DNA And Gene Expression Practice Test

Semiconservative replication means each daughter DNA molecule has one parental (old) strand and one newly synthesized strand. The key evidence comes from the Meselson-Stahl experiment, which used density labeling of DNA. Bacteria were grown in heavy nitrogen to label the old strands, then switched to light nitrogen. After one round of replication, the DNA appeared with intermediate density, indicating each molecule contained one old and one new strand. After a second round, the distribution showed both intermediate and light DNA, still consistent with one old and one new strand per molecule. This pattern rules out purely conservative replication (which would give two old or two new molecules after the first round) and dispersive replication (which would keep all molecules at an intermediate density). Therefore, the observed results support the idea that each daughter molecule contains one parental and one newly synthesized strand.