CHEMICAL PATHWAYS 117 



be regarded as likely intermediates, the study of which will be pursued with 

 great interest. 



2. Are Polypeptides Made on a Template? 



In its present state of development, biochemistry offers essentially one 

 type of explanation for each of the metabolic steps it describes, namely the 

 presence in the cell of an adequate enzyme which promotes the reaction 

 considered. When several possibilities exist from a purely chemical point of 

 view, the specificity of the enzyme or of the enzyme assortment is again 

 invoked. For instance, a great variety of fatty acids, terpenes, carotenoids, 

 steroids can be made from acetate ; the nature and assortment of the pro- 

 ducts elaborated by a cell is explained by its particular complement of 

 enzymes. 



If this type of stepwise assembly process is assumed to operate in the 

 specific synthesis of the individual proteins, a fundamental difiiculty is 

 raised by the number of enzymes which would be specifically involved in 

 the production of each single specific protein. This time, simply referring 

 to the specificity of the enzymes would amount merely to displacing the 

 problem, for the enzymes are specific proteins and they are also made in the 

 same cell. The difficulty has been appreciated by biochemists for more than 

 a quarter of a century, and it was suggested several times, on purely logical 

 grounds, that the structure of the enzymes or at least of their specific part 

 must be controlled by a model, a mould or a template, i.e. by a substance 

 having a structure complementary to that of the enzyme synthesized. 



Current developments of molecular biology (see Chapter I) give a great 

 vitality to this old idea. Indeed it is now established that the primary 

 structure of the proteins, i.e. the sequence of the amino acids in the 

 polypeptide chains, is controlled by mendelian genes. The information 

 relative to the arrangement of the amino acids is recorded on the genetic 

 material which is made of nucleic acids. The purine and pyrimidine bases 

 compose the ciphers in which this information is coded, since replacement 

 of one base by another or substitution of one keto for an amino group in one 

 base can change the information and result in the specific replacement of 

 one amino acid by another at one specified place in the polypeptide chain. 

 All the specific information relative to the primary structure of a protein 

 is comprised in a unique segment of the genetic material, the locus. The 

 size of the locus is such that it might contain enough information for con- 

 trolling every single amino acid in the polypeptide. These facts almost 

 inescapably lead to the conviction that the arrangement of the amino acids 

 in the polypeptide correspond point to point to that of the purines and 

 pyrimidines (or of their 6-keto and 6-amino groups) in DNA. Moreover, 

 the information concerning a protein molecule must be transferred to the 

 protein making system in one single package or in a very small number of 



