If a gene sequence is a "sentence" describing a protein, then its basic units are three-letter "words," or "codons," each of which translates into one of 20 amino acids or a "stop translating" signal. Cellular machinery transcribes DNA genes into RNA versions--whose nucleotide building blocks are represented by the letters A, C, G and U--and then translates the RNA genes, codon by codon, into a corresponding amino acid sequence. Nature's exact amino acid definitions (right) were worked out during the early 1960s. But the significance of patterns in the code would not be fully appreciated for several decades.

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This article was originally published with the title Evolution Encoded.

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