cytosine /ˈsaɪtəsiːn/ noun1 •A heterocyclic base, 4-aminopyrimidin-2(1H)-one, which pairs with guanine in DNA and RNA (by means of three hydrogen bonds).Then he found them, the substances that made up the four-letter alphabet in whose language all life is written — adenine and cytosine, guanine and thymine. — 1997 formscytosines
