Guanine Quartet
 
Telomeric DNA plays an important role in stabilizing chromosomes. Telomeres refer to the terminal protein-DNA complexes of eukaryotic chromosomes, and the DNA sequences found in telomeres are rich in G and contain a short highly-repetitive sequence, such as (TTGGGG)n. The G-rich strand at the 3′ end exists as a single-stranded overhang and is capable of forming a higher-ordered DNA structure, known as the guanine- or G-quartet, in which four strands associate. There are several ways in which the G-strand can form a very stable four-stranded structure. Indeed, the structures of a number of telomeric sequences have been determined, showing the various forms of the G-quartet structures. The d(GGGGTTTTGGGG) sequence from the protozoa Oxytricha forms a hairpin, and two such hairpins joins to make an antiparallel hairpin dimer, thereby resulting in a four-stranded helical structure, with the thymines forming loops at either end. The G-quartet structure was formed by the guanine residues being held together by a cyclic, head-to-tail network of hydrogen bonds and a metal ion  located at the center. The four guanine residues were found to have a glycosyl conformation that alternates between anti and syn. Another arrangement of the tetra-stranded G-quartet is a fourfold symmetric motif in which the four G-strands are in the parallel orientation (Fig. 1) (1). The polymorphism of the quadruplex structure has been reviewed elsewhere (2). More recently, DNA molecules containing a string of isoguanines have been demonstrated to form very stable parallel tetraplex structures as well. 

Figure 1. Side and end views of a parallel G-quartet DNA structure (Nucleic acid data base No. UDD023).
 
References
1. K. Phillips, Z. Dauter, A. I. H. Murchie, D. M. J. Lilley, and B. Luisi (1997) J. Mol. Biol. 273, 171-182.
2. J. R. Williamson (1993) Current Opin. Struct. Biol. 3, 357–362.