Formation of a hairpin structure by telomere 3′ overhang

The telomeres of most eukaryotes contain tandemly-repeated DNA sequences, with a cluster of G residues on one strand. Recent studies showed that the Oxytricha telomeric DNA oligonucleotide, d(G₄T₄G₄), dimerizes to form a quadruplex in the presence of Na⁺ or K⁺. We have observed that the oligonucleotide d(G₄T₄G₄) does not dimerize in the presence of Li⁺ ion at low sample concentrations. In the monomeric state, this molecule forms a simple foldback hairpin structure containing G · G reverse Hoogsteen basepairs in the stem region. This hairpin structure has a thermal stability which is well reconciled with telomere functions in vivo.