The Code Is Highly Degenerate

A triplet code, one made from four nucleotides taken three at a time, generates a total of 64 different triplet sequences or codons. Three of these codons, as we will see, are utilized to terminate translation and are not generally used to specify amino acids. The remaining 61 codons and the 20 amino 

As we have indicated, the codon AUG is the only one generally used to specify methionine, but it serves a dual function in that it is also used to initiate translation. Occasionally, GUG and UUG are also read as an initiating codon in bacteria, but in internal positions these codons are always read as valine and leucine, respectively. In eukaryotes, initiation at codons other than AUG is much less frequent than in prokaryotes. Weak initiation occasionally occurs at GUG, CUG, and ACG codons in eukaryotic systems. The UGA triplet also serves a dual function it is usually recognized as a stop, but on occasion it serves as a codon for selenocys-teine (box 29A). 

Source Robert F. Weaver, and Philip W. Hedrick, Basic Genetics. Copyright 1991 Wm. C. Brown Communications, Inc., Dubuque, Iowa. All Rights Reserved. Reprinted by permission. 

Wobble Introduces Ambiguity into Codon-Anticodon Interactions 

For 61 triplets to act as codons, tRNAs must interact specifically with each triplet. Strict Watson-Crick base pairing between codon and anticodon would require 61 different anticodons and, correspondingly, 61 different tRNAs. As the 

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The Code Was Deciphered with the Help of Synthetic Messengers The Code Is Highly Degenerate Wobble Introduces Ambiguity into Codon-Anticodon Interactions... 

All 64 codons have been deciphered (Table 5.4). Because the code is highly degenerate, only tryptophan and methionine are encoded by just one triplet each. The other 18 amino acids are each encoded by two or more. Indeed, leucine, arginine, and serine are specified by six codons each. The number of codons for a particular amino acid correlates with its frequency of occurrence in proteins. 

The genetic code is highly degenerate, with most amino acids represented by more than one codon. In many cases the 3 base in the codon may be altered without changing the amino acid that is encoded. 

As described above, DNA determines the structure of mRNA. The mRNA has 64 possible triplet codons. However, there are only 20 amino acids. The code is actually highly degenerate most amino acid residues are designated by more than one triplet. Only Trp and Met are designated by single codons. Table 14.7 (95) provides the genetic code of protein biosynthesis. Note that three combinations indicate a termination of the translation and the carboxyl end of the protein chain. 

Hydrophobic cluster analysis (HCA) is a very efficient method to analyse and compare protein sequences. Schematically, HCA uses a highly degenerated code for the sequences, where only two main states are initially considered hydrophobic andhdrophilic. 

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