Hello, everyone. In this lesson, we are going to be talking about DNA replication. So DNA replication has only relatively recently been understood. Before we actually knew exactly how DNA replicated, there were going to be three different hypotheses on how DNA double-stranded helices actually replicated themselves whenever the cell is going to divide. So before replication was incompletely understood, there were these three ideas about how DNA was replicated. And those three ideas are going to be the Conservative Replication Hypothesis, the Dispersive Replication Hypothesis, and the Semi-Conservative Replication Hypothesis.
Now, first off, let's start with the Conservative Replication Hypothesis. Basically, this hypothesis is saying that the old strand of DNA is conserved, and a new strand of DNA is made. The old double helix is conserved, and the new double helix is made. So the old double helix is going to separate and be utilized as a template to create the new double helix. And then those old strands of DNA are going to join back together to recreate or conserve the old double helix, and then the new strands of DNA are going to form a new double helix. So after replication, there is one old double helix and one new double helix. That is the conservative model. Basically, you can think of it as the old double helix is conserved and a new double helix is made.
Now the second one is going to be the Dispersive Replication Hypothesis. And this is basically stating that the old double helix separates, is utilized as a template to build these new strands of DNA, and then two double helices are made, and these are both going to be created from parts of the old and the new. You can kind of think of this as the old pieces of the double helix are dispersed throughout the new pieces of the double helix. I know this can be kind of confusing. But, basically, think of it as you have these two double helices, and old and new pieces of DNA are dispersed around these double helices. So after replication, the strands have some old sections and some new sections.
Now, the Semi-Conservative Replication Hypothesis is going to say that you have an old double helix that is going to separate, be utilized as the template for the new strands of DNA, and then you're going to create two double helices with one old strand and one new strand. So the old double helix is partially or semi-conserved in the two double helices. So after replication, each strand has one old strand and one new strand.
So now let's look at this visual representation because that's always easier. Okay. So we have semi-conservative at the top, conservative in the middle, and dispersive at the bottom. And just know that the original or old DNA is in red, and the newly synthesized DNA is in pink. So we have the old, and we have the new. In the semi-conservative replication model, you can see that each strand, or each double helix is made of one new strand and one old strand. Now, in the conservative model, you can see that the top double helix is made of two old strands. The old double helix was conserved, and the other double helix is made of two new strands.
Now we have the Dispersive model, which is always kind of interesting to look at, and you can see here that you have two double helix or double helices that are made of mixed DNA, old and new. So they are made of mixed DNA. And that's why it's dispersive because you have some old strands or some old components and some new components dispersed through these DNA double helices. Now, how did scientists figure out which of these models was the correct model? Which of these actually happened in biology, in living cells? How did DNA actually replicate?
Well, they narrowed it down by doing this experiment. The Meselson and Stahl experiment actually did an experiment to figure out which of these models it was. And they found out that it was the Semi-Conservative Replication Model. The Semi-Conservative Replication Hypothesis was the one that actually occurred during DNA replication. Now how did they figure this out? They're going to use E. coli bacteria because E. coli bacteria replicate very, very quickly, and they divide very, very quickly. And what they're going to do is they're going to take these e coli bacteria, and for a long time, they're going to grow them and allow them to replicate in this environment that only has heavy nitrogen or N15. So these bacteria are living in a petri dish, and all they have is nitrogen 15 to build their DNA. And nitrogen is a very important component of DNA.
So after a long time, these E. coli only have heavy DNA because all of their DNA has Nitrogen 15 in it. So, then what they're going to do is they're going to move the E. coli, and they're going to move it into another environment, another petri dish, another plate with normal nitrogen or light nitrogen. And this is N14. So, now, we have E. coli that have completely heavy DNA living in an environment where they only have light DNA. And these E. coli are going to begin to divide and grow like they normally do. But to build those new DNA strands that they have to build to replicate their DNA, they're only going to incorporate light nitrogen or N14 into their newly synthesized DNA strands.
This allowed Meselson and Stahl to track where the new DNA strands were being created because they could tell which ones were heavy, and they could tell which ones were light. So they could see which strands and how DNA replicated, and which strands were light, and which strands were heavy. So, the new strands, this is very important to understand this experiment, the new strands will have normal Nitrogen, and the old strands will have heavy Nitrogen. So they let these e coli go through one round of DNA replication. And with one round of DNA replication, there was only one band of a single mixed weight. So then they're going to let these E. Coli go through a second round of replication. So after two rounds of replication, there were two bands, one band of mixed weight, and one band of light weight. And this is going to lead them to believe that DNA in these E. Coli is replicating via semi-conservative replication.
So let me see if I can draw this out for you guys, just so you can understand what we're saying. So let's do the heavy Nitrogen in red, and let's do the light Nitrogen in blue. Okay. So, remember that all of these E. Coli are going to start off with DNA that is made of heavy nitrogen. It is going to be completely heavy nitrogen. And then, they're going to go through one round of DNA replication, but now they're in this light Nitrogen environment. So after one round of replication, what is going to be made? Well, we're going to have that heavy DNA double helix split in half and create two new double helices that are both made of heavy and light nitrogen strands. One old and one new strand because this is Semi-Conservative Replication. So, this is what's going to be made, and they have a heavy Nitrogen strand, and they have a light nitrogen strand. So this is what's happening right here.
After one round of replication, there was only one band of mixed weight. So they're saying that the DNA that was created had a mixed weight after one round of replication. All of the DNA had mixed weight. Well, that's because all of the DNA at this point, after one round of replication, is made of one old strand and one new strand. And then it's going to go through a second round of DNA replication. And what's going to happen? Well, you're going to get these mixed double helices to split apart, and then you're going to get the old strand is going to be used again to create a new strand. And then, the new strand is going to be utilized to make another new strand, and the same thing is going to happen for this other double, or the Double Helix over here. And then, we're going to get something different. Now, we have two double helices that are of mixed weight, and we have two double helices that are of light weight. And that is going to be dealing with the second round of replication that happens right here. You get two bands of mixed weight and of lightweight. So that is how they knew that this DNA replication process that is happening in these E. Coli is semi-conservative replication.
So now, let's go down and let's look at this other figure that I have. So these results suggested that this was semi-conservative replication. The reason that they know that it's not conservative or dispersive is because conservative replication would have one heavy band and one light band after a single round of replication. But that did not happen in their experiment, so it was not conservative replication. And dispersive replication would have one band of mixed weight after every round of replication. But remember, in their second round of replication, they have two bands. They don't have one mixed band. They have one mixed and one light. So they were able to rule out conservative and dispersive replication.
Now, this is another diagram, just a different one, to demonstrate the same experiment to you guys. I just want you all to have many different ways of seeing this experiment because it's a very famous experiment, a very important one to know. I'm sure you will be asked about it. So, this is just another representation of their same experiment. They had their heavy nitrogen that they grew the original e coli in, and then they had their light nitrogen where they grew the new strands of DNA. So, what happened? What are we looking at here? Well, here, we're looking at the density. So, as it gets farther down in the tube, it is more dense. We're talking about the DNA. So the farther down in the tube the DNA is, the more dense it is. So, after no rounds of replication, the e coli haven't replicated yet, we have very heavy DNA. And that's because it's all made of N15. Right? This is all N15 heavy DNA. So what's going to happen after one round of replication? Well, this is the same thing that I talked about above. After one round of replication, you have mixed weight. You guys can see that it gets less dense, and that's because this is mixed weight. It's made of N15 and N14. And that is because these DNA strands are now made of one old and one new strand of DNA. These double helices are semi-conservative, they are mixed. Then, after the second round of replication, which I talked about above as well, you're going to have one light band and one heavy band. And, you're going to see that all of these are made of N14 nitrogen, these are the light bands. And then you're going to have these right here be the N15s, the N14s that were made in the first round of replication. So we have our mixed band and our light band. And as the replications go on and on and on, and more DNA is being made, more light nitrogen DNA is being made overtime, you're going to get lighter and lighter DNA over time. So this is how they were able to determine which type of DNA replication was happening in living things. And they did find, because of this experiment where they measured the weight of the nitrogen inside of the DNA, they were able to determine that DNA replicates via semi-conservative replication.
Okay, everyone. Let's go on to our next topic.
