G quartets & quadruplexes
G quartets
Solutions of guanylic acid have a propsensity to form gels. This is because G can form planar Hoogsteen bonded rings, stabilised by a monovalent metal ion. These structures are called G quartets and can stack to form large macromolecular complexes.
G quadruplexes
In the context of a linear DNA sequence, runs of dG bases can form G quartets that can stack to form a stable secondary structure, the G quadruplex. G quadruplexes can act as significant replication impediments.