FATE OF INFECTING PHAGE PARTICLES 229 



particles partly in the form of large pieces each containing about 

 20 per cent of the DNA from a single parental particle. 



The possible implications of these and other experiments in 

 progress make exciting reading and are discussed by Levinthal 

 (1956), Hershey and Burgi (1956), and Delbruck and Stent 

 (1957) . The experimental results and interpretations are still too 

 fluid, however, to discuss here in detail. 



/. Efficiency of Transfer 



As previously described, the efficiency of transfer of labeled 

 parental DNA to offspring seldom exceeds 50 per cent even 

 under the best attainable experimental conditions. This fact 

 occasionally prom^pted speculation that the limited transfer 

 signified something fundamental about replication of DNA. 

 One such speculation was testable. Maal0e and Watson (1951) 

 performed the first two-cycle transfer experiment, which had been 

 suggested by S. S. Cohen. They found that the efficiency of 

 transfer during first and second cycles of growth from P^Mabeled 

 parents was the same. This result disposed of the possibility 

 that phage particles contain two kinds of DNA, one transferable 

 and one not. Hershey and Burgi (1956) present evidence that 

 the incomplete transfer should be attributed to random losses 

 that have nothing to do with the mechanism of transfer (Figure 

 8). Other developments support their contention to this extent: 

 it now seems clear that an explanation of the low eflficiency of 

 transfer will not contribute much to an understanding of the 

 mechanism of transfer. 



The experiments of Levinthal (1956), Hershey and Burgi 

 (1956), and Stent, Jerne, and Sato (1957) revive the idea that 

 the DNA of T2 is composed of functionally distinct parts. The 

 merits of this idea are still debatable. However, it is clear that 

 such parts, if they exist, are transmitted from parents to progeny 

 with similar efficiency and probably without intercon version. 



4. Summary 



Here we will summarize the contributions which the transfer 

 experiments give to our knowledge of phage reproduction. Al- 



