A Form of DNA - A-form of DNA is formed when relative humidity is 70%. It is right-handed, wider, incorporates more base pairs per helical turn and is less flexible than the B- DNA. The major groove is deep and narrow, whereas the minor groove is broad and shallow. The diameter of the DNA is 23 A. Eleven base pairs are present per turn with a distance of2.6 A between them. A-form of DNA is less soluble than the B-form. The helix height or pith is 31A.