Figure 1.

Telomeric guanine-quadruplex molecules. (a) Schematic drawing of a parallel guanine-quadruplex from Stylonychia telomeric DNA d(G₄T₄G₄). (b) The quadruplex is stabilized by guanine quartet layers of four cyclically hydrogen-bonded guanine residues (nitrogen atoms are shaded). The Stylonychia telomeric DNA can also adopt fold-back structures as deduced in the presence of Na⁺ (c) and in the presence of K⁺ (d). The thymine residues are arranged in four-nucleotide loops spanning either the diagonal (c) or the edges (d) of the guanine-quadruplex stem (reviewed in ref. 4). Arrows indicate strand polarity.