Meselson-Stahl Experiment

by Jason Amores Sumpter
417 views
4
Was this helpful ?
4
in this video, we're going to introduce the Meselson Stahl experiment. And so after the structure of DNA was revealed by James Watson and Francis Crick, scientists across the world began toe wonder. How is it that this DNA genetic material is actually replicated and passed on from one generation of cells down to the next generation of cells? And so, in 1958 the scientists Matthew Meselson and Franklin Stall demonstrated that the specific strain of bacteria called S Curricula coli or E. Coli will actually replicate its own DNA via a specific model that's known as the semi conservative model. And so the semi conservative model of DNA replication basically says that replicated DNA molecules consist or have one old or parental DNA strand and one brand, newly built DNA strand. And so it turns out that the old parental strands in a DNA molecule in the original DNA molecule during DNA replication, they will separate from each other and act as templates in order for the synthesis of brand newly built DNA. And that new DNA that's built is gonna be complementary to the old parental strands. And so we'll be able to talk more details about the mechanism of semi conservative DNA replication and go Maurin depth into this line of text here later in our course. But for now, what we're gonna do is focus on the measles install experiment and how the measles install experiment confirmed semi conservative DNA replication. And so if we take a look at our image down below, the first thing you should notice is over here on the left hand side, we have this little key. And so the darker blue bands that you see, the darker blue strands that you see throughout represent ah, specific DNA strand that's labeled using, uh, nitrogen 15 isotope or n 15. And so the n 15 isotope is again gonna be represented using this darker blue color and the end 15 isotope is usedto label DNA strands specifically the old or parental DNA strands the original DNA strand. And then there's another isotope of nitrogen. It e n 14, and the end 14 is gonna be represented using this light blue color, and the end 14 will also labeled DNA strands. But it's going to be used specifically toe label the brand newly built DNA strands. So basically the dark blue here is going to represent old or parental DNA strands from the original molecule, and the light blue strands are going to represent brand newly built DNA strands. And so in this experiment there were really three competing models for how DNA replicated. And at the time scientists did not know which of these three models was the correct model for how DNA replicates. And these three models were the conservative model of DNA replication, which is being represented here. In this first wrote, The second model is the model that turned out to be the correct model, which is the semi conservative model of DNA replication, and that's represented here in the middle row. And then the third model competing model was the Disperse It model, and that's represented here in this last row. Now again, Meselson install confirm that of these three models, it's actually the semi conservative model that is the correct model, and that's why we have this green check mark over here to confirm that Hey, the semi conservative model is the correct model, and it turns out that the conservative model and the disperse of model are not correct models for how DNA replication actually proceeds. And so that's why we have these Big Red X is over here on the far right to show you that this was not the result that they got. And so, in this experiment, what they did was again. They took these E. Coli, these bacteria, and they originally started the E. Coli grown in nitrogen isotope. And so in When these bacteria were grown in nitrogen 15 isotope, they used that nitrogen 15 to build their DNA molecules. And so all of the DNA molecules start off with nitrogen. 15 labeled DNA in the very, very beginning. Okay? And so then what they did was they took the E. Coli that were grown in nitrogen 15, and they suddenly switched it over to some media that had nitrogen 14 or n 14 media. And so basically, the old original strands would always be labeled with nitrogen. 15, as we mentioned over here, and the brand newly DNA strands that are grown in nitrogen, 14 are going to be labeled with nitrogen 14 and will be light blue color. And so the conservative model of DNA replication again, which is not the correct model. It basically suggested that the DNA replication was conservative and that the original DNA strand DNA molecule would always be conserved. That's why it's called the conservative model. And so what you'll notice is that this original DNA molecule remains conserved and new molecules are going to have brand new strands with nitrogen. 15. However, again, this is not the result that Meselson installed got. They did not get results that suggested a conservative model. Now the semi conservative model semi is a route that means partial. That means that it's partially conservative and that when DNA molecules replicate, uh, half of the molecule is going to be consisted of an old older strand, and the other half is going to consist of a new strain. And so what happens is these two strands are going to separate from one another as you see here, and a brand new strand will be built next to it, using it as a template. And again, we'll get to talk more about this specific mechanism of semi conservative DNA replication as we move forward in our course. And so if you grow another round again, what happens is each of these strands is going to separate from one another and new strands will be built. And so this is the result that they got. This result here was consistent with the actual results that they've got. And so again, the semi conservative model is the main model that you should be familiar with and know that DNA replicates to be of the semi conservative model. Now the disperse of model again. This is one that they proved to be incorrect and that basically says that the DNA molecule, the original DNA molecule, would be dispersed throughout the new molecules that were created. And so again, this was not the result that they got. This was not consistent with their results of the disperse of model was also not the result that they got. And so again, the main model that you want to be focusing on in this image is going to be the semi conservative model of DNA replication. And again, the biggest take away here is that semi conservative model of replication says that DNA molecules, when they're replicated, will have one old parental strand and one brand newly built strand. And we'll get to talk Mawr and Mawr about semi conservative DNA replication and the mechanism about how it works as we move forward in our course. But for now, this here concludes our introduction to the Meselson Stahl experiment and semi conservative DNA replication model, and I'll see you all in our next video.