Elaboration of Polypeptide Chains

Protein synthesis is a complex biosynthetic process in which the information stored in the DNA bases is translated into a specific amino acid sequence. We have already described both ends of this process DNA and the formation of amino acyl tRNA. We will now consider the intricacies of the reaction that brings the individual tRNA charged with this amino acid into proper alignment to form a specific polypeptide chain. 

The actual elaboration of the chain occurs at some distance from the nucleus, somewhere in the cytoplasm. Therefore, the DNA message must be carried to the cytoplasm messenger RNA performs this function. 

The complexity of the reaction involved in protein synthesis is so great that these reactions could not be efficiently carried out with substrate (charged tRNA) and template (messenger RNA) circulating free in solution. Successful protein synthesis thus requires that substrate and template be brought together in close proximity. The ribosomes are the pillars that support this intricate structural organization (see Fig. 2-14). 

The schematic presentation of the mechanism of protein synthesis that we have just presented immediately raises a number of problems. 

If the reaction occurs on ribosomes, transfer RNA must first attach to the ribosome. The amino acids that are attached to adjacent tRNA s become involved in peptide bond formation only after attachment. Thus, we need to discuss the mode of attachment of the tRNA to the ribosome and the mechanism of peptide bond formation. 

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