By the early 1950s, scientists knew that genes were made of deoxyribonucleic acid (DNA) and that specific proteins were the products of specific genes. The exact link between DNA and proteins was less well understood, however. Since proteins are considered the language of life, researchers believed that the DNA molecule, with its four nitrogenous bases, might be the code for this language. This is how the term genetic code originated.

Protein molecules are comprised of amino acids. There are twenty biologically important amino acids. Only four different bases--adenine (A), thymine (T), cytosine (C), and guanine (G)--are found in DNA. When each of these bases combines with a sugar and a phosphate molecule, a nucleotide unit is formed. How could only four different nucleotides code for twenty different amino acids? Scientists reasoned that if a single nucleotide coded one amino acid, only four amino acids could...