B-DNA is the most common form of DNA under physiological conditions. It is the familiar right-handed double helix associated with the classic Watson–Crick model of DNA.
B-DNA is favored in the relatively hydrated, salt-containing environment of living cells. Its sugar-phosphate backbones wind around the outside, while the nitrogenous bases stack inside and pair across the two antiparallel strands. The helix has major and minor grooves, and many DNA-binding proteins recognize sequences by interacting with these grooves, especially the chemically informative major groove.
DNA is structurally flexible, so B-DNA is not the only form it can adopt. A-DNA is also right-handed but is shorter and wider; it becomes more favorable under dehydrating conditions and can occur in some DNA–RNA hybrid structures. Z-DNA is left-handed and has a zigzag-shaped phosphate backbone. Short stretches can shift between forms depending on sequence, hydration, chemical conditions, protein binding, or torsional stress. Thus, “most common under physiological conditions” identifies the predominant overall form, not a claim that every base pair in every cell is permanently B-DNA.