The genetic code

There are 20 amino acids but only 4 bases in RNA, so a code is needed to turn a base sequence into an amino acid sequence. One base per amino acid gives only 4 combinations; two bases give 16, still too few. George Gamow argued for three bases, giving 4 × 4 × 4 = 64 combinations (codons), enough for 20.

Nirenberg and Matthaei (1961) cracked the first word. They added an artificial RNA made only of uracil, poly(U), to a cell-free system that can make proteins, and got a chain made only of phenylalanine: so UUU = phenylalanine. In the same way, poly(C) gave proline (CCC) and poly(A) gave lysine (AAA). Nirenberg, Matthaei, Leder, Ochoa and Khorana decoded all 64 codons.

61 codons code for the 20 amino acids. Three, UAA, UAG and UGA, are stop codons: they code for no amino acid and end the chain. A memory trick: "U Go Away, U Are Away, U Are Gone". AUG codes for methionine and also acts as the start signal.

Features of the code:

• Unambiguous: each codon means only one amino acid.
• Degenerate: most amino acids have several codons; only methionine and tryptophan have just one.
• Non-overlapping: each base belongs to one codon only; the message is read three bases at a time without overlap.
• Nearly universal: GUG means valine in bacteria and in humans alike, with a few exceptions in mitochondria and some protozoa.