Describe the concept of semiconservative replication with respect to the Meselson-Stahl experiment.

• A. All DNA remains heavy after one round.
• B. After two rounds, all DNA becomes light.
• C. DNA shows intermediate density after one round and then segregates to light density with one old strand per daughter.
• D. Semiconservative replication is demonstrated by separation of strands into two identical copies each containing an old and a new strand.

Answer: (C)

Semiconservative replication means each new DNA molecule contains one old (parental) strand and one newly synthesized strand. In the Meselson-Stahl setup, growing bacteria in heavy nitrogen first, then switching to light nitrogen, lets us follow strand origins by density. After one round, every DNA molecule is of intermediate density, showing one old heavy strand and one new light strand. After a second round in light nitrogen, the population splits: some DNA remains intermediate (still one old and one new strand), and some becomes light (both strands newly synthesized). This pattern aligns with the option: intermediate density after one round and a shift toward light density, reflecting that daughter molecules can retain one old strand while others do not.