G-DNA: A Twice-Folded DNA Structure Adopted by Single-Stranded Oligo(dG) and Its Implications for Telomeres

Our dimethyl sulfate modification experiments suggest that (dG)nstretches within single-stranded DNA fragments, which represent the simplest model for telomeric sequences, adopt a complex intrastrand structure other than a simple hairpin. We present a molecular model for the DNA structure that conforms to dimethyl sulfate methylation data. The principal element of this G-DNA structure is a quadruple helix formed by pairwise antiparallel segments of the twice-folded (dG)nstretch. This quadruple core has two wide and two narrow grooves connected by three loop-shaped segments. The strong stacking interactions of the neighboring guanine tetrads and the large number of hydrogen bonds formed can be the primary reasons that such structures are favored over a common hairpin for long (dG)nstretches. Such compact structures may be formed from (dG)nstretches of telomeric sequences.