Hi. In this video, we're going to be talking about DNA as the genetic material. So, DNA 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 whatever molecule is the 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 4 nucleotides could store that intense amount of information that is needed to encode a human, for instance. It has to be able to transmit that information, both to 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 must evolve with mutation and variation, and people just didn't think that 4 nucleotides could really confer this much ability to DNA 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, there are 20 amino acids. And there were so many more than just 4 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. It also didn't help that at the time there was this hypothesis called the tetranucleotide hypothesis, and it just stated that the DNA nucleotides, all 4 of them, were just repeated over and over: ACGT, ACGT, and on and on. And this was really what DNA was thought to be at the time; they didn't know why it was there, it was structural, it was something, but it wasn't the genetic material.
But around that time, in the 1940s and later, there were a number of experiments that began to prove that DNA was the genetic material. So the first one we're going to talk about is the Oswald Avery, MacLeod and McCarty experiments in 1944. 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 3 types of viruses. Right? And the virus that would normally be infectious but is non-infectious is called the heat-inactivated virus because when you put that infectious virus and turned up the heat, it became noninfectious. It ruined something in the virus that made it noninfectious. So what they did is they infected mice with various combinations of these. And so, this experiment is here. Right? So you infect it with a non-infectious, and the mouse lives. You infect it with an infectious, and the mouse dies because it's infectious. You infect it with a heat-inactivated infectious, so now it's non-infectious, right, because it's been heat-inactivated. The mouse lives 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, this was really shocking. And they were completely surprised why would two noninfectious viruses actually confer infection. And so they were saying, okay, well, what's the source of this infection? Is it protein? Is it fat? Is it RNA? Is it DNA that transformed that heat-inactivated virus into an infectious virus. And they did some experiments to figure this out, and they figured out it was indeed DNA that caused these mice to die. That was an important experiment.
Another experiment occurred in 1952, and this is the Hershey-Chase experiment. So this actually worked with bacteriophages. These are viruses that infect bacteria, and what they did is they took the bacteriophage and they labeled the protein and they labeled the DNA with different radioisotopes. So here we have our bacteriophage. This is the protein coat, it's labeled in red, and here we have the DNA that's labeled in green, and then they infected bacteria with these labeled viruses. And they saw then they looked at the bacteria, and they said, okay, well, what's getting into the bacteria? Is it the red protein, or is it the green DNA? And you can see here that they found that none of the red protein got in, but the green DNA did. So this suggested that, you know, at least to infect, for viruses, that DNA was what was being injected into the bacteria genetic material. And so after these experiments, people were really believing, okay, DNA 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 can replicate, how can it be passed on to offspring, how it stores information.
And so this is where Watson, Crick, Franklin, and Wilkins come in, and they are the people who discovered the 3D structure of DNA. Now I'm sure you've heard of Watson and Crick, but Franklin here was super important to, helping them actually, figure it out and she didn't, and 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. What Franklin did is she used X-ray diffraction, and this is a process that beams X-rays at a molecule. So in this case, they were using DNA, and when you beam X-rays at it, it deflects those X-rays. They made the DNA such that it would deflect those X-rays onto a screen, and when you get the screen, it presents this picture. It's almost the shadow of DNA, 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 at DNA, it reflects a shadow of the DNA, and they can use that shadow now to determine what the structure is. So Rosalind Franklin was the first to do this for DNA, 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 have this data that sort of proved that DNA was a double helix, they were able to come up with this 3D double helix model based on Rosalind Franklin's data that was given without her knowledge or permission. And, 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 DNA was the double helix. And, but usually, you hear about Watson and Crick and not Wilkins and Franklin, 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 couldn't have done it without Franklin. They came up with this model, 3D 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 like 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 she's an awesome scientist, important to know because she really led them to discover that it was this 3D double helix structure. And so by this time, everyone is on board, you know, DNA is the genetic material, not protein. So with that, let's now move on.
