Question

The left-handed helix, Z-DNA is 18 \AA \ (1.8 nm) in diameter which contains _____________________.

• A. 10 bp per turn
• B. 11 bp per turn
• C. 10.4 bp per turn
• D. 12 bp per turn

Hint:

Memorize the key stats for the three main DNA forms. \textbf{B-DNA} is the 'normal' one (~10.5 bp/turn). \textbf{A-DNA} is short and fat (11 bp/turn). \textbf{Z-DNA} is the 'weird' one: \textbf{Z}igzag, \textbf{Z}any (left-handed), and has the most base pairs per turn (12).

Answer 1

The Correct Option is D

Step 1: Concept Summary:
DNA exists in various conformations, including the common B-DNA, and less common A-DNA and Z-DNA, each appearing under different conditions. These forms are differentiated by their helical structure, diameter, and base pairs per helical turn. This question concerns the number of base pairs per turn in Z-DNA.
Step 2: Detailed Comparison:
Comparison of the major DNA forms:


B-DNA: The standard Watson-Crick DNA structure. A right-handed helix with approximately 10.4-10.5 base pairs per turn and a diameter of about 20 \AA. (Option C describes B-DNA).

A-DNA: A dehydrated DNA form. A right-handed helix, wider and shorter than B-DNA, containing 11 base pairs per turn and a diameter of roughly 26 \AA. (Option B describes A-DNA).

Z-DNA: A left-handed helix characterized by a zigzagging phosphate backbone. Thinner and more elongated than B-DNA, with a diameter of about 18 \AA. It contains 12 base pairs per turn (or 6 dimeric units).

Step 3: Final Answer:
Z-DNA, the left-handed helix, has 12 base pairs per turn.