Density-gradient centrifugation in cesium chloride: DNA labeled with heavy nitrogen (15N) will band where relative to DNA containing light nitrogen (14N)?

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:

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