Question

Which of the following statement(s) about A and Z DNA structure are correct?

• A. Z-form DNA is a right-handed helix; A-form DNA is a left-handed helix
• B. A-form DNA is a right-handed helix; Z-form DNA is a left-handed helix
• C. In A-form, the bases occupy the interior of the helix; In Z-form, guanosine rotates around the glycosidic bond
• D. In A-form, the bases rotate around the glycosidic bond; In Z-form, the bases occupy the interior of the helix

Answer 1

Answer: (B), (C)

In A-form, the bases occupy the interior of the helix; In Z-form, guanosine rotates around the glycosidic bond

A-form DNA is a right-handed helix and Z-form DNA is a left-handed helix.

In nucleotides the nitrogenous bases can rotate around the glycosidic bond to take a syn- or anti- conformation. Although they are commonly seen in the anti-conformation due to steric hindrance in the syn-conformation. However in Z-DNA, guanosine is found in the syn- conformation also. Thus the Z-DNA shows rotation around the glycosidic bond.