166 NUCLEIC ACIDS, DECODING, ETC. 



past, one of the principal marks of the specificity of the cell. The 

 approach to the problem of the sequence of nucleotides in a 

 nucleic acid is, however, much more difficult than in the case of 

 the proteins. The main reason is that we still lack a sufficient 

 arsenal of different enzymes breaking predictable internucleotide 

 bonds in a specific manner. So far, we have been compelled to 

 rely mostly on the differences in stabiUty of the purine and 

 pyrimidine nucleosides when exposed to chemical degradation. 

 It is indeed very regrettable that amazingly little work is being 

 carried out at present on the search for new and specific 

 nucleases. 



The basic fact which is at the origin of all our sequence studies 

 on DNA is the great difference in stability towards acid exhibited 

 by the purine and the pyrimidine nucleosides. Whereas the 

 glycosidic link between purine and sugar is cleaved with little 

 difficulty, pyrimidine nucleosides are quite stable under these 

 conditions. For this reason, it is relatively easy to deprive a 

 DNA polymer chain of its purine complement without disturbing 

 the arrangement of the pyrimidine nucleotides. We have called 

 degradation products of this type, i.e., chains which contain free 

 sugar phosphates at the places where purines were, apurinic acid. 

 Additional, slightly more energetic, degradation will now release 

 the various pyrimidine clusters occurring in the original DNA. 

 We refer to pyrimidines that find themselves interjacent between 

 purines as "solitary" pyrimidine nucleotides; longer pyrimidine 

 runs can be designated as "bunched", or more particularly as 

 "couples", "triplets", etc. For purposes of such an analysis, which 

 we have referred to as differential pyrimidine distribution anal- 

 ysis, it is not necessary to distinguish between the two strands 

 of a double helix, if it occurs, since such a DNA polymer could 

 be viewed as being joined at one end, or even at both ends, so 

 as to produce one continuous polynucleotide. The analysis is 

 based on the differential release of pyrimidine nucleoside 3',5'- 

 diphosphates by /^-elimination and hydrolysis. 



The principal conclusions from a large series of such studies 

 are: (1) The method permits the distinction of DNA specimens 



