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7. DNA and Chromosome Structure

DNA as the Genetic Material


History and Experiments

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Hi in this video we're gonna be talking about D. N. A. As the genetic material. So D. N. A. Actually wasn't always thought of as the genetic material. A lot of people didn't really know what its purpose was and they just thought it was sort of a structural filler or something that was much less important than what it actually is. And this is because the B genetic material, whatever molecule is that genetic material has to have certain qualities, right? It has to be able to store information. And a lot of people just didn't believe that only four nucleotides could store that, you know, intense amount of information that is needed to encode a human. For instance, it has to be able to transmit that information both to you know, an individual cell but also to the next generation it has to be able to replicate itself often with little errors and it has to be able to change over time. It has to be able to evolve mutation and variation. And people just didn't think that four nucleotides could really confer this much ability to D. N. A. To allow it to be the genetic material. It also didn't help that. Before the 1940s, many people believed proteins were the source of genetic material. There were so many more amino acids, right? I mean these 20 amino acids and there were so many more than just four nucleotides. And so it was believed that proteins were really the source of that genetic material and could have that diversity that would allow something to create a human and it also didn't help that at the time, there was this hypothesis called the Tetra nucleotide hypothesis. And it just stated that the DNA nucleotides, all four of them were just repeated over and over and over and over again. So that kind of looked like this, right? Like a C. G. T. A C G. T. And on and on and on. Um And this was really what at the time what D. N. A. Was thought to really be and they didn't know why it was there, it was structural, it was something that it wasn't the genetic material But around that time or in the 1940s. And later there were a number of experiments that begin to prove that DNA was the genetic material. So the first one we're going to talk about is the Oswald avery Maglione and McCartney experiments in 1944. And so what they did is they had viruses, they had a non infectious virus. They had an infectious virus and they had the ability to turn that infectious virus into a non infectious virus. So they had three types of viruses, right? And the virus that would normally be infectious but is non infectious. It's called the heat inactivated virus. Because when you put that infectious virus and turned up the heat, it became non infectious. It like ruined something in the virus that made it non infectious. So what they did is they infected mice with various combinations of these. And so this experiment is here. Right So you infected with a non infectious and the mouse lips. You infected with an infectious and the mouse the ice because it's infectious you infect it with a heat and activated infectious. So now it's non infectious right? Because it's been heat and activated the mouse lifts because it's been inactivated it's an inactivated virus. But if you infect it with a non infectious and the heat inactivated so both should be non infectious independently. Right? What happened is the mouse still died? And so um this was really shocking and they were completely surprised why would two non infectious viruses actually um Confer infection. And so they were saying okay well what's the source of this infection? Is that protein is that fat is an R. N. A. Is A. D. N. A. That transformed that he did not the virus into a infectious virus. And they did some experiments to figure this out and they figured out it was indeed D. N. A. That caused this these nice to die. That was a big important experiment. Another experiment occurred in 1952. This is the Hershey Chase experiment. So this actually worked with bacterial pages. These are viruses that infect bacteria. And what they did is they took the bacteria page and they labeled the protein and they labeled the D. N. A. With different radioisotopes. So here we have our bacteria pages. This is the protein coat. It's labeled in red. And here we have the D. N. A. That's labeled in green. And then they infected bacteria with these label viruses. And they saw then they looked at the bacteria and they said okay well what's getting into the bacteria? Is that the red protein or is it the green D. N. A. And you can see here that they found that you know none of the red protein got in. But the green D. N. A. Did. So this suggested that you know to at least to infect for viruses That D. N. A. Was what was being injected into the bacteria that would be used to create more viruses. So this was suggesting that D. N. A. Was indeed the genetic material. And so after these experiments um people were really believing okay D. N. A. Is the genetic material but there wasn't really a good idea of what its structure was or how it could do all these things that I said before how it could replicate how it can be passed on to offspring, how it stores information. And so this is where Watson, Crick, franklin and Wilkins comes in and they are the people who discover the three D. Structure of D. N. A. Now I'm sure you've heard of Watson and Crick but franklin here was super important to helping them figured out and she didn't Rosalind franklin ended up not receiving the nobel prize even though the other three men did because she had passed away before it was given. So she didn't really get the fame or the credit for it even though she really deserves it. So what franklin did is she used X ray diffraction and this is a process that beams x rays and a molecule. So in this case they were using D. N. A. And when you beam x rays at it it deflects those X rays they've labeled, they made the D. N. A. Such that it will deflect those X rays onto a screen. And when you get the screen it presents this like picture, it's almost the shadow of D. N. A. And you they use a bunch of math to be able to do that. It's way beyond this class. I don't even know the math to do it. But essentially X ray diffraction shoots those beams that D. N. A. It reflects the shadow of the D. N. A. And they can use that shadow and math to determine what the structure is. So Rosalind franklin was the first to do this for D. N. A. And her scientific partner was Wilkins. And when Rosalind franklin got her data she was really excited about it, I'm sure. And without her knowing her partner Wilkins actually showed that information to Watson and Crick. So she had no idea that her data was being shown to other people. And when it was Watson and Crick saw that data and they were astute individuals and now that they had this data that sort of proved that DNA was a double helix. They were able to come up with this three D double helix model based on Rosalind franklin's data that was given without her knowledge or permission. And um so Watson and Crick ended up publishing a paper and so did Wilkins and franklin at the same time published two separate papers showing that the D. N. A. Was the double helix. And but usually you hear about Watson and Craig and not Wilkins and franklin um in part because she was a female and it wasn't very accepted at that time for her to be doing this type of research. And so Watson and Crick are given the credit for it but they both really discovered it and Watson and Crick couldn't have done it without franklin. And so Watson, Crick and Wilkins got the Nobel prize for this but franklin had passed away from cancer before the Nobel prize is awarded and the Nobel prize isn't awarded to someone who's dead, so she didn't get it. But even though she probably deserved it. But anyway so they came up with this model, three D double helix model and Watson and Crick came up with this literally just by playing with models kind of like you do in your chemistry classes with those plastic models of different molecules that's exactly what they had and they created this huge structure with these plastic models that they made themselves and figured out that DNA must be a double helix. But again, Rosalind franklin, She's an awesome scientist important to know because she really led them to discover that it was this three D. Double helix structure. And so by this time everyone is on board, you know, D. N. A. Is the genetic material, not protein. So with that let's now move on.

The tetranucleotide hypothesis stated what?


The Avery, Macleod, and McCarty experiment in 1944 figured out what?


The Hershey/Chase experiments in 1952 use bacteriophages to study what?