68 



George Gamow and Martynas Ycas 



neighbors really exists in the known protein sequences, it is necessary to test 

 all possible assignments between the twenty amino acids and the twenty possible 

 base triplets. The number of all possible assignments of that type is 20! = 

 3.10^^, Since 3.10^^ represents the age of our universe (5 bilhon years) expressed 

 in seconds, the straightforward test of that kind would require quite a consider- 

 able time even if we could test one assignment each second ! However, as it 

 often happens in cryptographic problems, one can sometimes find parts of the 

 message which reduce quite considerably the amount of necessary work. Thus 

 the code messages sent by German spies during the war were likely to contain 

 the combinations of letters corresponding to various possible ports of embark- 

 ation of American expeditionary troops. The same happens in protein sequence. 

 For example, the adrenocorticotropin molecule contains the sequence: 



— Lys — Lys — Arg — Arg— Pro — Val — Lys — Val — 



In this sequence there are two identical amino acids in succession followed 

 by another pair of identical ones. In the English language there are not many 

 words having such a property. (Tennessee is one of the rare examples!) Then 

 lys repeats again three steps later, and has identical neighbors (val) on both 

 sides. These facts simplify the problem to such an extent that, instead of 

 spending five billion years, it was possible to find a single assignment between 

 the amino acids in the above sequence, and the base triplets in the course of an 

 afternoon. At first it was thought the problem had been solved, but, when one 

 tried to extend these assignments to the other parts of the ACTH molecule 

 and to the other known protein sequences, one was led to direct contra- 

 dictions. In the course of subsequent decoding work, other examples leading 

 to similar contradictions were found, and it became clear that the thing just 

 will not work. In fact, as Dr Yeas discusses in the following article, it seems 

 that there is no correlation between the neighboring amino acids whatsoever. 

 This negative result can only mean that the original hypothesis represented 

 in Fig. 2 was incorrect, and that in the process of protein synthesis the nucleic 

 acid molecule is not present in its extended form. If, as seems to be true, we 

 deal here with a "non-overlapping code" in which each amino acid is determined 

 by an individual base triplet of its own (Fig. 3), we are forced to assume that 



RNA-Template 



Fig. 3. 



the RNA molecule is shrunk by a factor of three. We can imagine, for example, 

 that during the process of protein synthesis the RNA molecule has the shape 

 of a spiral as shown in Fig. 4. 



Closely connected with the problem of a non-overlapping code is the problem 



