The modern science of genetics began in the mid-nineteenth century with the work of Gregor Mendel, but the nature of the gene itself was not understood until James Watson and Francis Crick announced their findings in 1953. According to the Watson and Crick model, genetic information is stored in molecules of DNA (deoxyribose nucleic acid) by means of certain patterns of nitrogen base that occur in such molecules. Each set of three such nitrogen bases were codes, they said, for some particular amino acid, and a long series of nitrogen bases were codes for a long series of amino acids or a protein.

Deciphering the genetic code and discovering how it is used in cells has taken many years of work since that of Watson and Crick. The basic features of that process, however, are now well understood. The first step involves the construction of...