BDNA

B-DNA is the canonical form of DNA under physiological conditions and the most common structure observed in living cells. It is a right-handed, antiparallel double helix in which two polynucleotide strands wind around a common axis. The helix comprises about 10 base pairs per turn, with a rise of roughly 3.4 Å per base pair and a diameter near 20 Å. Bases are stacked inside the helix, while the sugar–phosphate backbone is on the outside, forming prominent major and minor grooves that facilitate protein binding and recognition.

Stabilization of B-DNA arises from hydrogen bonds between complementary bases (adenine–thymine with two hydrogen bonds, guanine–cytosine with three) and from base stacking interactions. The stability is influenced by sequence context, hydration level, and the concentration of cations such as Na+ and K+. B-DNA is favored under neutral to slightly alkaline pH and moderate humidity and salt conditions, which reflect typical cellular environments. Temperature and chemical conditions can shift the DNA toward alternative conformations, such as A-DNA or Z-DNA, in specific sequences or under extreme environments.

In nature, B-DNA coexists with other forms but is generally regarded as the standard model of DNA structure for most genetic and molecular biology studies. Local deviations from the ideal B-form occur due to sequence-dependent structural variations, bending, and interactions with proteins, ligands, or chromatin. The form was identified in the 1950s through experimental data that led to the widely adopted depiction of DNA as a right-handed double helix.