* Differences between the milochrondiat and "universal" genetic code (Table 15-1) are shown by green shading.

t Each group of codons is shaded in gray and is read by a single tRNA whose anticodon, written 5'—*- 3', in parentheses. Each four-codon group is read by a tRNA having a U in the first (5'} position of the anticodon. Two-codon groups with codons ending in either U/C or A/G are read with GU wobble by tRNAs, with G or U, respectively, in the first position of the anticodon. The anticodons often contain modified bases.

t Note that the C in the first anticodon position engages in unusual pairing an amino acid is specified by four codons (with the same first and second positions), only a single mitochondrial tRNA is involved. (Recall that a minimum of two tRNAs would be required by nonmitochondrial systems.) Such mitochondrial tRNAs all have in the 5' (wobble) position of their anticodons a U residue, which is able to engage in pairing with any of the four nucleotides in the third codon position. In cases where purines in the third position of the codon correspond to different amino acids from pyrimidines in that position, a modified U in the first position of the anticodon of the mitochondrial tRNA restricts wobble to pairing with the two purines only.

Exceptions to the "universal" code are not limited to mitrochondria but are also found in several prokaryotic genomes and in the nuclear genomes of certain eukaiyotes. The bacterium Mycoplasma capricojum Lists UGA as a tryptophan codon rather than a chain-termination codon. Likewise, some unicellular protozoa use UAA and GAG. which are stop codons in the "universal" code, as glutamine codons. Finally, a codon (CUG) for one amino acid (leucine) in the "universal" code has become a codon for another amino acid (serine) in the yeast Candida.

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