134 NUCLEOTIDE SEQUENCE IN DEOXYRIBONUCLEIC ACIDS 



intermediate product in all procedures involving acid degrada- 

 tion^^. When a deoxyribonucleic acid is exposed to mildly acidic 

 conditions (pH 1.6 at 37°) all the purines are cleaved off gradual- 

 ly, leaving behind a polymer representing the original poly- 

 nucleotide, but composed of deoxyribophosphate units, at the 

 places of the previous purine nucleotides, and of the pyrimidine 

 nucleotides in unchanged ratio and at the same position as in the 

 native starting material. For instance, in the segment represented 

 in Fig. 11, positions 3, 5-7, 9, T, 2', 4', 8', 10' would remain 

 unchanged, whereas in the remaining places the purines would 

 now have made room for the reactive aldehydo groups of the 

 deoxy sugar. The presence of the aldehydo group^^ and, there- 

 fore, of a free hydroxyl at 4' brings about a remarkable labili- 

 zation of the polymer: it is broken not only by alkali, but even by 

 buffers (pH 8.6) containing primary amino groups^^. It is likely 

 that this susceptibility of the poly-sugar-phosphate backbone of 

 apurinic acid to amines is involved in the degradation reactions 

 employing diphenylamine under acidic conditions'^. 



Studies on the arrangement of the pyrimidine nucleotides in 

 apurinic acid'^ were, in fact, the first that made possible an ap- 

 proach to the problem of nucleotide sequence in deoxyribonucleic 

 acid. The information so obtained was limited, since it was only 

 qualitative, but it showed that a considerable portion of the 

 pyrimidine nucleotides, and therefore also of the purine nucleo- 

 tides, was arranged in the form of tracts of several nucleotides of 

 one kind. 



5. SEQUENCE STUDIES THROUGH DIFFERENTIAL 

 DISTRIBUTION ANALYSIS 



a. Mechanism 



The important discovery that among the fragments produced by 

 the acid degradation of deoxyribonucleic acids there are found 

 the 3',5'-diphosphates of deoxycytidine and thymidine is due to 

 the work of Levene, Thannhauser and their collaborators^^"^^. 

 A more recent re-investigation^* by improved techniques, which 



