Discovery of the Genetic Code

American biochemists Marshall Nirenberg and Heinrich Matthaei deciphered the first codon of the genetic code in 1961, showing in a cell-free system that an artificial RNA strand made up entirely of the base uracil directed the production of a protein containing only the amino acid phenylalanine, proof that a sequence of three RNA bases, a codon, specifies a single amino acid. Working separately in Severo Ochoa's laboratory, other researchers soon showed that a run of adenine bases coded for lysine and a run of cytosine bases for proline, and Indian-American chemist Har Gobind Khorana went on to synthesize defined RNA sequences that identified many of the genetic code's remaining codons, while American biochemist Robert Holley determined the structure of transfer RNA, the molecule that reads a codon and delivers the matching amino acid during protein synthesis. Nirenberg, working afterward with Philip Leder, eventually helped establish the meaning of 54 of the genetic code's 64 possible codons, and Khorana, Holley and Nirenberg shared the 1968 Nobel Prize in Physiology or Medicine for cracking the code by which genetic information directs the making of proteins.