Severo Ochoa (1905-1993) received half the 1959 Nobel Prize for Physiology or Medicine for his discovery of an enzyme in bacteria that he used to synthesize RNA. In 1955, he found, completely by chance, an enzyme that converts adenosine diphosphate into RNA-like polymers. Initially he hoped this enzyme, named polynucleotide phosphorylase, was responsible for the biosynthesis of RNA. But he realized that polynucleotide phosphorylase did not use a DNA template – which would have been needed for RNA synthesis – so he dispelled this idea. This initial disappointment was soon forgotten when Ochoa realized that, instead, polynucleotide phosphorylase synthesizes a variety of RNA-like polymers, and he identified nucleotide triplets that encode for amino acids. In addition to his work on RNA, Ochoa made significant contributions to the elucidation of the Krebs cycle, a series of enzymatic reactions that are the source of cellular energy.