FINAL REMARKS 35 



apurinic acid are contrasted in Table 14 with those of the calf 

 thymus DNA specimen that served as the starting material. 



3. PENTOSE NUCLEIC ACIDS 



Time does not permit an adequate discussion of the chemistry 

 of PNA. In the past few years, significant contributions to this 

 field were made by GuUand, Kerr, Schmidt, Allen, Loring, Cohn, 

 and many other workers. I should Uke to make brief mention of 

 some recent work done by Magasanik in our laboratory in which 

 the course of action of crystalUne ribonuclease on PNA from 

 yeast and from pig liver was studied with the use of chromato- 

 graphic and spectroscopic procedures. About 60-70%, or in 

 some cases somewhat more, of the nucleotides present in the 

 initial substrates were liberated by enzymic action as rapidly 

 dialyzable nucleotides of low molecular weight. This fraction 

 consisted of free cytidylic and uridylic acids, which comprised 

 a high proportion of the total pyrimidine nucleotides, and of 

 combined purine and pyrimidine nucleotides. All PNA prepara- 

 tions yielded a non-dialyzable residue, resistant to enzymic at- 

 tack, which was found to consist to about two thirds of guanyUc 

 acid and of varying amounts of the other nucleotides. A selection 

 of these experiments is presented in Table 15. These and other 

 findings have led us to certain conceptions concerning the 

 specificity of ribonuclease and the structure of PNA; but refer- 

 ence must be made to a more detailed article^. 



4. FINAL REMARKS 



It is fitting to conclude this all toa sketchy survey with a confes- 

 sion of ignorance. What our studies have taught us more than 

 anything else is how Httle we know as yet about the chemistry 

 of nucleic acids. The chemical specificity of macromolecules and 

 the interactions between them through which the organization of 

 the cell is maintained can only partly be understood in terms of 

 our present knowledge. In the approach to a scientific problem 



References p. 37 



