Pentose ring structures in nucleic acids. (a) The linear and closed-ring forms of ribose are in equilibrium in solution. When incorporated into nucleosides, nucleotides, or polynucleotides, the pentose exists only in the ring form. The pentose ring is formed by reaction of the hydroxyl group on C-4 with the aldehyde at C-1. (b) The pentose rings in nucleosides and nucleotides can exist in four predominant puckered conformations. In each case, four of the five ring atoms are nearly coplanar, but the fifth ring atom, either C-2′ or C-3′, is out of the plane. The C-2′ endo configuration, in which the C-2′ atom points in the same direction as the C-5′ atom, predominates in DNA. The C-3′ endo configuration, in which the C-3′ atom points in the same direction as the C-5′ atom, predominates in RNA.