114 Information Storage and Neural Control 



TMV, it has been shown that in one mutant proHne is replaced 

 by leucine, and threonine is replaced by serine. Since the effect 

 of nitrous acid is to convert cystosine to uracil, this would imply 

 that there are changes of 1U-2C to 2U-IC for the proline to 

 leucine replacement in the TMV protein and changes of 1U-2C 

 to 2U-1C for the threonine to serine replacement. The replace- 

 ment of serine by phenylalanine and of glutamine by valine in 

 another mutant is also consistent with the proposed genetic code 

 (Table XIV). 



The code need not contain the triplet CCA. The CCA is the 

 terminal sequence of transfer-RNA to which the activated amino 

 acid is attached during protein synthesis. 



The recent progress in the field of genetic coding is impressive. 

 However, many important questions must still be solved. Three of 

 these are: 1) How a triplet of nucleotides can sterochemically 

 account for the coding of an amino acid; 2) how ribosomal-RNA, 

 transfer-RNA, and messenger-RNA are held together on the 

 ribosomal particles; and, 3) the mechanisms by which ribosomal- 

 RNA, transfer-RNA, and viral RNA are synthesized. 



There is every reason to be optimistic that these and other 

 important questions will soon be resolved. 



REFERENCES 



1. Belozersky, A. N.: Nucleic Acids of Microorganisms. Nucleoproteins, 



p. 199-229. Proceedings of the Eleventh Institute International de 

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 Inc. 1960 



2. Belozersky, A. N. and Spirin, A. S.: Chemistry of the Nucleic Acids 



of Microorganisms. The Nucleic Acids III, ed. by E- Chargaff and 

 J. N. Davidson, New York. Academic Press, Inc., 1960, p. 147. 



3. Bergold, G. H.: Biochemistry of Insect Viruses. The Viruses I, ed. by 



F. M. Burnet and W. M. Stanley. New York, Academic press. 

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4. Brenner, S., Jacob, F. and Meselson, M.: An unstable intermediate 



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J. D.: Further studies on deoxyribonucleic acid dependent en- 



