64 Information Storage and Neural Control 



protein chemistry — in the effect of amino acid substitutions on 

 protein structure and specificity — and in the confirmation of the 

 correctness of the biochemical approach which we shall now 

 consider, rather than in the determination of the code for each 

 amino acid. 



THE MECHANISM OF PROTEIN SYNTHESIS AND THE 

 BIOCHEMICAL APPROACH TO THE GENETIC CODE 



The attempt to understand the intei mediate steps by which 

 genetic information is transferred into specific protein structure 

 obviously poses a very interesting biological problem. As recently 

 as a year ago, however, no one would have predicted that a crude 

 cell-free extract of E. coli could be forced, even in principle, to 

 yield precise information about the genetic code. The discovery 

 which revolutionized the coding search and opened what might 

 be called the biochemical approach to the genetic code was the 

 finding of Nirenberg and Matthaei (11) that one could trick the 

 E. coli extract into making a most unnatural protein — the polyamino 

 acid polyphenylalanine — by adding a most unnatural piece of 

 genetic material — the polyribonucleic acid of uridylic acid (poly U). 



To explain the significance of this experiment, it is necessary 

 first to describe briefly present ideas on the mechanism of protein 

 synthesis. There is believed to be a flow of information from DNA 

 through RNA to protein involving three classes of RNA: ribosomal 

 RNA, transfer RNA, and "messenger" RNA. Chemically, all of 

 these RNA's are polymers with a sugar-phosphate backbone like 

 DNA, but with ribose sugar instead of deoxyribose, and with the 

 base uracil (U) instead of thymine (T). 



Ribosomal RNA exists in the cell in cytoplasmic ribonucleo- 

 protein particles (ribosomes), which are generally considered to 

 be the cellular sites of protein synthesis (12). Messenger RNA is 

 assumed to carry the genetic information detailing the specific 

 amino acid sequence of the protein from the DNA to the ribosome. 

 Presumably the messenger RNA binds to the non-specific ribosome 

 (probably to ribosomal RNA) and serves as the information 

 bearing "template" for protein synthesis (13). Transfer RNA's 

 bind amino acids specifically (with the aid of enzymes). They are 



