Which statement about Z-DNA is true?

Which statement about Z-DNA is true?

• A. Z-DNA is left-handed.
• B. Z-DNA is right-handed.
• C. Z-DNA has a larger diameter than B-DNA.
• D. Z-DNA forms under high humidity.

Answer: (A)

Z-DNA is a left-handed double helix, twisting in the opposite direction from the common right-handed B-DNA. This left-handed twist comes from how the bases stack and orient themselves (purines often adopt a syn conformation while pyrimidines stay anti), which creates a zigzag backbone and a helix that turns left rather than right. Z-DNA tends to form in sequences with alternating GC steps and under conditions like high salt or negative supercoiling, where the DNA is stressed in a way that favors this unusual geometry. Its diameter is actually smaller than that of B-DNA, not larger. Humidity isn’t the factor that promotes Z-DNA; higher humidity tends to stabilize the usual B form. So the statement that Z-DNA is left-handed is the correct one.

Z-DNA is a left-handed double helix, twisting in the opposite direction from the common right-handed B-DNA. This left-handed twist comes from how the bases stack and orient themselves (purines often adopt a syn conformation while pyrimidines stay anti), which creates a zigzag backbone and a helix that turns left rather than right. Z-DNA tends to form in sequences with alternating GC steps and under conditions like high salt or negative supercoiling, where the DNA is stressed in a way that favors this unusual geometry. Its diameter is actually smaller than that of B-DNA, not larger. Humidity isn’t the factor that promotes Z-DNA; higher humidity tends to stabilize the usual B form. So the statement that Z-DNA is left-handed is the correct one.