In the Meselson-Stahl experiment, what was observed after one round of replication?

• A. DNA molecules consisted of one old strand and one new strand (hybrid).
• B. DNA remained entirely old.
• C. DNA became entirely new.
• D. DNA degraded.

Answer: (A)

The key idea is semi-conservative replication: each new DNA molecule keeps one of the old parental strands and builds a new complementary strand. After one round, every DNA duplex has one old, heavy strand and one newly synthesized light strand, so you observe hybrid molecules. In the Meselson-Stahl setup, this appears as an intermediate-density form, not as all-old or all-new DNA. Degradation isn’t part of this process, and a fully old or fully new molecule would contradict how replication copies both strands in a template-based manner.