NUCLEIC ACIDS. 



The first chemical examination of cell nuclei was made in 1868 

 by F. Miescher. Pus cells were digested with artificial gastric juice ; the 

 protoplasm dissolved and a residue consisting of the more resistant 

 nuclei was left as. an insoluble grey powder. This dissolved in dilute 

 sodium carbonate and was precipitated by dilute acetic acid. It was 

 found to contain phosphoric acid and to give the colour tests for 

 proteins. It was named nuclein. 



Eight years later Miescher examined the spermatozoa of Rhine 

 salmon. He found that they consisted almost entirely of a salt com- 

 posed of the base protamine (a protein) and an organic acid which he 

 termed nucleic acid ; this contained phosphorus. 



Nucleins were prepared from other tissues, yeast, red blood corpuscles, 

 etc., by other workers. They were analysed most carefully by Kossel 

 and his pupils to whom our knowledge of the constitution of nucleic 

 acid is almost entirely due. Kossel found that nucleins and also 

 nucleic acid, for which a method of preparation from thymus and other 

 organs was devised by Kossel and Neumann, on hydrolysis by acids 

 gave rise to the purine bases, guanine, xanthine, hypoxanthine and 

 adenine. The two bases, guanine and adenine, have since been shown 

 to be the only ones present in nucleic acid. In addition to the two 

 purine bases three other bases, the three pyrimidine bases, thymine, 

 uracil and cytosine, have been shown to be present in nucleic acid, and 

 besides these compounds there is also present a carbohydrate, a hexose 

 or a pentose. These compounds are obtained from animal or plant 

 nucleic acids. Nucleic acids consist of a carbohydrate, phosphoric 

 acid, two purine bases and two pyrimidine bases, as expressed in the 

 following scheme : 



Animal Nucleic Acid. . Plant Nucleic Add. 



Phosphoric acid. Phosphoric acid. 



Hexose (laevulinic acid). Pentose = ^/-ribose. 



Guanine. Guanine. 



Adenine. Adenine. 



Cytosine. Cytosine. 



Thymine. Uracil. 



Plant nucleic acid differs from animal nucleic acid in the nature of 

 the carbohydrate constituent and in the nature of one of the pyrimidine 

 constituents. 



It appears that all animal nucleic acids are the same and that all 

 plant nucleic acids are the same. 



The constitution of the nucleic acids has not yet been definitely 



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