In molecular biology,
G-quadruplexes (also known as
G4-DNA) are structures formed in nucleic acids by sequences that are rich in
guanine. Four guanine bases can associate through
Hoogsteen hydrogen bonding to form a square planar structure called a guanine tetrad, and two or more guanine tetrads can stack on top of each other to form a G-quadruplex. The quadruplex structure is further stabilized by the presence of a
cation, especially
potassium, which sits in a central channel between each pair of tetrads. They can be formed of
DNA,
RNA,
LNA, and
PNA, and may be
intramolecular,
bimolecular, or tetramolecular. Depending on the direction of the strands or parts of a strand that form the tetrads, structures may be described as
parallel or
antiparallel.