Degeneracy, genetic code

The degeneracy of the genetic code resides mosdy in the last nucleotide of the codon triplet, suggesting that the base pairing between this last nucleotide and the corresponding nucleotide of the anticodon is not strictly... 

A striking feature of the genetic code is that an amino acid may be specified by more than one codon, so the code is described as degenerate. This does not suggest that the code is flawed although an amino acid may have two or more codons, each codon specifies only one amino acid. The degeneracy of the code is not uniform. Whereas methionine and tryptophan have single codons, for example, three amino acids (Leu, Ser, Arg) have six codons, five amino acids have four, isoleucine has three, and nine amino acids have two (Table 27-3). 

If the amino acid sequence of a peptide is known, the possible nucleotide sequences of the mRNA and the complementary DNA may be deduced from the genetic code (Chapter 12). The number of possible DNA sequences is directly related to the extent of degeneracy of the genetic code. Unique sets of DNA oligonucleotides can be chemically synthesized, labeled at the 5 end with 32P, and used as probes to isolate a clone with DNA of specific interest. 

Figure 6.20. Probes Generated from a Protein Sequence. A probe can be generated by synthesizing all possible oligonucleotides encoding a particular sequence of amino acids. Because of the degeneracy of the genetic code, 256 distinct oligonucleotides must be synthesized to ensure that the probe matching the sequence of seven amino acids is present.

The first base of an anticodon determines whether a particular tRNA molecule reads one, two, or three kinds of codons C or A (one codon), U or G (two codons), or I (three codons). Thus, part of the degeneracy of the genetic code arises from imprecision (wobble) in the pairing of the third base of the codon with the first base of the anticodon. We see here a strong reason for the frequent appearance of inosine, one of the unusual nucleosides, in anticodons. Inosine maximizes the number of codons that can be read by a particular tRNA molecule. The inosines in tRNA are formed by deamination of adenosine after synthesis of the primary transcript. 

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