Experimental Design:

1. Labeling DNA with Heavy Nitrogen (15N):

   • E. coli cells were grown in a medium containing 15N (a heavy isotope of nitrogen) as the only source of nitrogen. This isotope was incorporated into the DNA, making it denser than normal DNA (which contains 14N, a lighter isotope).

   • After several generations in the 15N medium, all the DNA in the E. coli cells became fully labeled with the heavy 15N isotope.

2. Transfer to Light Nitrogen Medium (14N):

   • The E. coli cells were then transferred to a medium containing the lighter 14N isotope.

   • As the cells divided, they replicated their DNA using the lighter nitrogen (14N), resulting in new DNA strands made from the lighter isotope.

3. Centrifugation to Separate DNA:

   • After one and multiple generations of growth in the 14N medium, samples of DNA were extracted and subjected to density gradient centrifugation in a cesium chloride (CsCl) solution.

   • This technique separates DNA molecules based on their density, with 15N-DNA forming a distinct band at a heavier position in the gradient compared to 14N-DNA.

Results:

1. First Generation (After One Round of Replication in 14N):

   • After one round of replication, the DNA formed a single band at an intermediate density between 15N and 14N.

   • This result indicated that each DNA molecule consisted of one strand with 15N (old) and one strand with 14N (new), supporting the semiconservative model.

2. Second Generation (After Two Rounds of Replication in 14N):

   • After two rounds of replication, two distinct bands appeared: one at the intermediate density (15N/14N hybrid DNA) and one at the lighter 14N density (newly synthesized 14N/14N DNA).

   • This result further confirmed the semiconservative nature of DNA replication, as it showed that half of the DNA molecules were hybrids (one old and one new strand), while the other half were composed entirely of newly synthesized strands.

Conclusion:

The Meselson-Stahl experiment conclusively demonstrated that DNA replication is semiconservative. Each newly formed DNA molecule contains one strand from the original parent molecule and one newly synthesized strand. This was a groundbreaking discovery that provided insight into the molecular mechanism of genetic inheritance.

Alternative Models Tested:

1. Conservative Replication: This model suggested that the original DNA molecule would remain intact and produce an entirely new double-stranded DNA molecule. If this were true, the first generation would show separate heavy (15N) and light (14N) DNA bands, which was not observed.

2. Dispersive Replication: This model proposed that both strands of the new DNA would be a mix of old and new DNA segments. If this were true, the DNA in subsequent generations would always show intermediate density, but after the second generation, a light band (14N/14N) appeared, disproving the dispersive model.

Thus, the Meselson-Stahl experiment is widely considered one of the most beautiful experiments in biology, as it provided conclusive evidence for the semiconservative mechanism of DNA replication.