266 G. S. STENT 



progeny. In this "transfer experiment," bacteria are infected under conditions 

 of one-step grow-th with phage particles whose DNA is labeled with P^^ or C^^ 

 and the phage yield issuing from these infected cells is harvested, purified, and 

 assayed for its content of the parental radioisotopes. The outcome of Putnam 

 and Kozloff's transfer experiment was that about 30-40 % of the radioactive 

 atoms of the parental DNA are transferred to the progeny. Subsequent work 

 employing improved experimental techniques confirmed the transfer of an 

 appreciable fraction of the infecting DNA and revealed that the transfer 

 efficiency can amomit to as much as 50 %. The other, nontransf erred 50 % 

 of the labeled, parental DNA atoms of the lysate are usually fomid to be 

 either attached to bacterial debris (10 %), or liberated into the growth 

 medium in the form of macromolecular free DNA (30 %), or in low molecular 

 weight, acid-soluble compoimds (10 %) (Maaloe and Watson, 1951; French 

 et al., 1952; Hershey and Chase, 1952; Hershey, 1953a; Kozloff, 1953; Watson 

 and Maaloe, 1953; French, 1954; Hershey and Burgi, 1956). In view of the 

 inference that it is the DNA of the extracellular virus particle which carries 

 its genetic continuity into the host cell, it does not seem farfetched to imagine 

 that an understanding of the mechanism by which this transfer of DNA atoms 

 from parent to offspring takes place might afford valuable insights into the 

 nature of the reproductive processes. One may enquire, first of all, why the 

 transfer is incomplete. Why do only 50 % of the parental DNA atoms reappear 

 in the progeny? It is possible that phage reproduction involves no direct 

 transfer of specific parental DNA at all, but that the parental DNA is degraded 

 in the course of the latent period, the fragments introduced into the synthetic 

 pathways of the infected ceU and built up, along with other nucleic acid 

 precursors, into new bacteriophage-specific progeny DNA. That this ij^Q of 

 transfer can occur is demonstrated in experiments in which bacteria are 

 mixedly infected with unlabeled T4 and P^^.j^beled T3 phage particles. In 

 spite of the fact that phage T3 is completely excluded, and its DNA presumably 

 degraded under these conditions, some of its P^^ label reappears among the 

 T4 progeny (Kozloff, 1952, 1953; Watson and Maaloe, 1953). However, a 

 closer study of the chemistry of transfer makes the hypothesis of transfer 

 by breakdown and resynthesis less likely. For it has been found, not only 

 that the phosphorus and all four pyrimidine residues of the parental DNA 

 are transferred with equal efficiency, but also that addition of unlabeled 

 thymidine and uridine to the phage-infected bacteria during intracellular 

 phage growth does not suppress the transfer of parental, labeled pyrimidines 

 to the progeny (in contrast to the incorporation into phages of pyrimidines 

 derived from the bacterial host DNA, which incorporation is suppressed by 

 similar addition of pyrimidine nucleosides to the infected culture) (Hershey 

 eb al., 1954; Hershey and Burgi, 1956). The findings that the DNA from UV- 

 irradiated parental phages carries its lesions with it in mixed infection with 



