A. D. HER.SHEY 



tion away from damaged pieces, since both the rescue and au- 

 thentic recombination are subject to the same Hnkage rules. 

 The photosensitive target measured in these experiments is the 

 average size of pieces that can be rescued by recombination. 

 It measures one twenty-fifth of the total genetic material and is 

 large compared to the size of the individual markers, since 

 linked markers tend to be rescued together. 



Stent's conclusions are consistent with these and lead further 

 in one direction. Since the primary lethal damage produced by 

 assimilated P^^ results from one out of twelve atomic disintegra- 

 tions (30), at least this fraction of the DNA must be associated 

 with genetic material. The fraction must, in fact, be larger, 

 because the radiochemical efficiency is dependent on tempera- 

 ture (56). 



These methods are evidently capable of yielding rather pre- 

 cise descriptions of genetic material, possibly in physical terms, 

 certainly in radiochemical terms that can be compared with 

 genetic descriptions. Moreover, like the earlier results of Luria, 

 they call for interesting assumptions about the mechanism of 

 viral growth. The problem is to explain how rescue by re- 

 combination is achieved without greatly setting back the multi- 

 plication of the rescued piece relative to comparable genetic 

 material that does not require rescue. Such a setback does, in 

 fact, appear when the rescue is made more difficult by high 

 radiation dosages. In this situation we come face to face with 

 the problem of structure and function of vegetative phage. 

 Since both the facts and ideas bearing on the interpretation of 

 the radiogenetic experiments are changing rapidly, it would be 

 rash to predict how they are to be reconciled with each other. 



Relation between Virus and Cell Nucleus 



There are a number of indications from other phage- 

 bacterium systems that some bacteriophages engage directly 

 with components of the bacterial nucleus (5,29,37,45,70). 

 These are only beginning to be worked out and cannot be dis- 



22 



