In this video, we're going to introduce the Meselson-Stahl experiment. After the structure of DNA was revealed by James Watson and Francis Crick, scientists across the world began to wonder how this DNA genetic material is actually replicated and passed on from one generation of cells to the next generation of cells. In 1958, the scientists Matthew Meselson and Franklin Stahl demonstrated that the specific strain of bacteria called Escherichia coli or E. coli will actually replicate its own DNA via a specific model known as the semi-conservative model.
The semi-conservative model of DNA replication basically says that replicated DNA molecules consist of one old or parental DNA strand and one brand newly built DNA strand. It turns out that the old parental strands in a DNA molecule, in the original DNA molecule, during DNA replication, will separate from each other and act as templates in order for the synthesis of brand newly built DNA. This new DNA that's built is going to be complementary to the old parental strands. We'll be able to talk more details about the mechanism of semi-conservative DNA replication and go more in-depth into this line of text here later in our course. But for now, what we're going to do is focus on the Meselson-Stahl experiment and how the Meselson-Stahl experiment confirmed semi-conservative DNA replication.
If we take a look at our image down below, the first thing that you notice is over here on the left-hand side, we have this little key. The darker blue bands that you see represent a specific DNA strand that's labeled using nitrogen-15 isotope or N15. The N15 isotope is represented using this darker blue color, and it is used to label DNA strands, specifically the old or parental DNA strands, the original DNA strand. There's another isotope of nitrogen, the N14, and the N14 is going to be represented using this light blue color. The N14 will also label DNA strands, but it's going to be used specifically to label the brand newly built DNA strands.
In this experiment, there were really three competing models for how DNA replicated. At the time, scientists did not know which of these three models was the correct model for how DNA replicates. These three models were the conservative model of DNA replication, which is represented here in the first row. The second model is the model that turned out to be the correct model, which is the semi-conservative model of DNA replication, represented here in the middle row. The third competing model was the dispersive model, and that's repres...
