Figure 1.

(A) Scheme of Hoogsteen base-paring in G-quadruplex structures. The stacked tetrads of guanines (highlighted-purple, violet) are stabilized by a metal ion (M⁺, red) in the middle of the quadruplex; and (B) Quadruplexes can be formed within a single nucleic acid strand, from two strands (as a dimer of hairpins) or from four separate DNA or RNA strands. Green planes represent the guanine tetrads. Grey lines represent the sugar-phosphate backbone, with the arrows showing polarity of the nucleic acid chains.