BIOSYNTHESIS 



267 



4. The transmission of the code 



We have seen that the macromolecule of DNA is a double hehx in the 

 form of a spiral staircase in which the links between the different nucleic 

 acids form the banisters and the purine and pyrimidine bases form the 

 steps. As we have seen, each step is formed by one of the two combinations 

 adenine-thymine (or thymine-adenine) and guanine-cytosine (or cytosine- 

 guanine). This means that a particular sequence of bases on one of the 

 banisters is paired to one definite complementary' sequence on the other. 



Now if the chains separate in a medium containing free nucleotides of 

 deoxyribose and one of the bases (adenine, guanine, thymine or cytosine), 



formic acid 

 formaldehyde 

 methanol 

 glycine (a — C) 

 glycollic acid (a — C) 

 serine (a and /3 — C) 



threonine (a — C) 

 histidine (C — 2) 

 tryptophan (C — 2) 

 phenylalanine (^ — C and C — 3) 

 tyrosine (j8 — C and C — 6) 

 acetone 



Cx 



glycolysis 



fatty acids 

 pyruvic acid 

 amino acids 



acetic acid 

 ethanol 



r^\ 



\ 



oxaloacetate 





'■ 1 \/ 



succmyl-CoA | f 



/ 



a — ketoglutarate | / 



tricarboxylic 

 acid cycle 



sedoheptulose 



t 



hexose monophosphate shunt 



Fig. 82. The chief sources of the key-materials required for biosyntheses. 



each of the two chains of the parent double helix will become a template for 

 a complementary chain, the result being the formation of two daughter 

 double helices, carrying an exact copy of the code. How the sequence on 



