Figure 3.

G-quadruplexes at telomeres. The G-rich overhang of telomeres can form G-quadruplex structures involved in telomere end protection and telomeric DNA metabolism. (A) The long human G-rich overhang can form strings intramolecularly folded G-quadruplexes that may offer end protection against nucleases or regulate telomerase activity. (B) Ciliate telomeres form intermolecular G-quadruplex structures involving two telomeres promoted by the telomere-end binding protein TEBPβ. Telomeres are attached to a sub-nuclear structure (the nuclear matrix or scaffold) via an interaction of the telomere-end binding protein TEBPα. (C) Stabilizing of G-quadruplexes by G-quadruplex binding ligands (yellow stars) impairs telomere repeat synthesis by the telomerase enzyme and lead to telomere shortening (modified after (80)).