CH. III.] NUCLEIC ACID. 61 



broken down by a variety of ferments found in the tissues. 

 The final products of hydrolysis of thymus nucleic acid are 



Phosphoric acid. 



Purine bases, adenine, and guanine. 



Pyrimidine bases, thymine, and cytosine. 



An unknown hexose sugar. 



Yeast nucleic acid differs only in yielding uracil instead of 

 thymine and a pentose sugar (<f-ribose) instead of the hexose. 

 As to the composition of the nucleic acids, it has been 

 established that they consist of certain groups called 

 nucleotides, which can be liberated by the action of enzymes 

 found in the tissues, and called nucleotidases. There are 

 apparently four nucleotides to the molecule of nucleic acid. 

 The nucleotides consist of phosphoric acid-sugar-base, the 

 latter being either a purine base or a pyrimidine base. By 

 the action of an enzyme, called phospho-nuclease, on the 

 mononucleotides the phosphoric acid is split off, leaving the 

 carbohydrate attached to the purine or pyrimidine base. 

 These compounds are known as purine or pyrimidine 

 nucleosides. 



The nucleotides can, however, be attacked by another 

 enzyme, purine or pyrimidine nuclease, which splits off 

 the base from the phosphoric acid-sugar complex. 



The following scheme, suggested by Levene and Jacobs, 

 may represent the structure of thymus-nucleic-acid. 



Purine-nucleoside 



Phosphoric acid hexose --guanine 

 Phosphoric acid Hexose thymine 

 Phosphoric acid Hexose cytosine 



Phosphoric acid - hexose - adenine 



Mono-nucleotide. 



For further information on the subject the student is 

 referred to the valuable monograph by W. Jones.* 



* Nucleic Acids, by Walter Jones. (Longmans, Green & Co., London, 

 1914.) 



