DNA replication models — In a cesium chloride density gradient experiment (à la Meselson–Stahl), what bands are observed after two rounds of replication in light medium?

Introduction:
The Meselson–Stahl experiment used cesium chloride density gradients to distinguish conservative, semi-conservative, and dispersive DNA replication. This question asks you to recall the predicted banding pattern after two generations in light (14N) medium following initial growth in heavy (15N) medium.

Given Data / Assumptions:

• Gen 0 DNA is heavy (15N).
• Cells are shifted to light (14N) medium.
• Semi-conservative replication is assumed.

Concept / Approach:
After one replication in 14N medium, all DNA is hybrid (one heavy strand + one light strand), producing a single medium band. After two replications, half the molecules are hybrid (medium) and half are fully light, giving two bands: one medium and one light, with no heavy DNA remaining.

Step-by-Step Solution:

Verification / Alternative check:
Quantitative densitometry of the gradient shows increasing light fraction with successive generations, matching semi-conservative predictions and ruling out conservative or dispersive models under these conditions.

Why Other Options Are Wrong:

• Including a heavy band: impossible after replication exclusively in light medium.
• One medium band only: correct for one generation, not two.
• Three bands: not predicted by semi-conservative replication at two generations.
• Two heavy bands: not biologically plausible here.

Common Pitfalls:
Mixing up the first and second generation patterns; remember: 1st → medium only, 2nd → medium + light.

Final Answer:
One light and one medium band.