This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
The Z DNA helix 
Select and explain. 
A. Has fewer base pairs per turn than B DNA, B. Is ored by an alternating GC sequences, C. Tends to be found at 3 end to genes, D. Is inhibited by methylation of the bases
B: Is ored by an alternating GC sequences
Answer: B: Is ored by an alternating GC sequences 
Explanation: Z-DNA is one of the many possible double helical structures of DNA. It is a left-handed double helical structure in which the double helix winds to the left in a zig-zag pattern (instead of to the right, like the more common B-DNA form). Z-DNA is thought to be one of three biologically active double helical structures along with A- and B-DNA.