A, B and Z forms
of DNA
The right-handed
B form [middle] is
the standard form
in biological systems. The left-handed
Z
form [right] occurs occasionally in the middle of a B-form molecule.
Bases are co-planar in both the B
and Z forms. The right-handed A form [left] occurs in
crystalline
structures where the water concentration is reduced: the structure is
distorted and the bases are no longer co-planar.