442 D. M. BROWN AND A. R. TODD 



all, or almost all, the internucleotidic linkages must involve the 5 '-position 

 of a nucleoside residue. These facts all point clearly to an essential structure 

 of type XXX. 



Evidence from other enzymic studies supports this conclusion. The mode 

 of action of the crystalline deoxyribonuclease^^ is not yet understood, but 

 several workers^^''"^^^ have shown that a number of oligonucleotides of 

 varying size are produced when it acts on deoxyribonucleic acids. The use 

 of electrophoretic,^*^'^^^ paper chromatographic,^*^ and ion-exchange^*^ 

 methods have permitted the separation and identification of some of these 

 oligonucleotides.. Sinsheimer and Koerner^*' have described two dinucleo- 

 tides, one containing two cytosine residues and the other an adenine and a 

 cytosine residue, and have determined their structures by enzymic hydroly- 

 sis. The method used is indicated below, using the deoxycytidine dinucleo- 

 tide XXXIII as an example. Phosphomonoesterase treatment yielded the 



OPO3H2 



Cyt-C3-^C5. ^,^^^^^^^^ 



P *■ 2 mol. deoxycytidine- 5'-phosphate 



Cyt-Cg.— C5. 

 XXXIII 



monoesterase 

 T 



Cyt-Co.— C5. 



\ diesterase 1 mol. deoxycytidine and 



^\ 1 mol. deoxycytidine-5'-phosphate 



Cyt-Cg.— Cs- 



XXXIV 



dinucleoside phosphate XXXI V^** which in turn yielded, with snake venom 

 diesterase, both deoxycytidine-5 '-phosphate and the free nucleoside, deoxy- 

 cytidine. Direct enzymic hydrolysis of the original dinucleotide (XXXIII), 

 on the other hand, yielded only deoxycytidine-5 '-phosphate, indicating 

 that a phosphoryl group is attached at C5- in both residues. 



1. Nucleotide Sequence in Deoxyribonucleic Acids 



Although, as the above discussion shows, the nature and position of the 

 main internucleotidic linkage in deoxyribonucleic acids seems clear, the 

 problem of nucleotide sequence remains. [Cf. Chargaff, Chapter 10.] On 

 this no definite statement can be made. As yet no method of stepwise degra- 

 dation comparable to that proposed for ribonucleic acids"^ has been evolved. 



15" W. G. Overend and M. Webb, J. Chem. Soc. 1960, 2746. 



1" A. H. Gordon and P. Reichard, Biochem. J. 48, 569 (1951). 



162 J. D. Smith and R. Markham, Nature 170, 120 (1952). 



1" R. L. Sinsheimer and J. F. Koerner, J. Am. Chem. Soc. 74, 283 (1952). 



1" Cf. also J. D. Smith and R. Markham, Biochim. et Biophys. Acta 8, 350 (1952). 



