particular right-handed helical form of DNA (possessing 11 base pairs per turn) which is the form that DNA molecules exist in when they are partially
dehydrated. A-form DNA is found in fibres at 75% relative humidity and
requires the presence of sodium, potassium or cesium as the counterion.
Instead of lying flat, the bases
are tilted with respect to the helical axis, and there are more base pairs per turn.