Question

Which one of the following statements is not true about Z-DNA structure?

• A. Phosphate backbone in Z-DNA zig-zag left rather than to the right as in B-DNA.
• B. Z-DNA has only minor groove and major groove is absent.
• C. Z-DNA is much broader than B-DNA.
• D. There is alternate stacking of bases in anti- and syn- conformation.

Hint:

Comparison of DNA helix diameters:
A-DNA $\rightarrow$ Broadest ($2.3\text{ nm}$).
B-DNA $\rightarrow$ Intermediate ($2.0\text{ nm}$).
Z-DNA $\rightarrow$ Narrowest ($1.8\text{ nm}$) and is left-handed.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Double-stranded DNA can exist in several distinct helical conformations depending on sequence composition, hydration levels, and salt concentration.
The three primary structural forms are the right-handed A- and B-DNA, and the left-handed Z-DNA.

Step 2: Detailed Explanation:
Let us evaluate each of the statements concerning the structure of Z-DNA:
1. Statement A: Unlike right-handed B-DNA, Z-DNA is a left-handed double helix where the sugar-phosphate backbone follows a winding, zig-zag path. This statement is correct.
2. Statement B: Due to the orientation of the bases, the major groove in Z-DNA is flattened and barely discernible, while the minor groove is deep and narrow, meaning the distinct major groove is essentially absent. This statement is correct.
3. Statement C: Z-DNA is narrower (thinner) than B-DNA, not broader.
B-DNA has a helical diameter of approximately $2.0\text{ nm}$ ($20\text{ \AA}$), whereas Z-DNA is a stretched, narrow conformation with a helical diameter of only $1.8\text{ nm}$ ($18\text{ \AA}$).
Therefore, the statement "Z-DNA is much broader than B-DNA" is incorrect.
4. Statement D: Z-DNA contains alternating purine and pyrimidine sequences (such as alternating $\text{d(C-G)}_n$).
The pyrimidines (cytosine) adopt the standard *anti* conformation, while the purines (guanine) adopt the *syn* conformation, creating the repeating zig-zag backbone. This statement is correct.

Step 3: Final Answer:
The incorrect statement is that Z-DNA is broader than B-DNA, which corresponds to Option (C).