The organic chemist Phoebus Levene isolated and discovered the structure of the individual units of deoxyribonucleic (DNA) and ribonucleic acid (RNA). DNA and RNA consist of an acidic phosphate group, a nitrogenous base, and a sugar. Following Levene's notation, the base and the sugar are called a nucleoside; the addition of the phosphate completes the nucleotide. In the 1910s, Levene identified the sugar in RNA as D-ribose, the biologically active mirror image of commercially available L-ribose. Similarly, in the late 1950s, he identified the sugar in DNA as D-2-deoxyribose (D-ribose minus an oxygen at the 2-position). By 1935, Levene had determined that both sugars were furanose rings. Based on the structure of individual units, Levene proposed that DNA joined together in groups of four. Although his theory would later prove to be incorrect, Levene's tetranucleotide hypothesis long held sway as the model of DNA structure. A series of...