The genetic code is degenerate. Amino acids are encoded by either 1, 2, 3, 4, or 6 triplet codons (see Figure 13.7). An interesting question is whether the number of triplet codes for a given amino acid is in any way correlated with the frequency with which that amino acid appears in proteins. That is, is the genetic code optimized for its intended use? Some approximations of the frequency of appearance of nine amino acids in proteins in E. coli are given in the following:

Amino Acid       Percentage
  Met                        2
  Cys                        2
  Gln                         5
  Pro                         5
  Arg                         5
  Ile                           6
  Glu                         7
  Ala                         8
  Leu                        10

Analyze your data to determine what, if any, correlations can be drawn between the relative frequency of amino acids making up proteins and the number of codons for each. Write a paragraph that states your specific and general conclusions.

Key Concepts

Degeneracy of the Genetic Code

The genetic code is described as degenerate because most amino acids are encoded by more than one codon. This redundancy allows for some mutations in the DNA sequence to occur without altering the resulting protein, providing a buffer against potential harmful effects. Understanding this concept is crucial for analyzing how the frequency of amino acids relates to their codon usage.

Codon Usage Bias

Codon usage bias refers to the phenomenon where certain codons are preferred over others for encoding specific amino acids in a given organism. This bias can influence the efficiency and accuracy of protein synthesis, as well as the overall expression levels of genes. Analyzing codon usage in relation to amino acid frequency can reveal insights into evolutionary adaptations and metabolic needs of the organism.

Amino Acid Frequency in Proteins

The frequency of amino acids in proteins can vary significantly among different organisms and is influenced by factors such as evolutionary pressures and functional requirements. By examining the percentage of each amino acid in proteins, researchers can infer potential correlations with the number of codons available for encoding those amino acids. This relationship can provide insights into whether the genetic code is optimized for the organism's specific protein synthesis needs.