TWO TYPES OF NUCLEIC ACID 85 



Dr. E. Vischer and Mrs. C. Green, and I embarked on the study 

 of the chemistry of nucleic acids, the main known facts were 

 based principally on the fundamental work of Miescher, Kossel, 

 Levene, Steudel, Feulgen, Thannhauser, Hammarsten, Jorpes, 

 Caspersson, Brachet, Bawden, Pirie, Stanley, Avery, and their 

 collaborators^'-. At that period, "the present stage of our knowl- 

 edge of nucleic acid chemistry could perhaps be compared to 

 that in which protein chemistry found itself at the beginning of 

 this century"^. The composition of only two nucleic acid prepa- 

 rations, the deoxyribonucleic acid of calf thymus and the ribonu- 

 cleic acid of yeast, was more or less completely known, and this 

 only qualitatively. Although nucleic acids, at any rate calf thymus 

 DNA and the pentose nucleic acids of plant viruses, were known 

 to exist as high polymers, although viruses had been recognized 

 as nucleoproteins, although the principle active in bacterial trans- 

 formation had been identified as deoxypentose nucleic acid — and 

 I could go on listing a few more disconcerting facts — it was 

 almost generally assumed that nucleic acids were relatively simple 

 compounds composed of a series of "tetranucleotides". As no 

 methods were available to test this structural hypothesis, this 

 simpUfication contributed considerably to the peace of mind of 

 the chemist; and it continues to have this soothing effect on many 

 investigators even now. 



We approached the study of nucleic acid chemistry in the 

 belief that the nucleic acids were complicated macromolecules, 

 comparable to the proteins in their intricate and specific struc- 

 ture^. It was evident that for a discussion of the chemical specific- 

 ity of the nucleic acids to become possible, one question had to 

 be answered first: How much of what do they contain? This 

 required, first of all, the elaboration of precise micromethods for 

 both the qualitative and quantitative determination of all nucleic 

 acid components — procedures that because of the strong ultra- 

 violet absorption of the purines and pyrimidines were based on 

 filter paper chromatography and spectrophotometry in the ultra- 

 violet^-^. 



The principal nitrogenous constituents of the nucleic acids are 



References p. 98 



