Question

Which of the following is the most stable DNA configuration under normal physiological conditions?

• A. A-DNA
• B. B-DNA
• C. C-DNA
• D. Z-DNA

Hint:

Associate B-DNA with 'Biological' or 'Body' conditions. It's the standard, most common form you see in textbook diagrams. A-DNA is for 'Arid' (dry) conditions, and Z-DNA is for 'Zigzag' and is the odd one out, being left-handed.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
DNA can exist in several different helical structures, known as conformations. The most common are A-DNA, B-DNA, and Z-DNA. These forms differ in their helical sense (right-handed vs. left-handed), dimensions (width, pitch), and the orientation of the bases relative to the helix axis.

Step 2: Detailed Explanation:
- B-DNA: This is the classic right-handed double helix structure described by Watson and Crick. It is the most common form of DNA found in cells under normal physiological conditions, which involve a high degree of hydration and relatively low salt concentrations. Its stability under these conditions makes it the primary conformation for most DNA sequences in vivo.
- A-DNA: This is also a right-handed helix, but it is wider and more compact than B-DNA. It is primarily observed under dehydrating (low water) conditions. While not the dominant form for DNA in the cell, this conformation is adopted by DNA-RNA hybrids and double-stranded RNA.
- Z-DNA: This is a left-handed helix with a zigzagging sugar-phosphate backbone. It is less stable than B-DNA and forms only under specific conditions, such as high salt concentrations or in sequences with alternating purines and pyrimidines (e.g., GCGCGC). It is thought to play a role in gene regulation.
- C-DNA: This is a less common right-handed form observed under certain low-hydration and salt conditions.

Step 3: Final Answer:
The question asks for the most stable configuration under "normal physiological conditions." This directly refers to the aqueous environment inside a living cell. In this environment, the B-DNA conformation is the most stable and predominant form. Therefore, option (B) is the correct answer.