Nirenberg produced RNA composed of Uracil. Then he added the RNA into a cell free extract of Escherichia coli which contained DNA, RNA, Ribosomes, and other cellular machinery for protein synthesis. After that he added DNase, 1 radioactive amino acid, and 19 unlabeled amino acids to the extract. The result was a radioactive protein. Nirenberg was able to breack the genetic code and describe how it operates in protein synthesis.