A variety of experiments throughout the 1940s and 1950s showed that DNA was the molecule of basic genetic information. However, there were no breakthroughs in understanding how DNA controlled the formulation of proteins. There were also big questions as to the role RNA played in the process. While working for the National Institute of Health, Nirenberg teamed with Heinrich J. Matthaei to discover the relationship between these processes in 1959.
After creating an RNA sample comprised solely of the nucleotide uracil, they added it to an extract of Escherichia coli. This extract contained the cellular machinery needed for protein synthesis such as DNA, RNA and ribosomes. Utilizing DNase to ensure that no extra proteins would be produced, they added a radioactive sample of phenylalanine. After analyzing the extract they had determined that they had identified the genetic code for the amino acid: UUU. Nirenberg had established that three units of genetic material (DNA or RNA and known as codons) form an amino acid. Over the next few years, Nirenberg's team repeated the steps to decipher the three-base codons for many other amino acids.
Using this research, one of Nirenberg's colleagues, Phillip Leder, passed various combinations of messenger RNA through a filter, stimulating the binding of transfer RNA to ribosomes. The transfer RNA could then be associated to various amino acids, identifying the messenger RNA sequence.
