NUCLEIC ACIDS AND NUCLEOPROTEINS 123 



NUCLEOPROTEIN 

 I 



(gastric digestion) 



Protein Nuclein 



i 

 (pancreatic digestion) 



Protein Nucleic Acid 



The Nucleic Acids. The nucleic acids of the animal body occur 

 mainly in combination with protein material in the so-called nucleo- 

 proteins of which they form the characteristic radicals (see page 123). 

 The amount and character of the protein with which the nucleic acid 

 molecule is combined varies and the acid may^m certain cases be found 

 in cells in a free form. Naturally those tissues are richest in nucleic 

 acid which contain the largest amount of nuclear material and of 

 nucleoprotein. Such are the glandular tissues of the body as the thy- 

 mus, spleen, pancreas, liver, etc. The heads of the spermatozoa con- 

 sist almost entirely of nucleic acid in combination with protamine. 



The nucleic acids are a distinct class of substances, characterized 

 by their decomposition products. They are strongly acid in reaction 

 and contain considerable phosphorus. 'They may be divided into two 

 main groups, the animal and the plant nucleic acids. The two classes 

 differ in certain respects but all of the true animal nucleic acids appear 

 to be practically identical in composition. Animal nucleic acid is most 

 readily prepared from the thymus while plant nucleic acid is most 

 readily obtained from yeast. 



The nucleic acids are difficultly soluble in cold water, more readily 

 in hot water, insoluble in alcohol, but readily soluble in weak alkali 

 with the formation of the alkali salt. If pure they do not give the pro- 

 tein color reactions. They are optically active. They are precipitated 

 from their alkaline solutions by HC1, but only the plant nucleic acid is 

 precipitated by acetic acid. In weak acid solution they are precipi- 

 tated by protein the combination being considered a "nuclein." They 

 form insoluble salts with alkaline earth and heavy metals. The sodium 

 salt of animal nucleic acid in 4 per cent solution is liquid while warm but 

 solidifies to a gelatinous mass on cooling. Plant nucleic acid does not 

 do this. 



The nucleic acids on hydrolysis yield phosphoric acid, purine and 

 pyrimidine bases, and a carbohydrate or carbohydrate derivative. The 

 composition varies slightly with the type of nucleic acid. Plant nucleic 

 acids contain a pentose group (d-ribose) while animal nucleic acids con- 



