366 ERWIN CHARGAFF 



cytosine, ascending contents of adenine and thymine) into several distinct 

 "bands," in which all the regularities characteristic of the entire nucleic 

 acid are maintained. 



IX. Composition Studies and Structural Investigations 



The problem of nucleotide sequence has frequently been mentioned in 

 the preceding pages. (See, in particular, Section V.l.) It is the most elusive 

 and vexing, and also the most important, part of what may be considered 

 as the structural investigation of the deoxypentose nucleic acids. There was 

 never much doubt that the principal connecting links between the mono- 

 nucleotides were 3 ',5 '-phosphate bridges (see Chapter 12), though the 

 proverbial exceptions that should prove or test this rule have, perhaps, not 

 yet been looked for in a sufficient number of cases. But even complete 

 certainty regarding the points of attachment of the phosphate bridges 

 would contribute no more to the problem of structure than the statement 

 that all proteins contain peptide bonds. It must be admitted that the task 

 of sequence analysis, beyond our present means if a single macromolecular 

 polynucleotide chain is to be unriddled, becomes so gigantic, if the con- 

 clusions from the fractionation experiments (Section VIII) are justified, 

 as to discourage the most sanguine of optimists. 



At the present time, only the crudest form of mapping the order in which 

 the mononucleotides are aligned in a nucleic acid chain appears attainable. 

 The task is similar to that of an ancient geographer: no more than dim 

 contours, vague directions can be discerned. 



As has been pointed out before (Section V), several partial degradation 

 products of deoxypentose nucleic acids appear to offer an opportunity of 

 searching for the existence of certain general structural features; these are 

 the apurinic acids,"" which can be prepared readily from many deoxy- 

 pentose nucleic acids, and the various large fragments formed by enzymic 

 attack.^'-' ^-^ If the production of pyrimidine deoxyribonucleoside diphos- 

 phates^^*' 263-266 (^ggg g^jgQ Chapters 4 and 12) by acid hydrolysis of nucleic 

 acids can be standardized and a procedure for the quantitative estimation 

 of individual diphosphates developed, this may also contribute to the char- 

 acterization of structural differences, provided the method is applied to 

 several purified deoxypentose nucleic acids of different origin rather than 

 to commercial material. If these diphosphates really are indicative of those 

 positions in the original nucleic acid chain in which a pyrimidine nucleotide 

 is flanked by purine nucleotides,-^® it is conceivable that different nucleic 



2" P. A. Levene and W. A. Jacobs, J. Biol. Chem. 12, 411 (1912). 



26* P. A. Levene, J. Biol. Chem. 48, 119 (1921); 126, 63 (1938). 



2" S. J. Thannhauser and G. Blanco, Z. physiol. Chem. 161, 116 (1926). 



"« C. A. Dekker, A. M. Michelson, and A. R. Todd, J. Chem. Soc. 1953, 947. 



