Читать книгу Cell Biology - Stephen R. Bolsover - Страница 77

To introduce the terms degenerate and ambiguous, consider the English language. English shows considerable degeneracy, meaning that the same concept can be indicated using a number of different words – think, for example, of lockup, cell, pen, pound, brig, and dungeon. English also shows ambiguity, so that it is only by context that one can tell whether cell means a lockup or a living aqueous droplet enclosed by a membrane. Like the English language the genetic code shows degeneracy but, unlike language, the genetic code is unambiguous.

Figure 3.9. The genetic code. Amino acid side chains are shown in alphabetical order together with the three‐ and one‐letter amino acid abbreviations. Hydrophilic side chains are shown in green, hydrophobic side chains are black, and the minimal side chain of glycine is shown in gray. The significance of this distinction is discussed in Chapter 7. To the right of each amino acid we show the corresponding mRNA codons.

The 64 codons of the genetic code are shown in Figure 3.9 together with the side chains of the amino acids for which each codes. Amino acids with hydrophilic side chains are shown in green while those with hydrophobic side chains are in black. Glycine, which has a hydrogen for a side chain, is shown in gray. The importance of these distinctions will be discussed in Chapter 7. Methionine is encoded by a single codon: AUG. Tryptophan is also encoded by a single codon, but the other 18 amino acids are encoded by more than one codon and so the code is degenerate. Although there are 64 possible codons, there are only 20 amino acids. Sixty‐one codons specify an amino acid and the remaining three act as stop signals for protein synthesis (Figure 3.9). No triplet codes for more than one amino acid and so the code is unambiguous. Notice that when two or more codons specify the same amino acid, they usually only differ in the third base of the triplet. Thus single base substitutions in the third base can often leave the amino acid sequence unaltered. Perhaps degeneracy evolved in the triplet system to avoid a situation in which 20 codons each meant one amino acid and 44 specified none. If this were the case, then most mutations would stop protein synthesis dead.