36 



MAUKICE J. BESSMAN 



TABLE XVII 

 Nearest Neighbor Frequencies ok Mycobacterium phlei DNA"'*" 



« From Josse et al. (1961). 



* Chemical analysis of the base composition of the primer DNA indicated molar 

 proportions of thymine, adenine, cytosine, and guanine of 0.165, 0.162, 0.335, and 0.338, 

 respectively. 



'^ Identical Roman numerals designate those secjuence frequencies which should be 

 equivalent in a Watson and Crick DN.\ model with strands of opposite polarity; identical 

 lower case letters designate sequence frequencies which should be equivalent in a model 

 with strands of similar polarity. 



a template for the synthesis of a daughter strand with complementaiy 

 bases. The nearest neighbor frequencies of Table XVII provide inde- 

 pendent evidence for this scheme. ApA and TpT sequences are equal 

 as are GpG and CpC. In examining the other sequences, it is important 

 to realize that the predicted pairings of dinucleotides will depend on 

 whether the two chains of the double helix run in the same or opiiosite 

 directions, that is, have similar or opposite polarity. This difference is 

 made clear in Fig. 11 where a short segment of the replicating chain is 

 depicted. The expected pairing of dinucleotide-scquenccs as a conse- 

 quence of similar and opposite polarity in the nucleic acid chains are 

 shown, and it is obvious that decidedly different pairings would be 

 expected depending on which of the two possibilities obtained. In Table 



