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Figure 2.2. Codon dictionary (F.H.C. Crick, Cold Spring Harbor Symp. Quant. Biol. 31, 1-9, 1966).

Figure 2.2. Codon dictionary (F.H.C. Crick, Cold Spring Harbor Symp. Quant. Biol. 31, 1-9, 1966).

each. The remaining amino acids, with the exception of isoleucine, are coded either by two or by four codons; only isoleucine is coded by three codons.

It should be emphasized that the triplets coding for a given amino acid differ in most cases only in the third base. Only when the amino acid is coded by more than four codons do differences occur in the first and second positions of the triplet as well. A group of four codons differing only in the third nucleotide and coding for one and the same amino acid is often called the codon family. The code dictionary contains eight such codon families: for leucine, valine, serine, proline, threonine, alanine, arginine, and glycine.

The code presented in Fig. 2.2 is universal for the protein-synthesizing systems of most bacteria and for the cytoplasmic extraorganellar protein-synthesizing systems of multi-cellular Eukaryotes, i.e. animals, fungi, and plants.

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