REPLICATION AND CODE-SCRIPTS 



155 



(a) A group of three bases (or, less likely, a multiple ol three l>ases) along 

 the DNA helix codes one amino acid. 



(b) The sequence of bases is read from a fixed starting point along the 

 helix. This determines what groups of three in sequence code an 

 amino acid. 



(c) The triplets do not overlap each other. 



(d) Probably more than one triplet of bases will be found to code a single 

 amino acid (that is, the code is "degenerate"!. 



Lastly, Ochoa et al., in March 1962, disclosed a three-base coding for each of 

 the 20 amino acids, a code based on the increased rate of amino-acid uptake 

 by E. coli protein to which had been added the polymerized bases of known 

 composition. Other laboratories have been publishing partial codes also. 

 Although they may be revised even before this book is printed. Table 6-4 

 lists tentative codings published by four different laboratories. Underlined 

 are the codes in which the authors have expressed greatest confidence. 



TABLE 6-4. Triplet or Three-Base Codes for Each of the 20 Amino Acids of Proteins 



I ' lll'.ic ll 



*l tnderlined < odes are 



C i \ tosine 

 those thought l>\ the respe< live 



\ adenine 

 authors to be vei 



( , guanine 

 ible I ' 



