meselson and stahl experiment

Meselson-Stahl experiment
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A summary of the three postulated methods of DNA synthesisThe Meselson-Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl which demonstrated 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.

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. The 15N isotope is not radioactive, only heavier than common nitrogen.

E. coli were grown for several generations in a medium with 15N. When DNA is extracted from these cells and centrifuged on a salt density gradient, the DNA separates out at the point at which its density equals that of the salt solution. 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. This discovery was hugely important for the development of biology and is a majory aid to the treatment of disease (e.g. cancer).