For many years the genetic code was assumed to be universal, meaning that each codon specifies the same amino acid in all organisms. We now know that the genetic code is almost, but not completely, universal; a few exceptions have been found. Most of these exceptions are termination codons, but there are a few cases in which one sense codon substitutes for another. The majority of exceptions are found in mitochondrial genes; a few nonuniversal codons have also been detected in nuclear genes of protozoans (Table 15.3).
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