Which description best characterizes B-form DNA?

• A. B-form DNA is a right-handed helix with about 10.5 base pairs per turn, a wide major groove, and a narrow minor groove.
• B. B-form DNA is a left-handed helix with a zigzag backbone.
• C. B-form DNA is a right-handed helix that is very short with a deep major groove.
• D. B-form DNA is a right-handed helix with about 12 base pairs per turn and equal grooves.

Answer: (A)

B-form DNA is the common right-handed double helix found in cells under normal conditions. It has about 10.5 base pairs per turn, giving a distinctive helical pitch, and features a wide major groove alongside a narrow minor groove. This combination—right-handedness, roughly 10.5 bp per turn, and unequal grooves—best matches the description. The other possibilities describe a left-handed form with a zigzag backbone (Z-DNA) or propose incorrect dimensions and groove patterns (such as a very short molecule, a deep major groove, or about 12 bp per turn with equal grooves), which do not align with B-DNA’s known structure.