Three- and four-stranded DNA structures. (a) Base pairing in triplex DNA. Atoms participating in Hoogsteen pairing are in red; canonical Watson-Crick base pairs are in black. (b) Side view of a triple-helical DNA containing two poly(T) strands and one poly(A) strand. The dark blue and light blue strands in the foreground are antiparallel and engage in canonical Watson-Crick base pairing. The poly(T) strand in the background is parallel to the poly (A) strand and is paired through Hoogsteen hydrogen bonding. (c) One layer of a guanosine tetraplex (quadruplex) structure, showing hydrogen bonding of the bases. A K⁺ ion in the center of the tetraplex stabilizes the structure by coordinating the bases’ functional groups.