1894, from German cytosin (1894), from cyto- "cell" + -ose + chemical suffix -ine (2). "The name cytosine (due to Kossel and Neumann) is misleading. Cytosine is not, like adenosine and guanosine, a nucleoside but the sugar-free base." [Flood]
cytosine cy·to·sine (sī'tə-sēn')n. Abbr. C A pyrimidine base that is an essential constituent of RNA and DNA.