cracking the genetic code

By 1961 biologists were sure the genetic code existed and was read in triplets, but they had no idea which codon meant which amino acid. The dictionary's pages were blank. Cracking the code meant filling in all sixty-four entries — and it took one of the great detective stories of twentieth-century science to do it.

The breakthrough came from Marshall Nirenberg and Heinrich Matthaei in 1961. They made an artificial RNA consisting only of repeating U's — poly-U — and dropped it into a cell-free brew containing ribosomes and amino acids. Out came a protein made of pure phenylalanine. The conclusion was immediate: the codon UUU spells phenylalanine. That single experiment, the first word of the dictionary, cracked the code wide open. Severo Ochoa's lab made other simple RNAs (poly-A, poly-C) to read off more codons. Then Har Gobind Khorana learned to synthesize RNAs with known repeating patterns (like UCUCUC...) to pin down trickier codons, and Nirenberg and Philip Leder devised an elegant trick: feeding ribosomes tiny three-base RNAs and seeing which amino-acid-charged tRNA stuck to each one. By 1966 all sixty-four codons were assigned.

This was a turning point. It transformed the genetic code from a logical idea into a concrete, complete table that anyone could look up — the table still printed in every biology textbook. Nirenberg, Khorana and Robert Holley (who first sequenced a tRNA) shared the 1968 Nobel Prize. The cracking of the code is also a model of how biology can be done: not by staring at living cells, but by rebuilding the machinery in a test tube and feeding it designed inputs to see what comes out.