What did the Meselson-Stahl experiment demonstrate about DNA replication?

• A. It showed that after replication, DNA consisted of one old and one new strand per molecule.
• B. It showed that replication is conservative.
• C. It showed that replication is dispersive.
• D. It showed that RNA primers are required for replication.

Answer: (A)

This question tests how DNA is copied during replication. The Meselson-Stahl experiment showed that each new DNA molecule contains one old strand and one newly synthesized strand, demonstrating semi-conservative replication. By labeling parental DNA with heavy nitrogen and then switching cells to light nitrogen, they used density differences to track strand origins. After one round of replication, every molecule appeared at an intermediate density, meaning each DNA molecule had one old (heavy) strand and one new (light) strand. After a second round, both intermediate and light bands appeared, consistent with the idea that each generation yields molecules with one old and one new strand. This pattern doesn’t fit the conservative model, which would yield separate heavy and light molecules after the first replication, nor the dispersive model, which would keep all molecules at an intermediate density regardless of generation. The experiment focuses on the mode of strand copying rather than primer requirements for replication.