Density-gradient centrifugation in cesium chloride: DNA labeled with heavy nitrogen (15N) will band where relative to DNA containing light nitrogen (14N)?
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AAbove the 14N DNA band
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BBelow the 14N DNA band
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CExactly co-incident with the 14N DNA band
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DAll of these (depends on rotor speed only)
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EAt the meniscus independent of density
Answer
Correct Answer: Below the 14N DNA band
Explanation
Introduction / Context:CsCl density-gradient centrifugation separates macromolecules by buoyant density. Classic Meselson–Stahl experiments used 15N labeling to distinguish heavy vs light DNA and demonstrate semi-conservative replication. This question asks where 15N-labeled DNA bands relative to 14N DNA.
Given Data / Assumptions:
- 15N increases DNA buoyant density compared to 14N.
- In a stable CsCl gradient, denser species migrate to positions of higher density (farther from the axis of rotation).
Concept / Approach:Heavier (higher density) molecules band closer to the bottom of the tube (i.e., farther out in a swinging-bucket rotor). Therefore, 15N-DNA bands below 14N-DNA.
Step-by-Step Solution:
1) Recall that buoyant density determines equilibrium position.2) 15N substitution increases DNA density.3) Thus, the 15N band forms lower than the 14N band in the gradient.Verification / Alternative check:Published density values show a measurable shift (~0.016 g/ml) between 15N and 14N DNA, yielding resolvable bands.
Why Other Options Are Wrong:
- Above: would imply lower density, which is false for 15N DNA.
- Co-incident: contradicts the density difference.
- “All of these”: rotor speed influences resolution, not which one is denser.
- Meniscus: banding occurs at the density-matching point, not at the surface.
Common Pitfalls:Confusing tube orientation; in most diagrams, “below” denotes the higher-density region toward the outer radius.
Final Answer:Below the 14N DNA band