Which form of DNA is left-handed?

• A. A-form
• B. B-form
• C. Z-form
• D. C-form

Answer: (C)

DNA can adopt different twists, and the left-handed form is Z-DNA. This form twists in the opposite direction to the usual right-handed DNA, giving a distinctive zigzag backbone and a longer, slender helix with about 12 base pairs per turn. Z-DNA is favored by sequences with alternating purine-pyrimidine runs (like CG repeats) and by conditions such as high ionic strength or negative supercoiling. In contrast, the common right-handed forms—A-form and B-form—are the usual structures under physiological conditions (B-form being the canonical form, about 10.5 base pairs per turn). There is also a rarer left-handed form called C-DNA, but Z-DNA is the form most characteristic of left-handed DNA in typical contexts.