128 NITROGEN METABOLISM 



pyrimidines. Position C'-3 is phosphorylated in the deoxy- 

 pentose nucleotides, and though it is probably the same in 

 the pentose nucleotides, position C'-2 is a possible alterna- 

 tive. Since hydrolysis in certain conditions yielded four types 

 of nucleotides, apparently in equimolecular proportions, all 

 nucleic acids were at first thought to be polymers of units 

 each of which contained the four nucleotides arranged in a 

 straight chain or in a cyclic tetrad. An alternative suggestion 

 was that the overall equivalent proportions did not neces- 

 sarily imply such a regular arrangement but were due simply 

 to a statistical mean. Such theories required that the ratio 

 of purine-N to pyrimidine-N should be 2:1, but with the 

 accumulation of precise quantitative data came the realiza- 

 tion that few nucleic acids contained the four nucleotides, 

 or even purines and pyrimidines, in equimolecular propor- 

 tions. For example, three types of DPNA have been isolated; 

 in the one found in animals, yeast and most bacteria, adenine 

 and thymine predominate (AT type), in another, found in 

 only a few bacteria, guanine and cytosine predominate (GC 

 type), whereas in the third type isolated from strains of 

 Esch. colt, the bases are in equimolecular proportions [13]. 

 Although discovered over eighty years ago, detailed in- 

 vestigations of structure have been confined to only two 

 nucleic acids, a RNA from yeast and a DRNA from the 

 thymus, and of these most is known about the former. 

 Markham and Smith [38] believe that yeast RNA is a mix- 

 ture of comparatively short, straight chains of nucleotides 

 in which the internucleotide linkage is between the phos- 

 phate at C'-3 (or C'-z) of one nucleotide and the hydroxyl 

 group at C'-5 in the adjacent nucleotide. They have also 

 presented evidence that some of the chains terminate in 

 cyclic nucleotides, i.e. nucleotides in which the phosphate 

 group forms a bridge between C'-2 and €'-3. By using ion 

 exchange resins, previous workers had shown that each of 

 the four nucleotides in alkaline hydrolysates of yeast RNA 

 could be separated into two isomers, the 'a' and *b' nucleo- 

 tides, which were regarded as being the nucleoside 2'- and 

 3 '-phosphates. It is now evident that only one of these 

 isomers occurs naturally, and that the other is formed by 



