BACTERIOPHAGE GENETICS 301 



which are still concentrated in single particles remains constant at about 

 40 %, and the amount of P^^ in these particles is still about 20 % of that in 

 the original labeled parents. Thus, one can say that there is a piece of DNA 

 (P^^ labels only the DNA of the phage) comprising about 20 % of the total 

 amount in a phage which passes through two growth cycles without being 

 diminished iii size, although only about half of the total number of these 

 pieces are transferred in each cycle. 



The transfer experiment can be combined with a cross in which P^^- 

 labeled parents containing one set of genetic markers are crossed with un- 

 labeled particles with different markers. If the progeny are first separated 

 according to the genetic markers they contain and then examined for their 

 P^^ content, it is found (Levinthal and Thomas, 1957) that phage in the 

 second-cycle progeny, which contain the 20 % piece of DNA, also contain the 

 genetic marker of the labeled parent. 



In similar crosses between labeled and unlabeled parents, Garen (1954) 

 showed that not all of the transferred P^^ remains with the genotype with 

 which it entered the cross. This result showed either that the genetic struc- 

 ture is broken into several pieces by growth and mating or that only a fraction 

 of the DNA is in a single genetic structure, and the remainder is distributed 

 among the particles of all genotypes. Hershey and Burgi (1956) extended 

 these experiments by varying the relative input of the labeled and unlabeled 

 parent and showed that in a cross followed by one cycle of further growth 

 approximately 40 % of the transferred P^^ remained associated with the 

 parental genotype; the remainder was distributed at random. In a further 

 growth cycle the fraction associated with the parental genotype decreased 

 to about 20 %. 



These results seem to indicate that there is a piece of DNA in the phage 

 which carries the genetic markers and that this piece of DNA is not broken 

 down by mating. However, there are two reasons why this interpretation is 

 open to question. First, in aU the experiments so far reported the separation 

 of genetic ty|3es in the progeny has been done only with respect to a single 

 genetic marker — the host range character h — which might not be representa- 

 tive of the behavior of the entire genetic structure. In addition to this tech- 

 nical difficulty in the experiments, there is another problem which can affect 

 the interpretation. It is known that the bulk of the P^^ which is transferred 

 from parental phage to progeny appears in those particles which are formed 

 early in the latent period (Hershey, 1953), and it is these which have mated 

 the smallest number of times. Therefore, if one looks only at the total progeny 

 in a mass culture, the P^^ will be found in those particles which have mated 

 only a few times. It is possible that the observed correlation between the 

 high P^2 content and the parental genetic marker is due to the selection of 

 particles which have not in fact mated rather than being a reflection of the 



