In 1958, there were still speculations about the hereditary material and the structure of DNA. Scientists knew that DNA existed but did not know how it replicated itself. There were three different possibilities that came to mind: conservative, semi-conservative, and dispersive models. The conservative model postulated that the replication process created a whole new DNA molecule, and the template molecule stayed the same. The semi-conservative model postulated that the template model divided into two strands and each strand replicated their complementary strand, creating 2 DNA molecules with one newly synthesized strand and one old strand. The dispersive model postulated that the newly synthesized parts are combined with the template DNA, creating 2 identical DNA molecules with both newly synthesized oligonucleotides and older ones (Figure 1).
Figure 1: Conservative, semi-conservative, and dispersive models that were postulated for DNA replication mechanism.
Meselson and Stahl designed an experiment using the radioisotopes of nitrogen to figure out which of these models is the correct one. They used heavy (N15) and light (N14) nitrogen, which is found in the nucleotides that make up the DNA. They aimed to differentiate between DNA samples depending on the weight or the position on a density gradient. Density-gradient centrifuge was a technique developed by Meselson himself. When centrifuged, contents of a sample move toward the bottom according to their density. Denser molecules migrate more than the molecules with smaller densities. Inside the gradient solution, each molecules reaches and equilibrium where they do not migrate any further. By comparing the location of DNA molecules with heavy (N15) and light (N14) nitrogen, it is possible to find out which DNA Replication model is correct.
Meselson and Stahl cultured Escherichia coli in media that has only heavy (N15) nitrogen. After a few generations, all the DNA molecules would have nucleotides that possess heavy (N15) nitrogen. Then they subcultured the same E. coli in media that has only light (N15) nitrogen. This allowed them to differentiate between the parent and daughter strands of DNA since the parental DNA would possess heavy (N15) nitrogen while the daughter DNA would possess light (N14) nitrogen. The amount of bands at different generations (Figure 2), and the presence of an half heavy (N15) nitrogen half light (N14) nitrogen followed by a decrease in heavy (N15) nitrogen bands characterized the postulated semi-conservative DNA replication model.
Figure 2: Schematic depiction of Meselson-Stahl experiment.
This experiment was also consistent with the latest findings of Watson and Crick on DNA structure based on Rosalind Franklin’s studies with X-Ray Crystallography that was published in 1953. Semi-conservative DNA replication model was also first suggested by Watson and Crick.
Understanding the theoretical information, postulated models, and the experimental design of Meselson-Stahl is possible with VRLab Academy. You can explain and show 3D models of DNA molecules, work with radioactive isotopes without any concern for contaminating your biology laboratory or causing any harm to the students. You can get data from an experiment that takes months to complete in a matter of seconds.
Meselson, M., & Stahl, F. W. (1958). THE REPLICATION OF DNA IN ESCHERICHIA COLI. Proceedings of the National Academy of Sciences of the United States of America, 44(7), 671–682. https://doi.org/10.1073/pnas.44.7.671
Davis, Tinsley H. "Meselson and Stahl: The Art of DNA Replication." Proceedings of the National Academy of Sciences 101 (2004): 17895–6.
Meselson, Matthew, Franklin W. Stahl, and Jerome Vinograd. "Equilibrium Sedimentation of Macromolecules in Density Gradients." Proceedings of the National Academy of Sciences 43 (1957): 581–8. http://www.pnas.org/content/43/7/581.short (Accessed April 18, 2017).