Which experimental evidence supports semi-conservative replication, aside from Meselson-Stahl experiments?

• A. X-ray diffraction patterns
• B. Electron microscopy of replication forks
• C. Density-gradient experiments showing each new DNA molecule contains one old strand
• D. Observations of mutations

Answer: (C)

To understand semi-conservative replication, you need evidence that every new DNA duplex contains one strand from the parent molecule. Density-gradient centrifugation provides this directly. If you label the old DNA strands with a heavy isotope and let replication occur, the first generation of new DNA molecules ends up with one heavy strand and one light strand. That pattern exactly fits the semi-conservative model—each daughter molecule preserves one parental strand while a new complement is built.

Other options don’t directly show how parental strands are inherited by the daughters. X-ray diffraction patterns reveal the helical structure of DNA but not how strands are copied. Electron microscopy of replication forks can visualize fork movement and the unwinding process, but it doesn’t prove that each new molecule contains one old strand. Observations of mutations indicate replication accuracy and error rates, not the mode of strand inheritance.