196 ROBERT L. SINSHEIMER 



parental "stars" are observed. 53 Roughly one such half-size star is found 

 per parent phage particle. 



When these progeny are used to produce a second generation of progeny, 

 the half-size stars appear to be preserved intact in the second generation 

 progeny. 



(2) Evidence from Transfer Experiments with P s2 -Labeled Ultraviolet Ir- 

 radiated Phage. A second less direct line of evidence that the DNA of T2 

 is bipartite, at least in function, is to be found in the experiments of Her- 

 shey and Burgi, 61 and of Tomizawa. 62 



When P 32 -labeled phages are used to initiate an infection in unlabeled 

 medium, their progeny will contain a fraction of the P 32 from the parental 

 particles. This fraction may vary from 40-60 % dependent upon the condi- 

 tions of the experiment. (The reasons for recovery of less than 100 % of the 

 parental phosphorus in the progeny are believed to be matters only of 

 technique and not of intrinsic importance. 61 ) 



Such P 32 -labeled phage can be inactivated by irradiation with an ade- 

 quate dose of ultraviolet light. Such irradiated phage attach to their host 

 cells and inject their DNA into these cells, but, presumably because of 

 damage to some critical function, the infection is abortive. In such cases, 

 the transfer of P 32 to the progeny particles is clearly zero. When, however, 

 a mixed infection is made in which several live unlabeled particles are 

 caused to infect every cell infected by an ultraviolet irradiated P 32 -labeled 

 phage, it is then found that the P 32 of the ultraviolet treated particle ap- 

 pears in the progeny to the usual extent (i.e., to the same extent as would 

 be observed had the labeled phage not been irradiated). 



This first generation of progeny is now found to contain a small number 

 of noninfective particles which, however, carry approximately 50% of 

 the transferred P 32 . That the number of noninfective particles is small is 

 indicated by the fact that such phage preparations have, within a few per- 

 cent, as many infective units per gram of nitrogen or per optical density 

 unit 63 as do normal stocks. That a large portion of the transferred P 32 is 

 in noninfective particles is shown by a comparison in a second cycle of 

 propagation of the P 32 transferred from this first generation of progeny to a 

 second generation in single infection when only viable particles will trans- 

 fer, with the P 32 transferred in a mixed infection with a high multiplicity 

 of fully viable particles when all of the labeled particles will transfer. 



Experimentally, in such a second cycle of propagation, the ratio of P 32 

 transferred in single infection to that transferred in multiple infection de- 



61 A. D. Hershey and E. Burgi, Cold Spring Harbor Symposia Quant. Biol. 21, 91 

 (1956). 



62 J. Tomizawa, Virology 6, 55 (1958). 



63 A. D. Hershey, J. Dixon, and M. Chase, J. Gen. Physiol. 36, 777 (1953). 



