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



results of ribonuclease studies, as already pointed out. The small amount of 

 pyrimidine h mononucleotide present in the venom digests was tentatively 

 regarded^^^'^" as originating in side chains consisting of only one nucleotide 

 residue attached through triply-esterified phosphorus, as in the early struc- 

 tural proposals of GuUand and his colleagues^". Their own and other'^** obser- 

 vations indicate that some 10% of the phosphoryl groups in ribonucleic 

 acids are singly esterified. 



While these experiments favor a branched-chain structure for ribonu- 

 cleic acids, it would be unwise to rely entirely on evidence from only one 

 type of degradation and further confirmation would be welcome. In any 

 case, it is not impossible that among the natural ribonucleic acids both 

 linear and branched-chain polynucleotides may occur; such a possibility 

 has, in fact, been suggested^^^ and certainly there appears to be no evidence 

 against it at the present time. 



9. Nucleotide Sequence in Polyribonucleotides 



In previous pages the problem of the nature and position of the internu- 

 cleotidic linkage has been discussed in the light of recent work. A clear pic- 

 ture has emerged in which the nucleoside residues in the ribonucleic acids 

 are joined at their 3'- and 5 '-positions through phosphodiester linkages; 

 the possible types of branching have been indicated and evidence for 

 their occurrence has been reviewed. In brief, it appears that for a straight- 

 chain ribonucleic acid the general structure XVI will apply, and for 

 branched-chain acids structure XXIII, with the possible addition of some 

 branching through phosphotriester linkages. The degree to which branch- 

 ing may occur in ribonucleic acids generally is not yet established, but this 

 does not affect the general structures as far as the internucleotidic linkage is 

 concerned. 



In discussing these structures little attention has been paid to the order 

 in which the four different nucleotide residues occur in the polynucleotide 

 chain. It has, however, been pointed out that in a branched structure 

 XXIII the first residue in the branch must be a pyrimidine nucleotide resi- 

 due, and that in the more specific structure XXIIIa the branching point 

 in the main chain must also be a pyrimidine nucleotide residue. Only in 

 the case of the small oligonucleotides has residue sequence been deter- 

 mined and the results of such determinations do not shed a great deal of 

 light on the situation in the intact nucleic acids. Nucleotide sequence in 

 ribonucleic acids cannot be simply elucidated by partial degradation to 



1" W. E. Cohn, D. G. Doherty, and E. Volkin, Phosphorus Metabolism 2, 339 (1952). 

 1" G. Schmidt, M. Seraidarian, K. Seraidarian, and S. J. Thannhauser, Federation 



Proc. 11,283 (1952). 

 1" G. Schramm and B. von Ker6kjdrt6, Z. Natyrforsch. 7b, 589 (1952). 



