52 Inside the Living Cell 



one group of three from others? We need to know where the groups 

 of three which belong to the code start. Thus, in the example given 

 above, the signal might be given either by ABC or the adjacent BCA 

 or CCA. It would be impossible to make use of both ABC and BCA 

 as signals for separate amino acids, as they overlap. 



Crick, Orgel and Griffiths have offered a solution of this difficulty. 

 They pointed out that out of the sixty-four possible combinations of 

 A, B, C and D taken three at a time, it is possible to pick out twenty 

 which are distinctive in the sense that they will not overlap with any 

 others. They suggest that these twenty combinations are the ones 

 which Nature uses as code signals for the twenty amino acids of 

 which, with few exceptions, proteins are made up. 



The whole picture of protein synthesis which is beginning to emerge 

 is thus a very complex one.^ The genes or hereditary particles are com- 

 posed of the DNA of the cell nucleus and these carry the code for the 

 synthesis of the specific proteins of the cell. But the code is not ex- 

 pressed directly. In the first place it is transferred to the rna of the cell 

 particles. How this is done is not clearly known, as the process has 

 never been actually demonstrated. But we must suppose that the 

 two complementary strands of dna are separated and a complemen- 

 tary strand which is composed of rna is built up on similar principles 

 on one of them. This rna enters the microsomal granules where it 

 forms a 'template' for the construction of protein molecules on the 

 lines discussed above. Here the code functions by determining the 

 order of the amino acids in the way I have already mentioned. That is, 

 each amino acid is 'activated' by a specific enzyme which attaches 

 to it the 'nucleotide' groups, which are going to enable it to find its 

 place by means of the code signal. Thus, suppose that the code on 

 the RNA for a particular amino acid X is ABC. X is attached by its 

 enzyme, not to ABC itself, but to the exact complement of ABC 

 which we shall call A'B'C. This grouping can only combine with 

 its complement ABC on the rna template and thus the position of X 

 in the peptide chain is fixed by the position of ABC in the nucleic 

 acid template (Fig. 13). Similarly the amino acids Y and Z find their 

 positions by being combined with the code group which we may 

 suppose to be A'B'D' and B'B'A'. The next step is the combination of 

 X with its neighbours Y and Z. This will liberate the code groups 

 A'B'C, etc. which are free to act again in the same way. 



It should be made clear that this proposed mechanism has not been 

 completely established — many parts of it have not yet been found 

 experimentally. In fact, all we have are a series of hints. But im- 

 portant experiments carried out by Dr Zamecnik and his collaborators 

 1 Readers who do not want to go into details can omit this and turn to p. 54. 



