isolation and composition of deoxypentose nucleic acids 345 



3. "Cores" (Limit Polynucleotides) 



When a deoxyribonuclease acts on a deoxyribonucleic acid, we are dealing 

 with an enzyme of as yet unrecognized specificity attacking a polynucleo- 

 tide chain of as yet unknown sequence. It is a reaction whose study is 

 likely to spread more darkness than light. Moreover, the fact that deoxy- 

 ribonuclease is a phosphodiesterase of a closely circumscribed, as yet un- 

 definable, specificity"^ producing only a very small quantity of the ulti- 

 mate "monomers," the mononucleotides, in addition to a complex mixture 

 ranging from polynucleotides to dinucleotides,"® raises a problem of a 

 special kind. The enzymic attack on a substrate of the complexity of a 

 deoxypentose nucleic acid, which results in its partial cleavage, must go 

 through an intricate pattern: every break of the original molecule produces 

 substrates that are new and different; the enzyme must deal with kaleido- 

 scopic substrate changes. The attempt to solve this gigantic puzzle by 

 fitting the innumerable fragments. into a plausible sequence is doomed to 

 failure. Unless, however, deoxyribonuclease is specific only for the size, 

 and not the quality, of the oligonucleotide fragments which it is able to 

 cleave, in which case the first few random events would decide all subse- 

 quent ones, the order in which different pieces are detached, and the com- 

 position of those that are left behind, may serve as means of distinction 

 between different nucleic acids. 



The study of the action of crystalline pancreatic deoxyribonuclease [com- 

 pare Schmidt, Chapter 15] on calf thymus nucleic acid has, in fact, shown 

 a characteristic trend in the composition of dialyzable digestion products 

 and the existence of polynucleotides, distinguished by greater resistance to 

 enzymic attack and characterized by greatly increased ratios of adenine 

 to guanine, thymine to cytosine, and purines to pyrimidines.^'--" Similar 

 studies have been carried out with nucleic acids from other sources."' •^^'*' 

 228-230 j^ j^g^y Q^^fy |-,g meutioned that the "cores" produced by the action 

 of two different deoxyribonucleases, those of pancreas and of germinating 

 barley, on the same nucleic acid differ markedly in their composition.-'' 



VI. Constituents of Deoxypentose Nucleic Acids 



The chemistry of the ultimate breakdown products of the nucleic acids, 

 namely, the sugars and nitrogenous constituents, is discussed in Chapters 

 2 and 3; the nucleosides and nucleotides are considered in Chapter 4. In 



"6 C. Tamm and E. Chargaff, Nahire 168. 916 (1951). 



"6 R. L. Sinsheimer, /. Biol. Chem. 208, 445 (1954). 



2" S. Zamenhof and E. Chargaff, J. Biol. Chem. 187, 1 (1950). 



228 W. G. Overend and M. Webb, J. Chem. Soc. 1950, 2746. 



«9 K. Moldave and C. Heidelberger, J. Am. Chem. Soc. 76, 679 (1954). 



"" L. L. Uzman and C. Desoer, Arch. Biochem. and Biophtjs. 48, 63 (1954). 



"' G. Brawerman and E. Chargaff, J. Biol. Chem. 210, 445 (1954). 



