Which statement correctly describes Z-form DNA?

• A. Z-form DNA is right-handed with a smooth backbone.
• B. Z-form DNA is identical to B-form in groove dimensions.
• C. Z-form DNA is left-handed with a zigzag backbone.
• D. Z-form DNA is the form adopted by RNA-DNA hybrids exclusively.

Answer: (C)

Z-form DNA is a left-handed helix with a zigzag sugar-phosphate backbone. This combination—left-handed twist and a backbone that takes on a zigzag path—sets it apart from the usual right-handed B-DNA and explains its distinct groove geometry. Z-DNA tends to form in particular sequence contexts, often GC-rich or alternating purine-pyrimidine runs, and under certain conditions like high salt or negative supercoiling, which help stabilize this unusual shape. Because of these characteristics, describing it as left-handed with a zigzag backbone captures the essential features. The other statements misrepresent its handedness, backbone pattern, groove structure, or its occurrence in RNA-DNA hybrids.