Genetics as Science

If arranged in groups of three, or triplets, in aU possible combinations or permutations, these letter designators furnish the code or blueprint to manufacture any of the 20 common amino acids that combine to form peptides or proteins. Each such triplet is called a codon, with a given letter variously appearing thrice, twice, or only once in each triplet. 

More than one codon or triplet signifying genetic instructions may exist for synthesizing each amino acid. This information is provided in terms of a tabulation of the standard genetic code, which is given in most biochemical textbooks. That is, it turns out that the number of codons may be one, four, or six, any one of which may serve to encode the instructions for producing a particular amino acid. 

whereas the formation of some amino acids will be instructed by only one codon or triplet, for example, for tryptophan or methionine, the formation of other amino acids may be instructed by any one of as many as six codons, examples being the amino acids arginine, leucine, and serine. Thus, for arginine, genetic instructions are encoded as any one of the six combinations CGU, CGC, CGA, CGG, AGA, and AGG. Any one of the combinations encodes the genetic instructions for the cell to form arginine. 

The convention is that one of the codons or triplets is used as a start signal for protein synthesis, and three for a stop signal. The start signal is AUG, which also serves as the codon for the genetic instructions to produce methionine. The stop signals are UAA, UAG, and UGA, which are the instructions for no known amino acids, and are therefore designated as nonsense codons. 

The genetic code can be applied universally, except for unique uses in the mitochondria, the cellular organelles or compartments associated with respiration. Accordingly, a better descriptor is that use of the genetic code is widespread rather than universal. 

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