This is lead author Mark Sanders, and in this edition of Visualizing Genetics, we'll take a walkthrough Figure 7.9, that illustrates an experiment by Matthew Meselson and Franklin Stahl that demonstrated that DNA replicates by a semi-conservative mechanism.
Figure 7.9 shows the 1958 experiment by Matthew Meselson and Franklin Stahl that tested three possible mechanisms of DNA replication.
This experiment conclusively demonstrated that DNA replicates by a semi-conservative mechanism.
The experiment began with E. coli bacteria growing in a medium containing nitrogen-15. This is the so-called heavy isotope of nitrogen. It's denser than its more common counterpart, nitrogen-14, having one additional neutron.
In the experiment, bacteria were first grown in an N-15-containing medium. DNA was then collected from some bacteria, and the remaining bacteria were transferred to an N-14-containing medium for the remainder of the experiment.
Bacteria grown in the N-15 medium contained DNA that has incorporated the heavy isotope of nitrogen. These bacteria are identified as Generation 0 -- the starting point of the experiment.
Meselson and Stahl classified the DNA from Generation 0 bacteria as heavy, because both strands contained N-15, the heavy isotope. Meselson and Stahl analyzed DNA using cesium chloride gradient centrifugation and densitometry to assess the DNA composition.
The cesium chloride gradient separates molecules by their different migration rates.
Generally, heavier molecules move faster than lighter molecules, so they appear lower in the gradient. Because both strands of Generation 0 DNA contained N-15, this heavy DNA is written N15/N15 to indicate the nitrogen composition of each strand.
Densitometry uses light and a photoreceptor to pinpoint the location of material in the gradient. The single densitometry band for Generation 0 DNA signifies that all the molecules in the gradient have the same density.
In this case, both strands of DNA contain N-15 -- the heavy isotope.
Let's connect to Figure 7.8, which shows the three proposed models of DNA replication-- semiconservative replication, conservative replication, and dispersive replication.
Meselson and Stahl's experiment proved that semiconservative DNA replication was the correct model.
Remember that in this model, each replication cycle features the parental strands separating, and each directs the synthesis of a complementary daughter strand.
Here the parental strands are blue, and the daughter strands are red.
Let's return to Figure 7.9 and interpret the results cycle by cycle.
DNA isolated after one replication cycle in the N-14 medium shows that all the DNA has the same density.
However, cycle 1 DNA has a slightly lighter density compared to Generation 0 DNA.
The first cycle DNA is ON15/N14 or hybrid DNA, with N-15 in the parental strands, and the lighter N-14 in the daughter strands.
This overall result is consistent with the semiconservative and dispersive models of replication, but inconsistent with the conservative model. Replication cycle 2 DNA contains molecules of two different densities.
The densitometry scan shows one peak for each density, and because the peaks are of equal heights, it indicates that both are present in equal concentrations.
One density is hybrid -- that is, containing both N-15 and N-14 -- and the other is light, or N14/N14, meaning that both strands contain N-14.
This result is consistent with the semiconservative model, but not the dispersive model.
Replication cycle 3 DNA again contains molecules of two different densities, but this time the heights of densitometry peaks are unequal, indicating that the lighter molecule is in greater concentration than the heavier molecule.
The heavier molecule is hybrid DNA, and the lighter molecule is like DNA.
Ratio of light to hybrid DNA is 3 to 1, and this result remains consistent with the semiconservative model of DNA replication.
Through this experiment, known as the Meselson and Stahl experiment, the researchers conclusively demonstrate that the predictions of the semiconservative model of DNA replication are observed.
Furthermore, the results reject both the conservative and dispersive models of DNA replication.