parental isotope is 

 not randomly distributed among progeny particles. Approxi- 



