Semi-conservative DNA replication in Meselson-Stahl experiment

Steps:

• Start: All bacterial DNA is heavy (15N/15N).
• After first division in 14N: Each DNA molecule unwinds; each strand pairs with 14N, yielding 100% hybrid (15N/14N) DNA.
• After second division in 14N: Each hybrid replicates; one daughter is hybrid (15N/14N), the other is light (14N/14N), so total DNA is 50% hybrid and 50% light.
• Extracted DNA from this population shows 50% hybrid molecules.

Why B is correct:

• Semi-conservative replication ensures each new molecule retains one parental strand, yielding exactly 50% hybrid DNA after two generations in light medium.

Why the others are wrong:

• A: 25% hybrid occurs after three generations, when light DNA predominates further.
• C: 75% hybrid mismatches the equal segregation of strands, producing only 50% hybrid.
• D: 100% hybrid is the result after just one generation.

Final answer: B