Genetic Code

Once DNA (and RNA) had been established as the genetic material, the next question was how hereditary information is actually written into the sequence of a nucleic acid molecule, and how that written information ultimately specifies the particular amino acid sequence of a protein. The answer is the genetic code: a fixed set of rules relating a sequence of three consecutive mRNA bases, called a codon, to one specific amino acid among the twenty used in protein synthesis, following what is known as the triplet rule. Because there are four possible bases at each of the three codon positions, there are exactly 4 x 4 x 4 = 64 possible codon combinations; of these, 61 codons each specify one particular amino acid, while the remaining three (UAA, UAG and UGA) do not code for any amino acid at all and instead function as termination (stop) signals that mark the end of a protein-coding sequence. Because only 20 different amino acids need to be specified by 61 available codons, most amino acids are in fact represented by more than one codon, a property called degeneracy, made possible by two related mechanisms: first, most amino acids are served by more than one distinct tRNA molecule, each carrying its own matching anticodon; second, accurate codon-anticodon pairing is strictly enforced only at the first two positions of the codon, while considerable flexibility is tolerated at the third position, so that several codons differing only in their third base can still specify the same amino acid. Because scientists such as Marshall Nirenberg, Severo Ochoa (using the enzyme polynucleotide phosphorylase, sometimes called Ochoa's enzyme), Har Gobind Khorana and Francis Crick worked out which codon specifies which amino acid, biologists were eventually able to assemble the complete codon-to-amino-acid 'checkerboard' shown in the table above. The genetic code follows several well-established rules. It is a triplet code, in which three consecutive, non-overlapping bases together specify one amino acid; a sequence such as GUUGUC, for instance, is read strictly as the two separate, non-overlapping codons GUU and GUC, never as any overlapping combination. The code is comma-less, meaning there is no punctuation of any kind separating one codon from the next, so the whole message must be read as one continuous, unbroken string from a fixed starting point. It is largely universal: with only a small number of documented exceptions, chiefly in some prokaryotic, mitochondrial and chloroplast genomes, every known living organism uses the identical set of 64 codons to mean the identical set of amino acids, so that, for example, the codon UUU specifies phenylalanine in essentially every cell of every organism on Earth. The code is described as degenerate, since more than one codon can specify the very same amino acid (all four codons GUU, GUC, GUA and GUG, for example, code for valine), while at the …