What is the most noticeable difference between A-form and Z-form DNA?

• A. Z-form is left-handed, A-form is right-handed.
• B. Both forms are right-handed.
• C. Both forms are left-handed.
• D. Z-form is right-handed, A-form is left-handed.

Answer: (B)

Handedness of DNA helices is the key idea here. A-form DNA is right-handed, while Z-form DNA is left-handed. The A-form is a more compact, wider helix that arises under dehydrating conditions or in RNA–DNA hybrids, whereas Z-form has a zigzag backbone and tends to form in sequences with alternating purines and pyrimidines, often under higher salt. Because the twist direction differs, the most noticeable difference between these two forms is their handedness: A-form twists to the right, Z-form twists to the left. This contrasts with the common B-form, which is also right-handed, highlighting that A-form and Z-form differ specifically in how they unwind or twist in space.