62 



Information Storage and Neural Control 



of the DNA. Let us now discuss how a gene imparts catalytic 

 specificity to an enzyme. Enzymes consist of a linear chain of amino 

 acids (the primary structure), coiled in part into an a-helix (the 

 secondary structure), and folded into a compact and specific 

 three-dimensional structure (the tertiary structure) (Fig. 2). 



I 

 N 



0=C 

 H-C- 



H-N 

 I 



c= 



CH3-C- 



H 

 I 



CH,-Q 



=0 

 H 



[c3 



'' lb] 



ft] 



Fig. 2. The Structure of Protein, (a) Part of a polypeptide chain showing the 

 peptide bonded backbone with side gi'oups characteristic of individual amino 

 acids (here alanine and phenylalanine), (b) Schematic representation of the 

 a-helix showing the hydrogen bonds required to maintain it. (c) The folded 

 polypeptide chain in myoglobin providing the specific three dimensional struc- 

 ture of tlie protein (as determined by the x-ray crystallographic work of Kendrew 



and collaborators) (19). 



The working hypothesis for the past few years concerning gene 

 control over protein specificity, usually called the sequence hy- 

 pothesis (6), states that the base sequence of the DNA specifies 

 the primary structure of the protein — the sequence of amino acids. 

 The original argument was based primarily on two points: first, 

 the base sequence of DNA is linear, and the only corresponding" 

 linear object in the protein is the amino acid sequence; second, 

 since proteins diff'er widely in amino acid composition, it was 

 difficult to see how such differences could arise other than by 

 genetic specificity. The argument is now much stronger. A number 



