Meselson-Stahl experiment

  The Meselson-Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl to prove that DNA replication was semiconservative. Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strand coming from the original helix and one newly synthesized.

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Nitrogen is a major constituent of DNA. ¹⁴N is by far the most abundant isotope of nitrogen, but DNA with the heavier ¹⁵N isotope is also viable.

E. coli were grown for several generations in a medium with ¹⁵N. The DNA of the resulting cells had a higher density (was heavier). After that, E. coli cells with only ¹⁵N in their DNA were put back into a ¹⁴N medium and were allowed to divide only once. DNA was then extracted from a cell and was compared to DNA from ¹⁴N DNA and ¹⁵N DNA. It was found to have close to the intermediate density. Since conservative replication would result in equal amounts of DNA of the higher and lower densities (but no DNA of an intermediate density), conservative replication was excluded. However, this result was consistent with both semiconservative and dispersive replication. Semiconservative replication would result in double-stranded DNA with one strand of ¹⁵N DNA, and one of ¹⁴N DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of ¹⁵N and ¹⁴N DNA, either of which would have appeared as DNA of an intermediate density.

DNA was then extracted from cells which had been grown for several generations in a ¹⁵N medium, followed by two divisions in a ¹⁴N medium. DNA from these cells was found to consist of equal amounts of two different densities, one corresponding to the intermediate density of DNA of cells grown for only one division in ¹⁴N medium, the other corresponding to cells grown exclusively in ¹⁴N medium. This was inconsistent with dispersive replication, which would have resulted in a single density, lower than the intermediate density of the one-generation cells, but still higher than cells grown only in ¹⁴N DNA medium, as the original ¹⁵N DNA would have been split evenly among all DNA strands. The result was consistent with semiconservative replication, in that half of the second-generation cells would have one strand of the original ¹⁵N DNA along with one of ¹⁴N DNA, accounting for the DNA of intermediate density, while the DNA in the other half of the cells would consist entirely of ¹⁴N DNA--one synthesized in the first division, and the other in the second division.

Literature cited

• Meselson, M. and Stahl, F.W. (1958). "The Replication of DNA in Escherichia coli". PNAS 44: 671-82. PMID 16590258.