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?
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AB-DNA
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BA-DNA
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CZ-DNA
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DX-DNA
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EP-DNA
Answer
Correct Answer: B-DNA
Explanation
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:
1) Identify the structure historically described by Watson and Crick.2) Compare helical parameters of A, B, and Z forms.3) Select B-DNA as the standard cellular conformation.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.