638 METABOLISM 



in those of plants (e. g., yeast), it is a pentose (5 0-atoms). It has been 

 found necessary to introduce some terms to designate the different parts 

 of the nucleic acid molecule ; thus, the whole molecule is called a tetra- 

 nucleotide, each mononucleotide molecule of which is composed of a 

 phosphoric acid molecule plus a nucleoside, which again is composed of 

 a purine or pyrimidine nucleus attached to pentose or hexose. The 

 nucleoside is so named because it is similar in structure to a glucoside. 

 Apart from differences in the carbohydrate group, it appears that 

 there is a close similarity in the structures of nucleic acids from dif- 

 ferent cells. This would indicate a common function for them all, which 

 may be either of a skeletal or of a physiologic nature; that is, nucleic 

 acid may have to do with the sustentacular material that builds the 

 nucleus, or it may have to do with some physiologic function common 

 to all cells, such as irritability, or growth, or respiration. If nucleic 

 acid is merely a sustentacular material, then the study of the behavior 

 of chromosomes and chromatine in cells can not have the significance 

 that it would have were nucleic acid concerned in the more vital activ- 

 ities of the nucleus. All the so-called nuclear stains owe their specific 

 staining properties to the fact that they are of a basic nature and com- 

 bine with nucleic acid. Until we know more definitely what the exact 

 function of nucleic acid may be, it is unwise to place too much weight 

 on the behavior of the chromosomes in cytologic researches. 



The History of Nucleic Acid in the Animal Body. We shall first 

 of all study the manner in which nucleic acid may be broken down. As 

 is to be expected from its complex structure, various types of enzymes 

 are concerned in this process. The first to act are known as the nucle- 

 ases. They split the tetranucleotide molecule into two dinucleotides, 

 which immediately afterward split further into mononucleotides. Four 

 nucleotides, two of purine and two of pyrimidine, are thus formed from 

 each molecule of nucleic acid. Each nucleotide molecule may now un- 

 dergo decomposition in one of two ways: (1) either by the splitting off 

 of phosphoric acid, leaving a nucleoside (guanosine or adenosine), or 

 (2) by the splitting off of both phosphoric acid and carbohydrate, leaving 

 free purine bases. Nucleuses have been found which specifically effect 

 either of these decompositions, and they have been called phospho- 

 nucleases* (1), and purine-nucleases (2), respectively. In the decompo- 

 sition of nucleic acid all of the four purine compounds guanine, guano- 

 sine, adenosine and adenine may be formed. This is illustrated in the 

 accompanying schema, in which the nucleic acid is represented as a 

 purine nucleotide: 



The numbers refer to the enzymes indicated in the schema. 



