III. BACTERIOPHAGE DNA AND BACTERIAL DNA 139 



considered. This distribution of parental DNA should be related to the 

 structure of the phage molecule and its mode of replication. 



1. Transfer of </>.r DNA 



This virus shows no or very little transfer of parental DXA to 

 progeny phage (Sinsheimer, 1961). 



2. Transfer of T7 DNA 



When eoli are infected with T7 phage labeled with N^'^ and C", the 

 parental DNA that is labeled with these heavy isotopes is transferred 

 to progeny phage particles. When the progeny phage are examined by 

 density gradient centrifugation, there appears a band corresponding to 

 phage with 24% of their atoms substituted by heavy isotopes (Mesel- 

 son, 1960). Since T7 phage are about 50% DNA (Putnam, 1953; 

 Davison, 1962) this means that some phage particles are formed which 

 have half newly synthesized DNA and half parental DNA. The total 

 DNA content of this phage is approximately 20 X 10" daltons (Davison 

 and Freifelder, 1962) and there is evidence to support the notion that 

 it is in the form of a single molecule (Thomas and Pinkerton, 1962). 

 // the biological formation of hybrid T7 phage could be accomplished 

 it could now be established that these phage DNA molecules which have 

 hybrid density are simple duplexes of polynucleotide chains, and if it 

 could be shown that the heavy and light subunits cannot be separated 

 by breaking the molecule by shear, but that separation of the density 

 label does occur by denaturation and separation of the polynucleotide 

 chains, then it would be permissible to conclude that the transferred 

 subunits are single polynucleotide chains, and that they are associated 

 with a newly synthesized polynucleotide chain to form a hybrid duplex. 



3. Transfer of T2 or T4 DNA 



In contrast to the results of T7 the parental DNA of a T2 or a T4 

 phage is broken into smaller pieces and incorporated into the DNA 

 molecules of many progeny phage. The transfer is dispersive. The early 

 work on this subject has been reviewed by Sinsheimer (1960). These 

 experiments by Levinthal (1956), Hershey and Burgi (1956), and Stent, 

 Sato, and Jerne (1959) jiresented evidence that a sizable fraction (40 to 

 60%) of the parental DNA was transferred to a relatively few progeny 

 phage particles in units which were as large as 15 to 20% of the parental 

 phage. The remaining parental DNA appeared to be even more highly 

 dispersed to a large number of phages in pieces no larger than 0.3%. 

 This transfer of apparently very large pieces of DNA suggested that at 

 least part of the phage chromosome was replicating semi-conservatively. 



