Final answer:
The Meselson and Stahl experiment required a second replication round to disprove dispersive replication and conclusively demonstrate that DNA replication is semiconservative, where each daughter DNA molecule consists of one parental and one new strand.
Step-by-step explanation:
The Meselson and Stahl experiment required two rounds of replication to distinguish between conservative and semiconservative replication. Their experiment involved labeling parental DNA with a heavy isotope of nitrogen (¹5N) and then allowing the bacteria to replicate in the presence of a lighter isotope (¹4N). After the first round of replication, the DNA formed a single band at an intermediate density, which ruled out conservative replication but could not distinguish between semiconservative and dispersive replication. It was only after the second round of replication, which resulted in two distinct bands — one at the intermediate density and one at the light density of ¹4N DNA — that the semiconservative nature of replication was confirmed. This result supported a model where each daughter DNA molecule consists of one strand from the parental DNA and one newly synthesized strand.