Semiconservative Replication

Hello everyone in this lesson, we are going to be talking about D. N. A replication. So DNA replication has only relatively recently been understood and before we actually knew exactly how DNA replicated there were go going to be three different hypotheses on how D. N. A. Double stranded helix sees actually replicated themselves whenever the cell is going to divide. So before replication was completely understood, there were these three ideas about how DNA was replicated and those three ideas are going to be the conservative replication hypothesis, the disperse it replication hypothesis and the semi conservative replication hypothesis. Now first off, let's start with the conservative replication hypothesis. So basically this hypothesis is saying that the old strand of D. N. A. 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 D. N. A. 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 disperse it replication hypothesis. And this is basically stating that the old double helix separates is utilized as a template to build these new strands of D. N. A. And then to double helix is 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 and I'll show you visual representation of what I'm talking about in just a second. I know this can be kind of confusing but basically think of it as you have these two double helix is and old and new pieces of DNA are dispersed around these double helix is. So after replication the strands have some old sections and some new sections. Now 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 D. N. A. And then you're going to create two double helix is with one old strand and one new strand. So the old double helix is partially or semi conserved in the two. He'll see. 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 disperse it at the bottom. And just know that the original or old D. N. A. Is in red and the newly synthesized D. N. A. Is in pink. So we have the old and we have the new, So if you look at 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, so the old double helix was conserved and the other double helix is made of two new strands. I'm going to go out of the picture here so I can right next to the disperse it model. So now we have the dispersing model which is always kind of interesting to look at. And you can see here that you have to double helix or double helix is that are made of mixed DNA old and new. So they are made of mixed D. N. A. And that's why it's disperse it because you have some old strands or some old components and some new components dispersed through these DNA. Double helix is. So this is the disperse it model now, how did scientists figure out which of these models was the correct model which of these actual, it happened in biology in living cells. How did DNA actually replicate? Well they narrowed it down by doing this experiment and Meselson and stall 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? Well they're going to use E. Coli bacteria because 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 in 15. So these bacteria are living in a Petri dish and all they have is nitrogen 15. To build their D. N. A. And nitrogen is a very important component of D. N. A. So after a long time these E coli only have heavy D. N. A. Because all of their D. N. A. 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 in 14. So now we have E. Coli that have completely heavy D. N. A. Living in an environment where they only have light D. N. A. And these E. Coli are going to begin to divide and grow like they normally do. But to build those new D. N. A. Strands that they have to build to replicate their D. N. A. They're only going to incorporate light nitrogen or in 14 into their newly synthesized DNA strands. This allowed Meselson install to track where the new D. N. A 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 in 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 D. N. A 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 equal, I go through a second round of replications 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 D. N. A. 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 D. N. A. 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, while we're going to have that heavy D. N. A double helix split in half and create two new double helix sees 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 fans 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 helix is 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 helix or the double helix over here and then we're going to get something different. Now we have to double helix is that are of mixed weight and we have to double helix, is that our of lightweight 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 light weight. 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 disperse it 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 experiments, it was not conservative replication and disperse its 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 disperse it 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. Very important one to know, I'm sure you will be asked about it. So, this is just another representation of their same experiments. They had there heavy nitrogen that they grew the original cola in and then they had their light nitrogen where they grew the new strands of D. N. A. 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 D N A. So the farther down in the tube, the D N A. 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 in 15. Right? This is all in 15, 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 Application, you have mixed wait, you guys can see that it gets less dense and that's because this is mixed weight, it's made of in 15 and in 14. And that is because these DNA strands are now made of one old and one new strand of D N A. These double helix is our 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 in 14 nitrogen these are the light bands. And then you're going to have these right here, be the in 15s, The in 14's 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 over time, you're going to get lighter and lighter D. N. A. 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 D N. A. They were able to determine that D. N. A replicates via semi conservative replication. Okay, everyone, let's go on to our next topic.