Semi-conservative DNA replication in Meselson-Stahl experiment

Steps:

• All initial DNA is heavy-heavy (HH) from N15 medium.
• In first replication with N14, each HH unwinds; each heavy strand templates a new light strand, yielding two hybrid (HL) molecules per original.
• Population doubles; all molecules are HL.
• Each HL has one heavy and one light strand, so 50% of total strands are heavy; 100% of molecules are hybrid.

Why C is correct:

• Semi-conservative replication ensures one parental (heavy) strand per daughter molecule, resulting in 50% heavy strands and 100% hybrid molecules after one generation.

Why the others are wrong:

• A: Underestimates heavy strands (50%, not 25%) and hybrid molecules (100%, not 50%).
• B: Correct on heavy strands but underestimates hybrids (100%, not 50%).
• D: Overestimates heavy strands (50%, not 75%) despite correct hybrids.

Final answer: C