NonB

Non-B DNA, or nonB, is a collective term for DNA conformations that differ from the canonical B-form helix described by Watson and Crick. These alternative structures arise in response to sequence patterns, supercoiling, and cellular conditions, and they have been observed in vitro and, in some cases, in vivo. Major non-B forms include left-handed Z-DNA formed by certain alternating purine-pyrimidine repeats such as (CG)n, right-handed A-DNA which can occur in dehydrated or RNA-rich contexts, H-DNA or triplex DNA formed at homopurine-homopyrimidine tracts, cruciform structures at palindromic repeats, G-quadruplexes built from guanine tetrads, and i-motifs formed by protonated cytosine-rich sequences. The stability of these structures depends on sequence, superhelical density, and ionic conditions, particularly the presence of potassium or other cations.

Biological significance: non-B DNA structures can influence transcription initiation and elongation, replication fork progression, and recombination. They are associated with genetic instability in regions such as fragile sites and trinucleotide repeat loci, and they can serve as binding sites for structure-specific proteins and for therapeutic agents. G-quadruplexes and other non-B motifs have been investigated as potential targets in cancer therapy.

Detection and study: researchers use chemical probing, structure-specific antibodies, circular dichroism spectroscopy, NMR and X-ray crystallography, and genome-wide mapping approaches to identify non-B motifs. The study of non-B DNA continues to reveal its roles in genome organization and regulation, as well as its potential as a drug target.

See also: B-DNA, Z-DNA, A-DNA, triplex DNA, G-quadruplex, cruciform DNA, i-motif.