Final answer:
Meselson and Stahl's experiments demonstrated that DNA replication follows a semi-conservative model, where each new DNA molecule contains one old and one new DNA strand. Their use of nitrogen isotopes in E. coli helped to rule out conservative and dispersive models of replication.
Step-by-step explanation:
Meselson and Stahl's experiments were crucial in understanding how DNA replicates. They provided clear evidence that DNA replication is of a semi-conservative nature. This means that each new DNA molecule consists of one strand from the original DNA and one newly synthesized strand. The alternative models of replication, such as conservative replication, where the parental DNA is fully conserved in one of the two daughter DNA molecules, and dispersive replication, where both copies of DNA are interspersed with segments of parental and newly synthesized DNA, were ruled out by their experiment.
The Meselson-Stahl experiment used isotopes of nitrogen, 15N and 14N, to distinguish between old and new strands of DNA based on their density after ultracentrifugation. After one generation in 14N, the DNA formed a single band at an intermediate density, indicating semi-conservative or dispersive replication. The absence of a heavy band ruled out conservative replication. After a second generation, the presence of two bands, one intermediate and one light, confirmed the semi-conservative model beyond doubt.