14 CHEMICAL COMPONENTS OF CELLS (1) 



Exercise III 



nucleotide is itself composite, being made of a 

 nitrogenous purine or pyrimidine base, a 

 5-carbon sugar (ribose in RNA and deoxyribose 

 in DNA), and phosphoric acid, united to one 

 another by the same principle of elimination of 

 water between them. The hydrolysis of nucleic 

 acid not only may cleave the nucleotides, but 

 may sever all these linkages, leaving us with a 

 mixture of the free nitrogenous bases, 5-carbon 

 sugars, and phosphoric acid. The acid hydroly- 

 sis that we will perform releases nearly all the 

 purine bases, but only a small fraction of the 

 pyrimidines. 



The ultimate components of the two families 

 of nucleic acids are: 



RNA 



adenine! 

 guanine] 

 cytosine 

 uracil 

 ribose 

 phosphoric 

 acid 



purmes 



DNA 



(adenine 

 {guanine 



. ... (cytosine 

 pynmidmes { , 



(thymme 



deoxyribose 



phosphoric 



acid 



The first four substances named in each column 

 are the nitrogenous bases. A nucleotide can be 

 written: base-ribose-phosphoric acid, the nu- 

 cleotides of each nucleic acid differing only in 

 their bases. The fundamental arrangement of 

 nucleotides in nucleic acid is: 



base-sugar-phosphoric acid 



base-sugar-phosphoric acid 



base-sugar-phosphoric acid 



where each bond represents a point at which 

 molecules have been united by elimination of 

 water, and conversely can be hydrolyzed by the 

 insertion of water. 



The third class of macromolecules, the pro- 

 teins, is composed of up to 20 different amino 

 acids, joined together to form chains hundreds 

 to thousands of amino acids in length. Since 



proteins are of many sizes, and their amino 

 acids can be united in any proportions and in 

 any sequences, almost an infinite variety of dif- 

 ferent proteins can exist. Living organisms 

 take full advantage of this possibility, for as far 

 as we know every living species, animal and 

 plant, contains specific proteins different from 

 those of all other living species. Proteins account 

 for much of the internal structure of cells, and 

 all known enzymes are proteins. 

 An amino acid has the general formula: 



R 



I 

 H— C— NH2 



I 



c=o 



\ 



OH 



in which — NHo is the amino, — COOH the 

 carboxyl (acid) group, and R may be any one 

 of 20 different groups (— H, — CH3, — CH2OH, 

 etc.). Amino acids are joined to one another 

 by taking out a water molecule between the 

 — NH2 group of one and the — COOH group 

 of its neighbor. The joint that results, 



O 



II H 

 — C— N— 



is called a peptide bond. Groups of amino acids 

 linked together in this way are called polypep- 

 tides until they get big enough to be called 

 proteins. 



The general arrangement of amino acids in a 

 segment of polypeptide or protein follows. The 

 insertion of molecules of water at the places 

 indicated by arrows hydrolyzes the structure 

 into its constituent amino acids: 



Ri 



H 



HO— H 



O 



HO— H 



