200 



(XXXVI) 



The transfer of amino acid to template RNA from a bomid amino acid an- 

 hydride of a nucleotide may also be conceived to proceed with the simultaneous 

 transfer of nucleotide to the end of an unsubstituted polynucleotide chain. 

 This mechanism is suggested in Fig. 25 for the closure of breaks in polynucleo- 

 tide chains, but it may also be of primary interest in the extension of poly- 

 nucleotide chains. Many variations of such a process can be postulated, 

 although the existence of free mtermediates, other than the adenylates, is 

 not yet known. However, one such mechanism has been suggested by Simkin 

 and Work (1957), and is shown in formula (XXXVII). 



Guanine Uracil Alanina 

 + 

 Sugar Sugar O -valine 



Guanine Uracil / I / I / 



I I / P P 



Sugar -I- Sugar '■ — ( 



I I \ Guanine Uridylic acid 



P—O- alanine P-O— valine \ | 4- 



Sugar O- alanine -valine 



(XXXVII) 



Imphcit in this mechanism is the concept of a specific relationship between 

 bound amino acid and nucleotide, as well as the idea that this relationship 

 wiU permit the synthesis of specific RNA and polypeptide sequences. How- 

 ever, the number of different kinds of nucleotides (4) are evidently inadequate 

 for the number of amino acids involved. 



