Genetic Code

The sequence of nucleotides in DNA determines the sequence of amino acids found in all proteins. Since there are only four nucleotide "letters" in the DNA alphabet (A, C, G, T, which stand for adenine, cytosine, guanine, and thymine), but there are 20 different amino acids in the protein alphabet, it is clear that more than one nucleotide must be used to specify an amino acid. Even two nucleotides read at a time would not give sufficient combinations (4 × 4 = 16) to encode all 20 amino acids plus start and stop signals. Therefore it would require a minimum of three DNA nucleotides Figure 1. A messenger RNA is translated in triplets, beginning with the first AUG encountered by the ribosome. Translation stops at a stop codon, one of which is UAA. to "spell out" one amino acid, and indeed this is the number that is actually used. RNA also uses a four letter alphabet when it reads and transcribes DNA instructions during protein synthesis, but its set of nucleotides is somewhat different, substituting U (uracil) for T (thymine).

Any single set of three nucleotides is called a codon, and the set of all possible three-nucleotide combinations is called "the genetic code" or "triplet code." There are sixty-four different combinations or codons (4 × 4 × 4 = 64). We now know that three codons (UAA, UAG, and UGA) specify a "stop" signal, indicating the termination of the polypeptide chain being synthesized on the ribosome. Each of the remaining sixty-one codons encodes an amino acid. The "start" signal is the codon AUG, which also encodes the amino acid methionine. The codons are read from the messenger RNA molecule during protein synthesis, and, consequently, they are given in RNA bases rather than in the original DNA sequence. The reading of the codons is shown in Figure 1.