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Hershey and Chase: DNA is the genetic material

How Alfred Hershey and Martha Chase showed conclusively that DNA is the genetic material.

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Video transcript

- [Voiceover] In the last video we began to see some pretty good evidence that DNA was the molecular basis for inheritance and we saw that from the work of Avery, McCarthy and McLead where they tried to identify whether it was DNA or proteins that acted as a transformation principle in Griffith's experiments and I encourage you to watch that video if all of this sounds unfamiliar. But even their work in 1944 was not viewed as conclusive evidence. It was viewed as strong evidence, but not conclusive evidence because, remember how they did it, they took the heat killed smooth strain, the smooth strain you might remember from Griffith's experiment was the virulent one. The heat killed it. When you heat kill it in injected amounts, it didn't do anything to the mouse, but if you took the heat killed smooth strain and put it with the rough strain, it somehow transformed the rough strain in to the smooth strain, in to the virulent strain and so they took the heat killed smooth strain and they took out its different components and they eventually were able to isolate one that was able to transform the rough strain in to the smooth strain by itself and then they applied all sorts of chemical tests to it and said, "Hey, there's pretty good evidence "that this is DNA," but it wasn't conclusive because, well, maybe they didn't purify it properly or maybe there was still a little bit of protein. Maybe it was mostly DNA, but maybe it was a little bit of protein that was still left there that actually did the transformation. So the scientific community, they weren't just saying, "Hey, that looks pretty good, "Let's move on, let's just assume." They wanted to continue to test it and especially test it in different ways. And, the conclusive evidence didn't come until a few years later, until 1952 when Alfred Hershey and Martha Chase decided to study T2 bacterio phage. Let me write this down. T2 bacterio phage, this is phage that infects bacteria. Bacterio phage. When you hear the word phage, we're referring to viruses. Now they knew that T2 bacterio phage was composed of proteins and DNA and they didn't, well, we now know, that it's a protein shell and there's DNA inside, but they, from their point of view, they said, "Okay, it's made up and if we try to "look at the stuff that this virus is made of, "it's protein and DNA." So protein plus DNA, and they knew that this virus, when it infects bacteria, it injects something into that bacteria. So it injects something and that something is what hijacks that bacteria's genetic information to start producing more of the T2 bacterio phage. So they could identify the something that gets injected. If they could figure out if that something was either a protein or a virus, then they would have conclusively proven so sorry, if they could show that something was not protein or virus, if it was protein or DNA, if they could show that it was either protein or DNA, then they could show conclusively that it's either the protein or the DNA that forms the molecular basis and so they're actually quite sceptical of Avery, McCarthy and MacLead's experiments. They actually, Hershey and Chase, actually thought that they were gonna show that it was the protein, and remember, this whole time people were like, "Protein, we know it's these complex molecules "that have these different shapes "and all these different amino acids. "It seems like that's much more likely to encode "the complexity of genetic information than DNA." They didn't have an appreciation for the structure of DNA at this time. So they devised an experiment to figure out what is that something that the T2 bacterio phage is infecting. Is that something protein? Is it protein or DNA? So this is the question. So what they do is they take two batches or they developed two batches of T2 bacterio phage. One batch of the T2 bacterio phage they do it in the presence of radioactive phosphorus, phosphorus 32. In the other batch, I should say they grow that T2 bacterio phage in the process of another radioactive isotope, but this time it is of sulfur. This is sulfur 35. So why are they doing that? Well phosphorus is found in DNA. So in this first batch, the radioactive marker, you could say, is going to incorporate itself into the DNA. In the second batch, sulfur is found in the protein and not in the DNA and so this would actually tag the protein parts. And if you're wondering, well, how do you develop these radioactive batches, well, you let the viruses hijack cells in a medium that has either the radioactive sulfur or the radioactive phosphorus and as they reproduce, they are going to incorporate that radioactive material into either the protein or the DNA of the new viruses that get produced. So anyway, they were able to produce some of the T2 bacterio phage in the presence of the radioactive phosphorus and they knew that way that the DNA would get that radioactive material in it and then with the radioactive sulfur they said the protein would have that radioactive sulfur. And then for each of those batches, they then infected bacterio phage with them and they said, okay, they're going to inject something in to the bacterio phage, and to figure out what that something is that was injected in to the bacterio phage they take the products in either of the two scenarios, they first blend 'em up so that all of the stuff that's left outside gets taken off of the surface of the bacteria cells and then they stick it in to a centrifuge and the centrifuge is, you can imagine, it's kind of just a big spinning machine. If you were to take a test tube and take it sideways, one way to think about it, put it sideways like this. Put maybe a stopper in it so nothing leaks and then you spin it around really, really, really, really fast, what you're going to find, you can actually generate significant g forces and so the heavier stuff is going to gravitate to the bottom of the test tube or to the right when it is on its side, and the lighter stuff is going to gravitate to the left. And, it turns out that the bacteria, the actual bacterial cells, those are heavier so the bacterial cells are going to go towards the bottom of the test tube and they're gonna form a material that we call the pellet and then all of the other stuff, all of the fluid and the leftover phage parts those are going to go up to the top of the test tube and we call that the supernatent. I always have trouble pronouncing that. Supernatent. And so they said, "Look, if we look at the pellet," which they knew had the bacterial cells int here or you could even say the remnants of the bacterial cells, "if the pellet here "contains phosphorus, that means that the DNA, "our radioactive DNA or our tagged DNA "made it in to the bacteria, "but if it contains sulfur, that means that the protein "made it in to the bacteria." And, what they found is they found that the radioactive phosphorus was in the pellet which allowed them to conclude that, hey, it's the DNA from the virus that made it inside of the bacteria and not the protein and then they said, "Well, it must be! "Wow, the Avery, McCarthy and Maclead were correct. "It's actually the DNA that is this transformation principle "that can go in and hijack the genetic, "the machinery of the bacteria "to produce more of the actual virus, "so this is a really, really, really big deal." Once again, we started with Mendel saying, "Hey, we have these inheritable factors "and they seem to segregate and sort in certain ways. "They seem to be discrete." Bover and Sutton said, "Hey, "chromosomes seem to kind of, "their behavior during meiosis "when cells split "seem to kind of match up to that." Morgan starts to provide some evidence. We have Griffith's experiments with the mice and the bacteria and saying, "Hey, look, "there's some transformation principle." Avery, McCarthy and MacLeod say, "Hey, "looks like when we try to really purify "this transformation principle, "it seems like DNA is what really matters." And then Hershey and Chase validate that even more conclusively.