Author(s): Ajay Kumar

The Guanine bases form non-canonical structures based on G-quartets was found in 1962. It is now well established that G-rich nucleic acids forms higher order structures called guanine quadruplexes . G-quadruplex structures show right-handed helicity due to hydrophobic stacking of two or more G-quartets. Little attention was paid to the of guanine tetrads structure and for more than 25 years this structure remained in search of functions. In this review the bonding in G-quartet, general features of quadruplex topology, structure and polymorphic behavior of G- quadruplex is described. In the quadruplex structure four guanines establish a cyclic array of hydrogen bonds from the Watson–Crick and the Hoogsteen bonding. A G-quartet is a planar association of four guanines held together by four central H-bonds between H1 of nitrogen and O6 of carbonyl group and also by four external H-bonds between H2 of amine and N7. Four guanine bases in a G-quartet can be brought together in monomer, dimmer or tetramer with parallel or antiparallel adjacent strands and structures can link up to provide different loop connectivities. The effect of guanine base modifications and metal cations on the G-guartets stability is described. The evidence of G-quadruplexes occurrence in human cells and anticipiated biological functions are highlighted.