28 NUCLEIC ACIDS AS CELL CONSTITUENTS 



are not sufficiently significant to permit their use as the sole 

 criterion of differentiation. The most important question of the 

 sequence in which, in a particular nucleic acid, the nucleotides 

 follow each other has so far barely been approached. The 

 elaboration of methods for sequence analysis is, perhaps, one 

 of the most urgent problems in nucleic acid chemistry, since 

 differences in nucleotide sequence may very well be among the 

 determinants of chemical and biological specificity. 



b. No differences in composition have so far been found in 

 DNA from different tissues of the same species. This provisional 

 conclusion refers only to the over-all composition. It is in this 

 connection noteworthy that no chemical differences appear to 

 exist between the composition of DNA from normal human 

 tissue^* and that of preparations from human cancer tissue^s. 



c. The tetranucleotide hypothesis is incorrect. 



d. There exist a number of regularities. Whether these are 

 merely accidental cannot yet be decided. In almost all DNA 

 preparations studied until now the ratio of total purines to total 

 pyrimidines never was far from 1 . Similarly the ratios of adenine 

 to thymine and of guanine to cytosine were near 1. 



e. There appear to exist two main groups of DNA, namely 

 the "AT type", in which adenine and thymine predominate, and 

 the "GC type", in which guanine and cytosine are the major 

 constituents. The latter has so far been found only in certain 

 microorganisms^^. 



Data to support these conclusions have, in the main, been 

 presented in previous publications and were summarized recent- 

 lyi9,23 J should like to limit myself here to the discussion of a 

 few as yet unpublished results. The study of the composition of 

 the DNA of salmon sperm^^ has brought out some of the regu- 

 larities mentioned before particularly clearly (Tables 10 and 11). 

 This substance belongs to the "AT type". 



Another investigation, undertaken in collaboration with G. 

 Brawerman, deals with the DNA of wheat germ and the course 

 of its degradation by crystalline pancreatic deoxyribonuclease. 

 Previous work in our laboratory on the enzymic disintegration 



