Specific Mechanisms of Protein Synthesis in the Developing Chick Embryo 135 



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DISCUSSION 



Quastler: It is useful to compare the informational requirements of various alternative 

 methods of protein synthesis. 



If the whole protein is synthesized directly from amino acids, then each locus on the 

 template must carry sufficient information to specify a single amino acid, or approximately 

 four bits; this is well within the informational capacities of chemical reactions. If the incor- 

 poration occurs in two steps, as has been suggested, then each step might have to specify no 

 more than two bits. 



If the protein is synthesized from peptide chains, then the informational requirements are 

 much more stringent. Consider the linking of two peptide chains of, say, five amino acids each. 

 If each of the ten amino acids can be any one of the whole set of amino acids, then the linking 

 operation must, in some way, identify ten amino acids, for a total of about forty bits — which 

 is a very large amount of information to be processed in a single act. The requirements are 

 greater — in fact, almost certainly too great — if two chains of ten amino acids are to be linked. 

 The following possibilities exist which allow the use of large fragments without imposing high 

 informational requirements : (a) the terminal amino acid in a chain identifies automatically 

 the other members — this would imply very strong sequential dependencies within peptide 

 chains, and consequently a low informational capacity of the whole amino acid sequence; 

 (b) linkages are formed without reference to the nature of residues remote from the locus of 

 linkage, and the resulting proteins are torn down again if not functional — in this case, the 

 probability of producing functional sequences by chance is small, and the efficiency of protein 

 synthesis is low; or (c) the protein studied is such that the exact sequence of residues is irrelevant. 



