According to the classical Watson–Crick model observed under physiological conditions, which DNA helix form represents the standard right-handed duplex of most cellular DNA?

Introduction / Context:
DNA can adopt multiple helical conformations depending on hydration, sequence, and ionic conditions. The historically famous Watson–Crick structure corresponds to a particular form predominant in vivo.

Given Data / Assumptions:

• Physiological (aqueous) conditions.
• Canonical duplex with antiparallel strands and complementary base pairing.

Concept / Approach:
B-DNA is the right-handed helix with about 10 base pairs per turn, wide major groove and narrow minor groove, matching the Watson–Crick model. A-DNA is a more compact right-handed form favored in lower hydration; Z-DNA is a left-handed helix occurring transiently in certain GC-rich sequences under high salt or negative supercoiling.

Step-by-Step Solution:

Verification / Alternative check:
X-ray fiber diffraction and crystallography support B-form parameters under physiological hydration. Biochemical footprinting also aligns with B-DNA groove geometry for protein binding.

Why Other Options Are Wrong:

• A-DNA: Right-handed but dehydrated form; common in RNA–DNA hybrids.
• Z-DNA: Left-handed, rarer, transient regulatory contexts.
• X-DNA and P-DNA: Not the classical cellular helix described by Watson–Crick.

Common Pitfalls:
Assuming any right-handed helix equals the classical form, or conflating A-DNA conditions with in vivo chromosomal DNA.

Final Answer:
B-DNA.