Step 1: Understanding DNA Conformations:
DNA can adopt different helical structures called conformations, primarily A-DNA, B-DNA, and Z-DNA. These differ in handedness (right or left), dimensions (width, pitch), and base orientation relative to the helix axis.
Step 2: Conformation Details:
- B-DNA: The classic Watson-Crick right-handed double helix. It's the most common form in cells under normal conditions (high hydration, low salt). Its stability makes it the primary in vivo conformation.
- A-DNA: A wider, more compact right-handed helix, mainly observed under dehydrating conditions. While not dominant in cells, it's adopted by DNA-RNA hybrids and double-stranded RNA.
- Z-DNA: A left-handed helix with a zigzag backbone. It's less stable than B-DNA and forms under specific conditions (high salt, alternating purines and pyrimidines like GCGCGC). It may regulate genes.
- C-DNA: A less common right-handed form seen under specific low-hydration and salt conditions.
Step 3: Conclusion:
The question concerns stability under "normal physiological conditions," i.e., the aqueous environment inside a living cell. B-DNA is the most stable and predominant form in this environment. Therefore, option (B) is correct.