138 CHARLES A. THOMAS, JR. 



periment!;!. So fai-. tlic handing studies of Bcrns and Thomas (1901) 

 indicate that tlie polynucleotide chains are continuous and nonintei'- 

 ruptt'd. Howevei'. tlieii' conclusions refer to the nidjority of the molecules. 

 Moreover, their techni(|Ue would not detect intei-rujjtions near the ends 

 of the molecule. Finally, an o\'erlap structure of the kind shown in 

 Fig. 10 could result from the closure of a linear molecule on itself to 

 form a circular structure, a hypothesis which would allow only a single 

 heterozygous region of this fonn per molecule. Therefore, at this point 

 it is not clear whether there is a conflict between the genetic and physical 

 experiments or not. 



5. Conclusions 



At the present time one may picture the phage DNA molecule as 

 comprising all of the DNA in the phage particle. After release the T2 

 molecule is found to be linear and over 50 /x in length. The genes in 

 lambda appear to be arranged on a linear molecule and can be separated 

 by breaking the molecule by shear; one expects (but as yet no evidence) 

 that the genes in T2 are in map sequence along the T2 molecule. There 

 are two aspects of the genetic structure of T2 which as yet have no 

 physical interpretation: (1) the formal circularity of the T2 linkage 

 map, and {^) the existence of a partly stable heterozygous region which 

 displays genetic discontinuities. It is important to note that the com- 

 bined length and mass determinations of the T2 DNA molecule only 

 allow one to conclude that the majority of the molecule is duplex. Direct 

 visualization in the electron microscope supports this conclusion. It is 

 still possible that as much as 10% of the molecule may have a more 

 special or elaborate structure which may perhaps correspond to the 

 physical heterozygous region, although it must be said with equal 

 weight that there is at the moment no physical evidence in favor of 

 these regions. 



I). THK DISTRIBUTION OF PARF:NTAL PHACE DNA TO PROGENY PHAGE 



With the finding that the DNA in mitotic chromosomes is in the 

 form of two subunits which, upon replication, separate and become 

 associated with newly synthesized subunits (Taylor et al., 1957; Mesel- 

 son and Stahl. 19o8a,b), attention turned quite naturally to the question 

 of the physical and chemical structure of these subunits. In view of the 

 small amount of DNA in bacteriophage, it seemed likely that phage 

 chromosomes were of a relatively simple structure. This has to a large 

 extent proved so, and has been the subject of previous sections of this 

 chapter. In this section the manner in which the DNA from parental 

 phage segregates and is distributed into i)rogeny i)hage molecules will be 



