The genetic code is universal. The genetic code is the set of biochemical rules governing how genetic information encoded in nucleic acids is converted into polypeptides (proteins). Basically, in a translational process that takes place on a cell’s ribosomes, successive triplets of nucleotides in a nucleic acid specify precisely which amino acids are to be added to a growing polypeptide chain. For example, the triplet AAA in an RNA sequence normally specifies the incorporation of a lysine, whereas the triplet CGC specifies arginine. With four different types of nucleotides (A, T, C, and G), a total of (4)3=64 different nucleotide triplets exist. The genetic code typically is presented as a table showing which amino acid is specified by each such codon. The genetic code was painstakingly deciphered in the early 1960s (an accomplishment for which Har Khorana, Robert Holley, and Marshall Nirenberg later shared a Nobel Prize), and for the next two decades it was assumed to apply universally to all forms of life on Earth.
genetic code; mitochondrial DNA
The genetic code is universal. The genetic code is the set of biochemical rules governing how genetic information encoded in nucleic acids is converted into polypeptides (proteins). Basically, in a translational process that takes place on a cell’s ribosomes, successive triplets of nucleotides in a nucleic acid specify precisely which amino acids are to be added to a growing polypeptide chain (see Chapter 42). For example, the triplet AAA in an RNA sequence normally specifies the incorporation of a lysine, whereas the triplet CGC specifies arginine. With four different types of nucleotides (A, T, C, and G), a total of (4)3=64 different nucleotide triplets exist. The genetic code typically is presented as a table showing which amino acid is specified by each such codon. The genetic code was painstakingly deciphered in the early 1960s (an accomplishment for which Har Khorana, Robert Holley, and Marshall Nirenberg later shared a Nobel Prize), and for the next two decades it was assumed to apply universally to all forms of life on Earth.
In 1979, researchers discovered exceptions that broke the standard coding rules. Specifically, mammalian mitochondrial (mt) DNA was shown to depart from the “universal” genetic code in several molecular details. Subsequently, additional variations in the genetic code were discovered among mitochondrial genomes of various animal taxa, fungi, and plants, and in the genomes of protozoans and bacteria.
Although this paradigm shift was both unambiguous and unanticipated, it has had limited impact on the field of evolutionary genetics, apart from stimulating interesting conversations on how changes in the genetic code might have transpired during the evolutionary process (see, for example, Osawa et al., 1992). Because alterations in the genetic code are individually rare events in evolution, in some cases they have also proved to be useful phylogenetic markers of particular organismal clades.
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