The genetic code was deciphered by a series of ingenious experiments carried out mainly by Marshall Nirenberg, along with his colleague Heinrich Matthaei. These researchers added contents of bacterial cells to 20 test tubes. This was done to provide the “machinery” needed to synthesize proteins. They also added all 20 amino acids to the test tubes, with a different amino acid “tagged” by a radioactive element in each test tube. Then they added synthetic RNA containing just one type of base to each test tube, starting with the base uracil. They discovered that an RNA molecule consisting only of uracil bases produces a polypeptide chain of the amino acid phenylalanine. The researchers used similar experiments to determine that each codon consists of three bases and eventually to discover the codons for all 20 amino acids.