198 ROBERT L. SINSHEIMER 



These experiments will be described in detail when the evidence concerning 

 viral DNA synthesis is discussed. 



(3) Evidence from P 32 Transfer Experiments. A third, indirect line of 

 evidence suggesting a bipartite nature of the DNA of T2 phage is provided 

 by the P 32 transfer experiments of Stent and Jerne, 64 and Stent, Sato, and 

 Jerne. 65 These experiments, as do the Hershey experiments, attempt to in- 

 quire into the distribution among the progeny phage particles of P 32 atoms 

 from parental particles. As do the experiments of Hershey, these experi- 

 ments also suggest that a large fraction (roughly half) of the transferred 

 P 32 is in a few progeny particles, while the remainder is distributed over 

 many particles. The former DNA fraction, which presumably suffers little 

 dispersion through the first generation of propagation, is found to remain 

 essentially intact through a second generation. 



In these experiments use was made of the "P 32 suicide" technique. When heavily 

 P 32 -labeled phages are prepared and are used immediately in unlabeled medium to 

 produce a first generation of progeny (involving an increase of twentyfold in phage 

 titer), those progeny particles, if any, with an appreciable P 32 content will be in- 

 activated upon storage. Experimentally it is observed that the decrease in phage 

 titer of the progeny upon storage is so small that although on the average each par- 

 ticle should contain at least 2% of the parental P 32 , the great majority must contain 

 less than 0.2% of the parental P 32 . Much of the transferred P 32 must reside in a few 

 particles. 



Such particles, containing a large number of P 32 atoms, might be expected to die 

 rapidly. Once dead they would not be able to transfer in single infection their re- 

 maining P 32 atoms to further progeny. 



An estimate of the P 32 in the few heavily labeled progeny can be obtained, then, 

 by measurement of the rate of decrease of transfer, after storage, of P 32 by the first 

 generation of progeny to a second generation. The results of such an experiment are 

 shown in Fig. 2. These results can be analyzed to show that of the P 32 in the first 

 generation of progeny, approximately 60% is in two particles that die out at such a 

 rate that they must each contain about 15% of the P 32 of the original particle, while 

 the other 40% is spread over the remaining particles which do not individually contain 

 enough P 32 to cause them to die off at an appreciable rate (less than 0.3% of the P 32 

 of the original particle). These calculations depend upon the assumption that the 

 decay of transferred P 32 inactivates the heterogeneously labeled phage particles of 

 the first generation of progeny with the same efficiency (0.1) as has been determined 

 for phage particles homogeneously labeled by incorporation of P 32 from the medium 

 and bacterial constituents. 



Experiments measuring the distribution of P 32 after still another generation indi- 

 cate that the large fragments of P 32 -labeled DNA continue to persist without further 

 dispersion. 



It is of interest that all of these experiments indicate a transfer of large 

 macromolecular pieces of DNA from parent to progeny without significant 

 molecular disintegration. This evidence that the DNA molecules remain 



64 G. S. Stent and N. K. Jerne, Proc. Natl. Acad. Set. U. S. 41, 704 (1955). 



65 G. S. Stent, G. Sato, and N. K. Jerne, J. Mol. Biol. 1, 134 (1959). 



