Degeneracy of the 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... 

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. 

The genetic code consists of 64 different codons that specify all 20 amino acids as well as codons that function to initiate and terminate translation. More than one codon may specify the same amino acid, which is called degeneracy of the genetic code. Finally, every organism from bacteria to human uses the same codons to specify the same amino acids this is why the genetic code is said to be universal. 

Most of the mutations shown result in a missense effect, with a different amino acid being incorporated into the same site in a protein. This may or may not have an effect depending upon its location. Some single-base mutations are harmless because of the degeneracy of the genetic code, whereby more than one triplet code exists for all amino acids except tryptophan and methionine. Choice a contains two mutations, one degenerate and the other missense. 

While you can go directly and predictably from a nucleotide sequence to one and only one amino acid sequence, the reverse is not true because of the degeneracy of the genetic code. 

For a fully degenerated oligonucleotide, each triplet will code for all 20 amino acids with no bias beyond what is due to the unequal degeneracy of the genetic code. At each coupling reaction an equal mixture of all four nucleotides (N) will be used for the first and second positions of each triplet. The third position will have a mixture of dC and dG or dG and T (NNK or NNS) [5,32]. In this way, the mixture will contain only 32 triplets instead of 64, but all 20 amino... 

Mutations occur by substitution, insertion, or deletion of bases. Substitution mutations are the most common types of mutation. A substitution mutation, involving the substitution of one base by another, changes one codon in mRNA. This may or may not alter the amino acid residue specified by that codon because of the degeneracy of the genetic code. 

How does degeneracy of the genetic code (different codons that code for the same amino acid) enable bacteria to have different genomic base compositions yet to code for similar proteins ... 

With degeneracy of the genetic code it is possible to have different base sequences in a gene and yet code for the same sequence of amino acids. In coding for leucine, e.g., the RNA codons for leucine are CUU and CUA which correspond to 33.3% (G -I- C), while its other two RNA codons are CUC and CUG and these are 66.7% (G + C). If the first codon only were used to code for leucine in a gene, and all other amino acids were coded with a similar bias in G -I- C, then the gene would have a G C/A T ratio of 1 2. This extreme situation never actually arises in nature because not all amino acids have four different codons. 

Specificity of aminoacylation The faithful translation of genetic information depends on the correct matching of amino acids to their cognate tRNA that is accomplished by aRS catalysis. The degeneracy of the genetic code versus the nearly general provision of only one aRS per amino acid poses an intriguing question about the specificity of aRS cataly-... 

---