Genetic Control of Protein Synthesis 



61 



NH..f l,Ot°' 



[a] 



[b] 



Fig. 1. The Structure of DNA. (a) Part of a polynucleotide chain showing the 

 sugar-phosphate backbone with purine (adenine) and pyrimidine (thymine) 

 side groups, (b) Schematic representation of base pairing between the two chains. 

 The sugar-phosphate chains are represented by the parallel vertical lines and 

 the bases by horizontal lines, (c) The double-helix. Base pairs are represented 



by horizontal lines. 



the bases adenine (A) and thymine (T) and between guanine (G) 

 and cytosine (C) (Figs, lb and Ic). A and T are called comple- 

 mentary bases because of this pairing phenomenon, and, similarly, 

 G and C are complementary. This model is now supported by 

 evidence from a variety of chemical and physical experiments. 

 Since the double helix model reveals no new irregularities in 

 topography, one feels reasonably confident that the mode of 

 storage of genetic information in DNA is in the linear sequence 

 of the four bases A, G, T, C along the DNA chain. The linear 

 aspect of the information storage mechanism is supported by 

 genetic studies which indicate linearity of the fine-structure genetic 

 map (5), the order of mutations within a genetic region controlling 

 a sinsie metabolic function. 



GENES AS DETERMINANTS OF PROTEIN STRUCTURE 

 (The Coding Problem) 



We have sketched briefly the evidence that genes are DNA and 

 that the "genetic code"' consists chemically of the base sequence 



