DEVELOPMENT 205 



The others are also rather simple in the chemical sense and so 

 the substance called a protamine is, relatively to many other 

 proteins (such as the globulins, haemoglobin, the albumens, etc.), 

 a very simple substance. 



The nucleic acid can be separated from its protamine base. 

 It is chemically complex but far less so than most of the substances 

 of the animal body. Its chemical composition has been found 

 to be as follows : — 



Phosphoric acid — a sugar — guanine 



phosphoric acid — a sugar — thymine 



I 



a sugar — cytosine 



.1 



phosphoric acid — a sugar — adenine 



phosphoric acid 

 (Thymus nucleic acid). 



We can simplify this by imagining it to be made up of the 

 four nucleotides. Each nucleotide consists of the three groupings 

 of atoms : 



phosphoric a hexose _ a purine 



acid carbohydrate base. 



There are various nucleins and the main differences between 

 them are in the kinds of carbohydrates and purines (or nitro- 

 genous bases) that they contain. The structure of a nucleotide is, 

 chemically, very suggestive : 



(i) The phosphoric-acid groups are annectant substances, 

 linking, releasing and re-linking the other groups ; 



(2) The carbohydrates are in such states that they may be 

 highly reactive, being stable when linked on to the nitrogenous 

 bases, but oxidizing at once when set free. 



(3) The purines, or nitrogenous bases are the simplest organic 

 groups carrying reactive carbon and nitrogen. 



The key substance of these nitrogenous bases is urea, which 



may be represented as 



/NH„ 

 O = C< 



\NH2 



