Inside the Cell 51 



determiner of the order of the amino acids and it was therefore sug- 

 gested that the rna provides a template to guide protein synthesis. 

 The great difficulty was to see how it could function in such a way. 



Very recently suggestions have been made which may offer the 

 solution of this puzzle. It appears that the synthesis of protein occurs 

 in two stages. It has been shown by Lippman that in the first stage 

 the amino acids are 'activated' by combination with the high energy 

 phosphate compounds, which, as was mentioned above, are the final 

 products of the oxidation of sugars. It has also been found that, for 

 each amino acid there is in the cell a specific enzyme which can acti- 

 vate it and it alone and this enzyme enables it to be combined with 

 some of the component parts of the nucleic acid (rna) present in the 

 microsomes. 



A very interesting suggestion as to how the specificity is brought 

 about has been made by Drs Crick, Orgel and Griffiths, of Cambridge 

 University. 



It has long been thought that the order of the bases in nucleic 

 acid provided a 'code' which is stored up in the dna of the chromo- 

 somes and transmitted to give rise to the actual templates on which 

 the proteins are formed. The great difficulty was to see how such a 

 code could be constructed making use only of the four different bases 

 which are present in the nucleic acids. With these bases there can . 

 only be four different ways of filling one place of the nucleic acid 

 threads, so that a code based on single places can only give four 

 different signals and this is not enough to indicate the order in struc- 

 tures of twenty amino acids. However, if we can allow two places 

 in the nucleic acid thread for each signal, we can fill them in sixteen 

 different ways and if we allow three places in the nucleotide thread 

 for a signal, we can have sixty-four different combinations, because 

 there are sixty-four different ways in which four kinds of units can 

 be arranged in groups of three. 



The minimum signal which has enough possibilities to cope with 

 about twenty amino acids is therefore one which uses three places 

 in the nucleic acid thread as a code for the placing of a single amino 

 acid. However, if we were to adopt some such code as this, we should 

 need some way of distinguishing the three places which constitute 

 a signal from adjacent places. So far as we know in a nucleic acid 

 thread, all the places are equivalent to each other and there is no way 

 in which one group of three are distinguished from another group of 

 three. For example, if the four bases are represented by A, B, C and D, 

 we might have the series : 



ABCCADBCACDBAC 

 If groups of three are to constitute a signal, how are we to distinguish 



