Non-B DNA Secondary Structures and Their Resolution by RecQ Helicases
Figure 2
Schematic representation of certain non-B-form DNA structures. DNA can assume various alternate conformations depending upon the sequences. (a) Cruciform structures are formed at inverted repeats and also as intermediates of homologous recombination pathway, (b) Intermolecular triplex are formed by a triplex forming oligonucleotide (TFO, shown in blue) which binds to the purine-rich strand of the target duplex through the major groove. (c) Intramolecular triplex DNA structures can form at homopurinehomopyrimidine sequences with mirror symmetry, where a single-stranded region can bind in the major groove of the underlying DNA duplex to form a three-stranded helix. (d) G4 DNA is formed via parallel arrangement of four G-rich DNA strands; green boxes represent four guanine bases in planar arrangement via Hoogsteen base pairing. (e) Intermolecular G4 conformation is formed by DNA sequences with G-rich repeats forming hairpins that dimerize to stabilize bimolecular structure. (f) Intramolecular G4 DNA (or fold-over G quadruplex) is formed by single DNA strand with either four G-rich repeats or longer G tract that can fold upon themselves to form the G4 structure.
