III. BACTKKI()l'HA(iE DNA AND BACTERIAL UNA 131 



size. On the basis of these arguments, and others based on the critical 

 rates of shear required to break molecules of different sizes, these authors 

 come to the conclusion that there are no weak points located at either 

 special or random locations along the molecule. 



5. Conclusions 



The studies on the DNA from T2 and T4 show that there is a single, 

 stable, linear DNA molecule which comprises all of the nucleic acid in 

 the phage particle. Insofar as is known, all bacteriophage may contain 

 a single molecule of nucleic acid. The length of the molecule (in T2) 

 is approximately what one would expect for a duplex molecule. When 

 the polynucleotide chains are separated by denaturation in the presence 

 of fomialdehyde, the high molecular weight of the polynucleotide chains 

 indicates the absence of interruptions in the continuity of the chains. 

 Thus, the bacteriophage DNA molecule could be a simple duplex over 

 its entire length. 



C. THE GENETIC STRUCTURE OF PHAGE 



A central concept that has developed in microbial genetics is that 

 the genetic structure, as revealed by the compiled data from recombina- 

 tion experiments, is related in an understandable way to the physical 

 structure of the DNA molecule, and that the jihysical structure is 

 reflected by the processes of synthesis and recombination in the form of 

 a genetic linkage map. The processes of "synthesis" and ''recombina- 

 tion" are themselves not understood, and the clarification of these events 

 are among the major objectives of molecular genetics. In this section 

 some features of the genetic structure of phage will be considered in their 

 relation to the physical structure of the phage DNA molecule. 



1. The Circular Genetic Map in T2 and T4 



On the basis of a sensitive linkage test, Streisinger anfl Bruce (1960) 

 have come to the conclusion that all of the known genetic markers in 

 T2 and T4 are linked to form a continuous genetic map. In these 

 experiments three markers were employed, two of which were known 

 (or could be demonstrated) to be linked, and a third the linkage of 

 which was to l)e tested. By selecting for observation a c'ass of progeny 

 phage which showed recombination between the two markers that were 

 known to be linked, they could then examine these phage for the third 

 character. If this third character were totally unlinked, one would 

 expect to find it associated with the most proximal of the linked pair 

 with a frequency depending solely on the relative multiplicities. The 

 experiment can be done with greater sensitivity by comparing the results 



