In structural biochemistry, which description correctly characterizes the B-DNA form observed in aqueous solution, including strand orientation and approximate helical pitch per turn?

Introduction / Context:
B-DNA is the predominant DNA conformation in physiological aqueous solution. This item tests recognition of its handedness, strand polarity, and approximate base pairs per turn—core facts for molecular biology and genetics.

Given Data / Assumptions:

• Context: nucleic acid structure at near-neutral pH and moderate ionic strength.
• We must select the accurate structural description of B-DNA.
• Approximate helical repeat is acceptable (~10 bp/turn commonly cited; many texts use 10–10.5).

Concept / Approach:
Canonical B-DNA is a right-handed helix with two antiparallel polynucleotide strands (one 5'→3', the other 3'→5'). Under physiological conditions, its helical repeat is about 10–10.5 base pairs per turn, with bases roughly perpendicular to the helix axis and distinctive major/minor grooves.

Step-by-Step Solution:
1) Handedness: B-DNA is right-handed (Z-DNA is left-handed; A-DNA is right-handed but is a dehydrated form).2) Strand orientation: DNA strands in duplex form are antiparallel, not parallel.3) Helical pitch: accept ~10 bp/turn as the standard simplified value.4) The description that matches all three properties is the right-handed antiparallel double helix with ~10 bp/turn.

Verification / Alternative check:
Fiber diffraction and high-resolution structures consistently show right-handedness and antiparallel polarity for B-DNA; only Z-DNA is left-handed and it has a zig-zag backbone but is not the predominant solution form.

Why Other Options Are Wrong:
Left-handed antiparallel: describes Z-DNA, not B-DNA.Left-handed zig-zag: again Z-DNA (special sequences/conditions), not the common solution form.Right-handed parallel: the strands in DNA are not parallel; they are antiparallel.

Common Pitfalls:
Mixing Z-DNA traits (left-handed, zig-zag backbone) with B-DNA, or assuming parallel strand orientation.

Final Answer:
Right-handed double helix of antiparallel chains (~10 bp/turn)