NUCLEUS 213 



cytosine, thymine) or purine rings (guanine, adenine) can occur. 

 Cytidylic acid, a nucleotide obtained from yeast, has the structural 

 formula shown in Fig. 122 c. 



Because of the purine and pyrimidine rings, the nucleic acids show 

 a strong ultraviolet absorption, having its maximum at 2600 A. This 



Phosphoric acid < '^"a'' 



Ribose- purine base { I \^ '^\n/ 



0^ ,0H ■ 



■h\h Ch\'\ Phosphoric acid-- -< ' y^Q W^ 



\C^- 



0. ^'"^ 



CH 



N^ .CH N. X-^ / o . . . 



^ru \u NH Pyrimidme base 



t-" •'-" ribose I A/ - ^ 



a) b) \ \ OH yO 



■' ^ Phosphoric acid < ^^P^ 



r 

 OH ^'tbose J %-0. 



0—P=0 



N \n 

 OH „/'^0 



0^ 



0. ^^^ 



Cytosine . Ribose Phosphoric acid !\i ' 







Fig. 122. Molecular structure of the nucleic acids, a) Pyrimidine base; b) purine base (the 



rings are usually represented in the form of rectangles, but this might be incorrect from a 



morphological point of view); r) cytidylic acid = nucleotide cytosine-ribose-phosphoric 



acid (from Fischer, 1942); d) nucleic acid = polynucleotide. 



property has been very skilfully utihzed in cytology by Caspersson 

 (1936). 



In cells, the nucleotides do not occur in the free state. A mutual 

 esterification to polynucleotide has taken place, the latter representing 

 the actual nucleic acids. The esterification takes place between an OH- 

 group of the phosphoric acid and an alcohoUc hydroxyl group of the 

 ribose. It is possible that periodically, say after every fourth nucleo- 

 tide, other kinds of bonds also occur. For example, in the nucleic 

 acid of yeast, four nucleotides (adenine, uracil, guanine and cytosine 

 nucleotides) are combined into a tetra-basic acid. This nucleic acid 

 however, apparently does not occur in the nucleus but in the cyto- 

 plasm. 



The nucleic acids from the nucleus differ from the nucleic acids of 



