Semi-conservative DNA replication in Meselson-Stahl experiment

Steps:

• Bacteria start with all heavy DNA (both strands 15N).
• In first generation after transfer to 14N medium, replication produces all hybrid DNA (one 15N strand, one 14N strand).
• In second generation, hybrids replicate to 50% hybrid and 50% light DNA (both strands 14N).
• In third generation, the 50% hybrids produce 25% hybrid and 25% light, plus the existing 50% light remains, yielding 25% hybrid and 75% light overall.

Why B is correct:

• Semi-conservative replication ensures first-generation DNA is fully hybrid; by third generation, dilution yields 25% hybrid (one quarter) and 75% light DNA.

Why the others are wrong:

• A: Describes first and second generations, not third.
• C: First generation has no half light or half heavy; third isn't one quarter each of light and hybrid.
• D: First generation has no light DNA; third isn't three quarters light without specifying hybrid.

Final answer: B