The genetic code is degenerate, with two to six codons specifying

The genetic code is degenerate, with two to six codons specifying each of the amino acids except for methionine and tryptophan. To explore this idea use the codon database and find data on the six leucine codons.

Go to the codon database complied from GenBank data: http://www.kazusa.jp/codon/ . Search Homo sapiens, Escherichia coli, Arabidopsis thaliana, and Caenorhabditis elegans in the query box. Answer each question fully.

1. Are all of the codons specifying a given amino acid used with equal frequency, or are some codons used more frequently than others? For example, the codons UUA, UUG, CUU, CUC, CUA, and CUG all specify leucine. Are these six leucine codons present with equal frequency in the coding regions of mRNAs?

2. Are the six codons specifying leucine used with equal frequency in mRNAs transcribed from human nuclear genes? From human mitochondrial genes? Are these codons used at the same frequency in nuclear and mitochondrial genes?

3. Are the leucine codons used at about the same frequencies in the four different species? Is there any bias in codon usage (preferred use of specific codons) related to the AT/GC content of the genomes of different species?