( 'hapter .'54 



GENETIC AMINO ACID CODING 



I 



f single ribotides in messenger 

 RNA were translated into dif- 

 ferent amino acids, only four 

 amino acids would be specified or coded. 

 Since there are twenty common amino acids, 

 we are presented with the problem of how 

 RNA codes for amino acids. To resolve this 

 problem, we can assume that an amino acid 

 is coded by a sequence of two nucleotides — 

 a situation comparable to having an alphabet 

 of four letters and a language of two-letter 

 words. In this case, assuming the RNA 

 code can be read only in one direction, we 

 would have four times four, or sixteen, pos- 

 sible doublets (words). (Unidirectional 

 reading seems reasonable since a single 

 strand of RNA is polarized just as a single 

 strand of DNA.) However, sixteen dou- 

 blets are still too few to specify twenty 

 amino acids, so other assumptions must be 

 made. We might hypothesize that a given 

 doublet encodes more than one kind of 

 amino acid, in which case the code would 

 be ambiguous. Alternatively, we could as- 

 sume an amino acid is coded by a sequence 

 of three messenger ribotides — a triplet. 

 Such a triplet code would give us four times 

 four times four, or sixty-four, different, uni- 

 directional sequences — more than enough to 

 encode twenty amino acids. Should more 

 than one triplet encode the same amino 

 acid, the code would be degenerate. Thus, 

 this introductory discussion suggests that a 

 sequence of two or three ribotides encodes 

 an amino acid — that is, acts as a codon. 

 436 



Other characteristics of messenger RNA 

 may affect amino acid coding. For exam- 

 ple, since the number of consecutive ribo- 

 tides can be in the hundreds or thousands, 

 no spacing — that is. no non-nucleotidc punc- 

 tuation — is provided to indicate where one 

 codon stops and the next begins. Conse- 

 quently, we are dealing with what is called 

 a comma-free code. Suppose six ribotides 

 are arranged linearly in positions 123456. 

 If triplet 123 specifies amino acid A and 

 456 specifies amino acid B, errors are pos- 

 sible due to overlapping triplets 234 or 345. 

 The problem of overlapping codons can be 

 avoided if only successive doublets or tri- 

 plets are read starting at one distinct point 

 on messenger RNA. In this case, the punc- 

 tuation is provided by the mechanism for 

 reading the code. 



The rll Region and the Code 



The genetic fine structure of the rll region 

 of </>T4 has already been discussed in Chap- 

 ter 26. We recall that the rll region is 

 composed of two genes (or cistrons), A 

 and B, both of which must function cor- 

 rectly to yield the r+ phenotype. From the 

 last chapter, it is inferred that these genes 

 produce messenger RNA which specifies the 

 two different polypeptide chains required for 

 the r + phenotype. In the case of hemo- 

 globin, the protein gene product is readily 

 collected and analyzed, but the genetic basis 

 for globin variants is difficult to study; the 

 converse is true for the r+ phenotype. In 

 other words, even though the presumed pol- 

 ypeptide chains involved in producing r + 

 have not been detected, the genetic basis 

 for rll mutants can be readily determined. 

 We would, of course, prefer to study a sys- 

 tem whose genetic and polypeptide conse- 

 quences both are easily investigated; never- 

 theless, other genetic studies ' of the rll 



1 The discussion follows the work of F. H. C. 

 Crick. L. Barnett. S. Brenner, and R. J. Watts- 

 Tobin (1961), and of others. 



