ABSTRACT
A 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 fibers 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. The A-form is biologically interesting because it is probably very close to the conformation adopted by DNA-RNA hybrids or by RNA-RNA double-stranded regions. The reason is that the presence of the 2
hydroxyl group prevents RNA from lying in the B-form.
