Figure 7.

Quadruplex folds.A, Schematic figures of three distinct quadruplex topologies, showing different loop types. The backbones in each case are colored blue, with strand directions shown by arrows. Human telomeric quadruplexes are polymorphic in solution and can adopt these and several other topologies (59–61). B, cartoon representation of the quadruplex formed from a promoter sequence in the c-KIT gene (76). Note the large A:G base-paired loop at the top of the structure. C, cartoon representation of the antiparallel chair arrangement formed by a human telomeric quadruplex (65), with three lateral loops. D, cartoon representation of the crystal structure of the c-MYC quadruplex (78). The two independent molecules in the crystallographic asymmetric unit are shown. Both have all-parallel topology, as in the human telomeric quadruplex (66), but with distinctive loops suitable for selective ligand binding. E, cartoon representation of a left-handed quadruplex (81). Note the narrow zig-zag groove parallel to the G-quartets. PDB, Protein Data Bank.