Correct option is D
The Z-form of DNA is characterized by a unique structural conformation in which the double helix takes a left-handed turn, in contrast to the right-handed turns of A-DNA and B-DNA. One of the most distinguishing features of Z-DNA is its flat or almost non-existent major groove, which differentiates it markedly from the other DNA forms. The minor groove, in contrast, becomes narrow and deep. This unusual geometry contributes to the distinctive zigzag sugar-phosphate backbone that gives Z-DNA its name.
The flatness of the major groove in Z-DNA results from the alternating purine-pyrimidine sequence (typically CG repeats) and the syn conformation of purines (especially guanine), which causes the sugar-phosphate backbone to protrude and obscure the major groove.
Additional Information:
- (a) Narrow and deep major groove: This is characteristic of A-DNA, not Z-DNA. A-DNA is a right-handed helix with a deep major groove and shallow minor groove.
- (b) Wide and deep major groove: This is a hallmark of B-DNA, which is the most common form of DNA under physiological conditions. B-DNA has a wide major groove that allows easy access for protein-DNA interactions.
- (c) Narrow and shallow major groove: This structure is not a standard characteristic of any common DNA conformation and does not describe Z-DNA’s groove structure accurately.
