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: Understanding the Concept:
DNA can exist in several different conformations. The most common is B-DNA, but others like A-DNA and Z-DNA exist under specific conditions. These forms differ in their structural parameters, including handedness, diameter, and the number of base pairs (bp) per helical turn. The question asks for the number of base pairs per turn for Z-DNA.
Step 2: Detailed Explanation:
Let's compare the key features of the major DNA forms:


B-DNA: The classic Watson-Crick model. It is a right-handed helix with about 10.4-10.5 base pairs per turn. Its diameter is about 20 \AA. (Option C refers to B-DNA).

A-DNA: A dehydrated form of DNA. It is a right-handed helix that is wider and shorter than B-DNA, with 11 base pairs per turn. Its diameter is about 26 \AA. (Option B refers to A-DNA).

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

Step 3: Final Answer:
The left-handed Z-DNA helix contains 12 base pairs per turn.