18 I The Process of Evolution 



131, 132, 133, 231, 232, 233, 141, 142, 143, 144, 241, 242, 243, 244, 

 341, 342, 343, 344. 



Recently several groups of workers have been able to show that 

 the RNA code consists of nonoverlapping triplets, each of which 

 determines the position of an amino acid. At this writing, codes for 

 19 of the 20 amino acids have been partially worked out. For in- 

 stance, the amino acid alanine is coded as some sequence of uracil, 

 cytosine, and guanine, and the amino acid serine as a triplet con- 

 taining two uracils and a cytosine. 



It now appears likely that the nonoverlapping nature of the code 

 is determined not by the structure of the triplets themselves but 

 rather by the existence of some device for designating starting 

 points for "reading" the code. For instance, in the sample sequence 

 above (141131312) there is no ambiguity if the left end is designated 

 the starting point and the code exists only as triplets; it becomes 

 clearly 141-131-312. If the code works in this manner, there are 

 then 4^ = 64 different possible sequences, a plethora for determining 

 only 20 items. It is quite possible that each amino acid can be coded 

 by more than one triplet combination and that certain combinations 

 indicate "capital" triplets (those starting a sequence). Work on the 

 decoding problem is now proceeding so rapidly that it seems in- 

 evitable that some of these questions will be settled before these 

 words are published. Whatever the answers, they will surely contain 

 fascinating hints as to the evolution of the code itself. 



Now, how do the amino acids "read" the messenger RNA sequence 

 code so that they condense into proteins containing the proper order 

 of amino acid residues? Amino acids become bound to relatively 

 small soluble RNA molecules (transfer RNA) before the acids are 

 linked together into proteins. Further data are very suggestive of 

 the following pattern of protein construction. There is a separate 

 transfer RNA molecule for each amino acid. In one part of the 

 molecule is a sequence of nucleotides that determines with which 

 amino acid it may react, and in another part is the sequence (a 

 triplet?) that determines the position on the RNA template to be 

 assumed by the transfer RNA unit. The compounds of RNA and 

 amino acids are formed with energy supplied by ATP. This enzyme- 

 mediated reaction is diagrammed in Fig. 1.5. The transfer RNA- 

 amino acid units then find their places on the long RNA template, 

 presumably pairing with complementary sequences on the template. 

 The amino acids are still in an activated state; when they are 

 brought into close proximity in the manner outlined above, they 

 condense (probably with the aid of an enzyme) to form a protein 

 with the proper sequence of amino acid residues. 



