Final answer:
Meselson and Stahl continued their experiment to two rounds of DNA replication after ¹⁵N labeling to prove that DNA replication is semiconservative, which they showed by observing an intermediate and a light band after the second round, corresponding to hybrid DNA and new DNA, respectively.
Step-by-step explanation:
The reason it was important that Meselson and Stahl continued their experiment to at least two rounds of replication after isotopic labeling of the starting DNA with ¹⁵N is because the results after one round of replication were not enough to distinguish between the semiconservative and dispersive models of DNA replication. After the second round, two distinct bands were observed: one at the intermediate density (indicating one old and one new strand, which is characteristic of the semiconservative model), and one at the density of ¹⁴N DNA (indicating two new strands). These results confirmed that DNA replication is semiconservative and each daughter cell receives one parental and one newly synthesized DNA strand.The Meselson-Stahl experiment was pivotal in demonstrating how DNA replication is a semiconservative process, where one strand from the parent DNA molecule is conserved in each of the daughter DNA molecules formed.