108 UNITY AND DIVERSITY IN BIOCHEMISTRY 



polynucleotide chains isolated from ribonucleic acids, this is an indication 

 of the complexity of what is customarily called a "ribonucleic" acid. The 

 nucleosides entering into the composition of the ribonucleic acid from 

 yeast are /3-D-ribofuranosides in which adenine and guanine are linked at 

 N-9 and cytosine and uracil at N-3. It appears that ribose is always the 

 sugar present in what were formerly known as pentose-nucleic acids and 

 they can, therefore, be named ribonucleic acids. 



The internucleotide bond which is predominant in the ribonucleic acids 

 is the phosphoric ester bridge between C-3' and C-5' of two adjacent 

 nucleotides. 



Cs— OH Ca—OH C=— OH C3-OH 



I I i I 



Cs.-O OH Cs'-O OH Co— O OH C.—O OH 



\ / 

 P 



\ / I \ / I \ / 



P P P 



// \ // \ ! // \ I // \ 



_C,, O O C.. O O C.. O O C5' o o 



In such a scheme, — C-2' — C-3' — C-5' — represents a nucleotide residue. 

 The presence of an -OH on C-2' explains how, by analogy with the results 

 obtained with mononucleotide esters, the alkaline degradation proceeds 

 through the intermediate formation of a cyclic structure in which nucleo- 

 side-2', -3' cyclic phosphates are formed by cleavage of the C-5' — O — P 

 bond, as shown above. 



c,' — o •'" o Ca — o ..-■ o 



The isolation of ribonucleic acids is difficult which makes the determi- 

 nation of their structure and composition difficult also. However it can 

 be shown that the ribonucleic acid of animals differs from that of yeast, and 

 that the ribonucleic acids from different organs of the same species are less 

 alike than are those from a given organ obtained from several species. 

 Unlike the desoxyribonucleic acids, the ribonucleic acids differ not only 

 from species to species but also from tissue to tissue in the same species. 

 Moreover, the ribonucleic acids of the nucleus differ from that of the 

 cytoplasm, and external conditions also cause variations. 



(b) Desoxyribonucleic Acids (DNA) 



The desoxyribonucleic acids which do not have an -OH group on 

 C-2', differ from the ribonucleic acids in possessing a more stable inter- 

 nucleotide bond. Although the linkage is between C-3' and C-5' of adjacent 

 sugar molecules, the cyclization described above for the ribonucleic acids 



