PROTEINS 



123 



is known, each nucleic acid contains but a single kind of sugar, either 

 D-ribose or D-deoxyribose. 



Both types of nucleic acids appear to be chains with alternating 

 sugar and phosphate groups. The purine and pyrimidine bases are 

 distributed along this chain, one for each sugar residue. The chain 

 itself depends upon ester bonds between phosphates and the adjacent 

 sugars with the hydroxyl groups of positions three and five involved 

 as follows: 



Hv CH2OPO2O Hx ^CH2()P020, Hx /CH2OPO2O, 



HO 



B ^H 



where B represents cytosine, uracil, adenine, or guanine. The in- 

 dicated negative charges arise by loss of protons from the phosphoric 

 acid groups and are present at physiological values of pH. Only strong 

 acids can restore the proton, converting the salt form into the acid. 

 Thus the nucleic acids normally occur as salts rather than as true 

 acids. 



As shown above, it is now believed that the second hydroxyl group 

 of ribose is free. Formerly the ribonucleic acids were thought to be 

 branched with the branches attached at position two. Present evi- 

 dence, however, suggests that branching does not occur. 



On the basis of X-ray data it has been postulated that this sugar 

 phosphate chain is arranged in a spiral with the bases fitted inside this 

 spiral. This idea is somewhat analogous to that for the peptide chains 

 of proteins. The sequence of the bases is just beginning to yield to 

 present methods of study. It is known that the four bases in the 

 ribonucleic acids occur in different relative amounts, and hydrolysis 

 studies show that the distribution is not uniform. Rather, there are 

 regions of the molecule containing only the pyrimidines, others only 

 purines, and others with the two types at more or less alternating sites. 



It is assumed that cells may contain ribonucleic acids of different 

 types. Certainly different ones can be isolated from different organs 

 and animals. However, sensitive biological tests for different ribo- 

 nucleic acids have not been found. Thus it is not possible to perform 

 sensitive fractionation studies. In addition, the ribonucleic acids are 

 very sensitive to degradation processes when in solution. This com- 

 plicates the isolations, so that, even when separations are achieved, the 

 fractions obtained may not represent the structures originally present 

 in the source materials. 



