Semi-Conservative DNA Replication Steps:

• Generation 0: All DNA is heavy-heavy (HH), 100% with heavy strands.
• Generation 1: Each HH unwinds; new strands are light (L), yielding all hybrid heavy-light (HL) DNA, 100% with heavy strands.
• Generation 2: HL templates produce 50% HL (heavy strand retained) and 50% light-light (LL), so 50% of molecules contain heavy strands.
• Generation 3: HL (50%) yields 25% HL and 25% LL; LL (50%) yields 50% LL, totaling 25% with heavy strands. Why C is correct:
• Matches semi-conservative model: each replication distributes one parental heavy strand to half the daughter molecules, halving the percentage with heavy strands per generation. Why the others are wrong:
• A: First generation is 100% hybrids, not 50%.
• B: First generation is 100%, not 75%.
• D: Second generation is 50% hybrids, not 75%. Final answer: C