What experimental evidence supports semiconservative replication?

• A. Autoradiography using radioactive nucleotides showed continuous semiconservative replication.
• B. Electron microscopy showed all new DNA strands are completely new.
• C. Genetic code degeneracy proves semiconservative replication.
• D. Meselson-Stahl experiment using heavy 15N DNA showed one old strand in each daughter after first generation, supporting semiconservative replication.

Answer: (D)

The key idea is how DNA replication preserves one parental strand in each daughter molecule. The Meselson-Stahl experiment tested this by labeling bacterial DNA with a heavy nitrogen isotope (15N) so all original strands were heavy, then shifting cells to a lighter 14N medium and watching how density of DNA changed over generations. After one round of replication, they found a single band corresponding to DNA of intermediate density—each molecule contained one heavy strand and one newly synthesized light strand. That pattern shows one old strand is retained in each daughter, with a new strand complementary to it, which is the defining feature of semiconservative replication. If replication were conservative, the first generation would yield one heavy (old) double helix and one light (new) double helix, producing two bands, which did not occur. If replication were dispersive, the first generation would also produce an intermediate density, but the second generation would eventually show a different distribution; the observed shift in subsequent generations matched semiconservative behavior. This experiment provides direct, quantitative evidence that each daughter DNA molecule inherits one parental strand and one newly synthesized strand.