34 



MAURICE J. BESSMAN 



be calculated. This value, multiplied by the molar proportion of dAMP 

 in the particular nucleic acid studied (this may be determined independ- 

 ently or it can be derived from the nearest neighbor data) gives the 

 fraction of the particular dinucleotide in the DNA. The jirocedure is 

 repeated for the other three reaction mixtures and the frequencies of all 

 possible dinucleotides may be determined. 



P--P--P 



SYNTHESIS 



(by polymerase) 



DEGRADATION 



(by micrococcal DNase 

 and splenic diestcrasci 



Fig. 9. Synthesis of a P^"-Iabeled DNA chain and its subsequent enzymatic 

 degradation to 3'-deoxyribonucleotides. The arrows indicate the linkages cleaved by 

 micrococcal DNase and calf spleen phosphodiesterase, yielding a digest composed 

 exclusively of 3'-deoxvribonucleotides. (From Josse et al., 1961.) 



Table XVI illustrates the data obtained from a typical nearest 

 neighbor analysis. In this experiment, Mycobacterium phlei DNA was 

 used as primer and the actual radioactivity j)resent in each of the frac- 

 tions is recorded. In the columns labeled "sequence" are reported the 

 dinucleotides represented by the incorporated label. In the column.'^ 

 labeled "fraction" are the values calculated as the fraction of the total 

 counts in a particular reaction digest found in the particular 3'-deoxy- 

 ribonucleotides. When these fractions are multiplied by the base-incorpo- 

 ration factors (mole fraction of the particular base in the given DNA) 

 the nearest neighbor frequencies recorded in Table XVH result. Earlier 

 work showing that the base composition of the newly synthesized DNA 

 reflected that of the primer was consistent with the scheme in Fig. 10 

 in which each strand of the proposed Watson-Crick double helix acts as 



