Mitochondrial genes
Exceptions to the Code
The genetic code is almost universal. The same codons are assigned to the same amino acids and to the same START and STOP signals in the vast majority of genes in animals, plants, and microorganisms. However, some exceptions have been found. Most of these involve assigning one or two of the three STOP codons to an amino acid instead.
Mitochondrial genes
When mitochondrial mRNA from animals or microorganisms (but not from plants) is placed in a test tube with the cytosolic protein-synthesizing machinery (amino acids, enzymes, tRNAs, ribosomes) it fails to be translated into a protein.
The reason: these mitochondria use UGA to encode tryptophan (Trp) rather than as a chain terminator. When translated by cytosolic machinery, synthesis stops where Trp should have been inserted.
In addition, most
- animal mitochondria use AUA for methionine not isoleucine and
- all vertebrate mitochondria use AGA and AGG as chain terminators.
- Yeast mitochondria assign all codons beginning with CU to threonine instead of leucine (which is still encoded by UUA and UUG as it is in cytosolic mRNA).
Plant mitochondria use the universal code, and this has permitted angiosperms to transfer mitochondrial genes to their nucleus with great ease.
Nuclear genes
Violations of the universal code are far rarer for nuclear genes.
A few unicellular eukaryotes have been found that use one or two (of their three) STOP codons for amino acids instead.
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