If an E.coli with heavy DNA (N15) is put in a medium with NH4Cl[N14] then, what type of DNA will be obtained after two generations/replications ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → N14 N15 and N14 N14 type
Meselson and Stahl showed that DNA replicates semi-conservatively: each new molecule keeps one old strand.
Start: N15N15 (heavy).
After 1 generation in N14: all N15N14 (hybrid).
After 2 generations: half N15N14 (hybrid) and half N14N14 (light).
Option (1) N14 N14 type — Incorrect. The two old heavy strands are still present, in two hybrid molecules.
Option (2) N14 N15 and N14 N14 type — Correct.
Option (3) N14 N15 and N15 N15 type — Incorrect. Fully heavy DNA disappears after the first generation.
Option (4) N15 N15 and N14 N14 type — Incorrect. This would be conservative replication, which the experiment ruled out.