Semi-conservative DNA replication in the Meselson-Stahl experiment

Steps:

• At 0 min: All DNA is heavy-heavy (15N-15N), lower band.
• After 50 min (1 generation): All DNA replicates to heavy-light hybrid (15N-14N), middle band.
• After 100 min (2 generations): Hybrids produce 50% hybrid and 50% light-light (14N-14N); upper band is 50% light.
• After 150 min (3 generations): Previous light stays light; hybrids produce 25% hybrid and 25% light; total upper band (light) is 75%.

Why C is correct:

• Semi-conservative replication ensures each new strand pairs with an old one, yielding 3/4 light-light DNA molecules after three generations.

Why the others are wrong:

• A: 25% is the hybrid DNA proportion, not light.
• B: 50% is the light DNA after two generations.
• D: 100% ignores persistent hybrid DNA from original strands.

Final answer: C