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 nucleotidesto "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.
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