The genetic code is degenerate. That is, many amino acids are encoded by more than one codon. For example, CCU, CCA, CCC, and CCG all encode proline. In some of these cases different tRNAs recognize the different codons, but certain tRNAs recognize several different codons. Crick suggested that the base at the 5' end of the anticodon does not have as strict base-pairing requirements as the other two base pases, allowing it to form hydrogen bonds with several bases at the 3' end of the codon [Crick, 1966]. This is especially true when the base at the 5' end of the anticodon is inosine (abbreviated I), it is particularly "wobbly". (Recall that tRNAs contain a number of unusual bases that are not normally found in DNA or other RNAs.) Unlike the standard base pairing rules, the base pairing rules for this "wobble position are shown below:
There are 86 tRNA genes on the E. coli chromosome, but only 47 different tRNAs are required to recognize all of the possible sense codons. For example, based upon the wobble base pairing rules and the codon table, you can predict the minimal number of tRNAs required as shown in the following table.
For each amino acid, the codons are shown to the left (written 5' to 3') and the cognate anticodons are shown on the right. Note that the first two bases of the codon and anticodon interact by standard Watson-Crick base pairing rules, but the third base of the anticodon can pair by the wobble rules. Based upon these rules a minimum of 32 tRNAs are needed to recognize all of the sense codons in mRNA. The amino acids are attached to the cognate tRNA via a specific aminoacyl-tRNA synthetase or via a tRNA-dependent amino acid modification [Woese et al., 2000].
tRNA genes. There are 86 tRNA genes on the E. coli chromosome [Blattner et al., 1997]. Thus, many tRNA genes are redundant. The tRNA genes that are redundant correlate well with the tRNAs that are most abundant in the cell (tRNAs which recognize the most frequently used codons) and the single copy tRNAs encode less abundant tRNAs (i.e. tRNAs which recognize codons that are rarely used).
The relative amount that different codons are used differs in different organisms. A table of codon usage in Escherichia coli and Salmonella enterica is shown below.