118 THE BIOSYNTHESIS OF PROTEINS 



packages : the information required for the synthesis of a piece of protein 

 of the size of a complete polypeptide chain cannot be used unless it is 

 delivered en bloc. This almost certainly means that all the building blocks 

 which make up a polypeptide are assembled under the influence of one 

 single organizer macromolecule. 



The remarkable results of the studies on the mechanism of DNA 

 replication, in providing evidence for the operation of a rigid template 

 process in this case (Kornberg, 1958; Lehman et aL, 1958), indirectly sup- 

 port the conviction that a mechanism of this nature operates in protein 

 synthesis. Unveiling of the template mechanism is the task for the coming 

 years ; it is felt that our curiosity will not have to wait for a very long time 

 to be satisfied. 



Current ideas about this most important step of protein synthesis have 

 been deeply influenced by the brilliant theory presented by Crick (1957, 

 1958) as a development of the coding principle called 'code without com- 

 mas'. For reasons that have been discussed in Chapter I of the present 

 book, Crick suggested that each amino acid of a protein is coded in the 

 corresponding DNA by a sequence of three bases ; there are twenty such 

 sequences, each coding for an amino acid. The coding sequences for each 

 amino acid are such that if they were located in a row next to one another 

 no reading mistake could be made: the triplets corresponding to each 

 amino acid are such that any triplet formed from contiguous parts of any 

 two of them is nonsense, i.e. does not correspond to any amino acid. This 

 is the coding principle. 



The gene DNA which carries this coded information transmits it to some 

 RNA by controlling the arrangement of the nucleotides in RNA by a tem- 

 plate process of the kind discussed by Stent and by Zubay (1958). This 

 specific RNA is the template for protein synthesis. The difiiculty here is 

 that amino acids must find their right place along a polynucleotide in order 

 to obey the coded information. Crick suggests that specific enzymes, able to 

 recognize amino acids and short nucleotide sequences, bind the individual 

 amino acids to specific trinucleotides which are complementary to the cod- 

 ing sequences in the RNA. These serve as adaptors which will carry the 

 amino acids and will find their right place by forming hydrogen bonds with 

 the complementary bases on the RNA template. When the template is 

 covered with adaptors carrying amino acids, the amino acids unite into 

 a polypeptide chain, which has thus the primary structure specified by 

 the gene. 



If there are only two coding digits (6-keto and 6-amino) instead of four 

 as first assumed by Crick (see p. 36), triplets should be replaced by 

 groups of five nucleotides, but this does not change the principle of this 

 extremely attractive template mechanism. 



The experimental data reviewed before establish that amino acids are 



