228 BACTERIOPHAGES 



The DNA is not randomly dispersed among a large fraction 

 of the progeny particles. This was first suggested by the transfer 

 experiments already discussed which showed that most of the 

 parental isotope is incorporated into the earlier formed progeny. 

 More recently Stent and Jerne (1955) confirmed this point in 

 some remarkable experiments employing the P^^-decay principle. 

 In their experiments nonradioactive bacteria were infected with 

 heavily P^2_i3i3eig(^ particles of phage T2, and a progeny of early 

 formed particles secured. These particles proved to be stable 

 insofar as repeated plaque-counts could determine, although they 

 contained large amounts of P^^. Stent and Jerne realized that 

 this must reflect an extreme concentration of P^- in very few 

 particles, so few that their loss by suicide could not be detected 

 by reductions in plaque count. To prove their inference, they 

 performed the following experiment. 



Unlabeled bacteria were infected with a highly radioactive 

 parental generation of phage particles as before, to obtain a. first 

 generation of particles that was likewise radioactive, but contained 

 only P^^ derived from its parents. As a test of viability of the 

 labeled first generation particles, they measured the transfer of P^^ 

 from particles of the first generation to those of a second. They 

 found about 50 per cent transfer as expected. However, if a 

 sample of the first generation progeny was tested again, a few 

 days later, the eflficiency of transfer to a second generation had 

 markedly decreased. Thus the first generation progeny must 

 contain much of its P^^ concentrated in a few particles that are 

 subject to radioactive decay. Other particles also contain P^^^ 

 but are not subject to decay at appreciable rate. Stent and 

 Jerne concluded that part of the transfer of DNA from parental 

 to off'spring particles preserves large pieces of parental DNA 

 among the offspring particles. 



Levinthal (1956) developed an autoradiographic method for 

 determining the amount of P*^ in individual phage particles. 

 This furnishes a powerful technique for investigating the distribu- 

 tion of transferred P^^ among offspring phage particles. By it he 

 shows that the phage DNA is indeed transmitted to off'spring 



