Which description best characterizes A-form DNA?

• A. A-form is right-handed and similar to B-form with 10.5 base pairs per turn.
• B. A-form is left-handed with a zigzag backbone.
• C. A-form is right-handed and very narrow with a shallow major groove.
• D. A-form is shorter and wider with a deep major groove and a shallow minor groove.

Answer: (D)

A-form DNA is a right-handed, more compact helix than the common B-form. It is shorter along the axis and wider in diameter, completing about 11 base pairs per turn. This geometry makes the major groove deep and the minor groove shallow. Therefore, describing A-form as shorter and wider with a deep major groove and a shallow minor groove best matches its actual structure, unlike forms that are left-handed or have different groove depths.