Genetic code synonyms

As the genetic code provides 4 = 64 codons for the 20 amino acids, there are several synonymous codons for most amino acids— the code is degenerate. Three triplets do not code for amino acids, but instead signal the end of translation (stop codons). Another special codon, the start codon, marks the start of translation. The code shown here is almost universally applicable only the mitochondria (see p. 210) and a few microorganisms deviate from it slightly. 

Most amino acids in proteins are specified by more than one codon (i.e. the genetic code is degenerate). Codons that specify the same amino acid (synonyms) often differ only in the third base, the wobble position, where base-pairing with the anticodon may be less stringent than for the first two positions of the codon. 

Note that synonyms are not distributed haphazardly throughout the genetic code (depicted in Table 5,4). An amino acid specified by two or more synonyms occupies a single box (unless it is specified by more than four synonyms). The amino acids in a box are specified by codons that have the same first two bases but differ in the third base, as exemplified by GUU, GUC, GUA, and GUG. Thus, most synonyms differ only in the last base of the triplet. Inspection of the code shows that XYC and XYU always encode the same amino acid, whereas XYG and XYA usually encode the same amino acid. The structural basis for these equivalences of codons will become evident when we consider the nature of the anticodons of tRNA molecules (Section 29.3.9). 

The G + C content of bacterial DNA ranges from less than 30% to more than 70%. DNA molecules with quite different G + C contents could encode the same proteins if different synonyms of the genetic code were consistently used. 

Natural variation in the genes that encode adrenergic receptors (ARs) have been identified. The variations of major interest for common diseases are those that occur with allele frequencies >1% and are termed polymorphisms. Within the coding region, polymorphic variation can result in either a change in the encoded amino acid (nonsynonymous) or, because of the redundancy of the genetic code, have no effect on the encoded residue (synonymous). The most common variants are single nucleotide polymorphisms (SNPs), but insertions and deletions are also found. AR polymorphisms have been considered as poten-... 

Figure 14.5. The standard genetic code minimizes the effects of point mutation better than the vast majority of alternative plausible genetic codes that could assign 20 amino acids to 64 codons in synonymous codon blocks (adapted from Freeland and Hurst 1998 [94]).

Explain what the genetic code is and list its major characteristics. Define the terms triplet code (codon), nonoverlapping, degenerate, synonym, triplet, and reading frame as they apply to the genetic code. Recognize the initiation and termination codons. 

---