IX. GENETICS AND HUMAN HEMOGLOBIN CHEMISTRY 435 



genetic experiments of Crick et al. (1961). These autliors have obtained 

 evidence that the code is a three-letter, or a multiple of three letters, 

 code. In agreement, the minimum number of nucleotides in coding units 

 of copolymers of three different ribonucleotides, which stimulate the 

 incorporation of some amino acids, is obviously three. 



Triplet code letters have been experimentally obtained for 19 amino 

 acids (Matthaei et al., 1962; Speyer et al., 1962). An experimental value 

 for the glutamine coding unit is still unavailable, but the composition of 

 this coding unit has been predicted from amino acid replacement data in 

 nitrous acid mutants of tobacco mosaic virus (Speyer et al., 1962). 

 Smith (1962a) has examined the relationships between the proposed 

 triplet code and the amino acid substitutions in polypeptides or proteins 

 of known structure, pointing out that all the known amino acid replace- 

 ments in the abnormal human hemoglobins are consistent with a single 



TABLE III 

 The Amino Acid Code" 



" The code is constructed according to Smith (1962b), assigning to glutamic acid 

 the base sequence UAG. The triplets composition is from Matthaei e( al. (1962) and 

 from Speyer et al. (1962). The sequence of the triplets in parenthesis cannot presently 

 be established. This is one of the six possible codes, based on the permutations of the 

 sequence for glutamic acid arbitrarily assigned. 



* Abbreviations: A, C, G, and U are used for adenylic, cytidylic, guanylic, and 

 uridylic nucleotide residues in RNA, respectively. 



