Success came when they added to their cell-free system an RNA molecule composed only of uracil, one of the four chemical units in RNA. The protein that emerged consisted only of phenylalanine, one of the 20 kinds of amino acids in proteins. Because the genetic code was known to consist of triplets, the experiment showed that UUU is the codon for phenylalanine, U being the symbol for uracil.

Image Marshall W. Nirenberg Credit... National Library of Medicine

Dr. Nirenberg and Dr. Matthaei were such outsiders that they had not heard of messenger-RNA, made to transfer DNA’s instructions to the cell’s protein-making machinery. While biologists in the club were producing the first evidence for the existence of messenger RNA, Dr. Nirenberg and Dr. Matthaei had independently synthesized one.

By rights, their experiment should not have worked at all because natural messenger-RNAs carry at their front end a special codon that says to the ribosomes, “Start here,” a fact not known at the time. But the recipe for protein synthesis used by Dr. Nirenberg and Dr. Matthaei happened to contain twice the natural amount of magnesium, an anomaly that was later found to override the need for a start codon.

Dr. Nirenberg presented their findings at the next big conference of molecular biologists, held in 1961 in Moscow. His talk was given to an almost empty room, Horace Judson writes in “The Eighth Day of Creation,” his history of molecular biology. But one of the few participants recognized its significance and told Dr. Crick, who arranged for Dr. Nirenberg to give his talk again, this time in a large hall attended by an audience of hundreds.

Then followed the race to identify all the other codons, a prize that Dr. Nirenberg’s talk had placed in full view of a hall of better financed rivals like Severo Ochoa of New York University.

“It was a David-and-Goliath situation in which a young investigator without resources came into competition with a distinguished Nobel laureate like Ochoa,” said Philip Leder of Harvard, who joined Dr. Nirenberg’s laboratory after Dr. Matthaei had left.

Credit for the genetic code is often assigned to Dr. Crick and Dr. Brenner, who resolved its general nature through theorizing and with a clever experiment. But it was Dr. Nirenberg and Dr. Matthaei who cracked the code itself.