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



substantially iinbranched, and in which the individual nucleotides are 

 joined by phosphodiester linkages.-" Certainly most of the available evi- 

 denced^ ^ is in agreement with this although there has been an isolated sug- 

 gestion,^" on titrimetric results, that chain-branching through phosphotri- 

 ester linkages (about once in every 10-20 linkages) occurs and some slight 

 degree of branching has been also suggested from work using light-scatter- 

 ing techniques.^" These suggestions are at present unsubstantiated by other 

 physical studies, and no evidence for branching has been found in degrada- 

 tive studies. The observations on which they rest may be susceptible of 

 other interpretations in the light of recent structural models proposed by 

 other workers^^**'^^^ in which hydrogen-bonding between associated poly- 

 nucleotide chains plays a major role. For the purposes of the present discus- 

 sion, therefore, the deoxyribonucleic acids will be considered as essentially 

 unbranched polynucleotides. 



In their discussion of the hydrolytic behavior of the nucleic acids. Brown 

 and Todd''- adopted for deoxyribonucleic acids a general structure of type 

 XXX (Base — C3-— Cs- represents a nucleoside unit), in which the deoxy- 

 nucleoside units are linked by 3 ',5 '-phosphodiester groupings. This struc- 

 ture, they pointed out, was in accord with the alkali-stability of deoxyribo- 

 nucleic acids, and it is also borne out by the results of chemical and enzymic 

 degradation. 



Base Base Base Base 

 C3. C3. C3. C3. 



Cs' Cg- Cs' Cg- 



XXX 



It was claimed many years ago by Levene and Jacobs^*" that acid hydroly- 

 sis of thymus deoxyribonucleic acid under fairly vigorous conditions yielded, 

 among other products, two substances which were diphosphates of the two 

 pyrimidine deoxyribonucleosides, thymidine and deoxycytidine ; this claim 

 was subsequently disputed by other workers. ^^^ Dekker, Michelson, and 

 Todd^^2 have reinvestigated this matter using deoxyribonucleic acid from 

 herring sperm and have vindicated the claim of Levene and Jacobs."" "^'^^^ 



138 Inter al., R. Signer and H. Schwander, Trans. Faraday Soc. 46, 790 (1950). 



1" M. E. Reichmann, R. Varin, and P. Doty, J. Am. Chem. Soc. 74, 3203 (1952); P. 



Doty and B. H. Bunce, ibid. 74, 5029. 

 138 L. Pauling and R. B. Corey, Nature 171, 346 (1953). 

 "9 J. D. Watson and F. H. C. Crick, Nature 171, 737 (1953). 

 1" P. A. Levene and W. A. Jacobs, /. Biol. Chem. 12, 411 (1912). 

 1^1 H. Bredereck and G. Caro, Z. physiol. Chem. 253, 170 (1938). 

 "2 C. A Dekker, A. M. Michelson, and A. R. Todd, J. Chem. Soc. 1953, 947. 

 1" See also P. A. Levene, J. Biol. Chem. 48, 119 (1921); 126, 63 (1938). 



