Indeed, "for their discovery of the mechanisms in the biological synthesis of ribonucleic acid and deoxyribonucleic acid," Marshall W. Nirenberg, Robert W. Holley, and Har Gobind Khorana were awarded the 1968 Nobel Prize in Physiology or Medicine. Working independently, Dr. Holley (Cornell University) had discovered the exact chemical structure of transfer-RNA, and Dr. Khorana (University of Wisconsin in Madison) had mastered the synthesis of nucleic acids. Dr. Nirenberg showed - excluding nonsense codons - every combination of a triplet (i.e. a codon) composed of four different nitrogen-containing bases found in DNA and in RNA produces a specific amino acid.