268 G. S. STENT 



Hershey, 1953a). At no time during the latent period does there appear to 

 exist an important fraction of the parental DNA which forms part of structm^es 

 intermediate in size between free DNA fibers and matm^e bacteriophage 

 particles (Watanabe et al., 1954). The fact that the parental DNA persists as 

 free nucleic acid and that it, like all other viral DNA precursors present in the 

 host cell at early stages of infection, reaj)pears preferentially in the first 

 progeny phages to mature, leads one to infer that the transferred parental 

 DNA simply forms part of the general intrabacterial pool of viral percursor 

 DNA already discussed in previous sections. 



2. Distribution of Transferred Material 



Although the experiments just described point to the fact that most of the 

 transferred parental DNA atoms remain part of specific polynucleotides 

 throughout the intracellular reproductive processes, they do not permit any 

 conclusions as to whether the integrity of the parental phage DNA is really 

 conserved. That is, do the DNA atoms transferred from one parental virus all 

 wind up in a single progeny particle or are they dispersed over many of the 

 descendants? It has been the hope that the answer to this question might 

 give some clue to the mechanism by which the parental phage DNA goes 

 about its own manifold reduplication, since an important operational distinc- 

 tion between different conceivable DNA replication schemes is their prediction 

 of the distribution of the atoms of the parental molecule over the replica 

 structure (Stent, 1953; Delbriick and Stent, 1957). 



The problem of the distribution of the transferred DNA atoms has been 

 solved by two rather different techniques, each capable of measuring the P^^ 

 content of individual virus particles in a heterogeneously labeled population. 

 The more direct of these techniques, an autoradiographic method developed 

 by Levinthal (1955, 1956), involves measurement of the P^^ content of 

 individual phages by embedding the radioactive particles in a sensitive 

 photographic emulsion, allowing radioactive decay to proceed, and counting 

 the number of ^-ray tracks found to be emanating from point sources after 

 development of the exposed emulsion. Since each track represents the decay 

 of a single P^^ atom, the total P^^ content of each source can be calculated 

 from the number of tracks and from the exposure time and known decay rate 

 of the isotope. A second, much less direct method, measures the P^^ content 

 of the individual phages by observing the lethal effects of the decay of the 

 radioactive atoms on the particle in which the decay has taken place (Stent 

 and Jerne, 1955; Stent et al., 1959). Since one out of every ten such P^^ 

 disintegrations is lethal (Hershey et al., 1951a), the rate of death with radio- 

 active decay of individual particles is proportional to their content of the 

 labeled isotope. Therefore, in order to measure the distribution of the 

 transferred parental DNA, bacteria are infected with highly P^^-labeled 



