Biology: Avery, MacLeod & McCarty/Hershey & Chase

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When Griffith showed the world that there was this thing inside of bacteria that was a transforming principle. The world knew it was on the track. You know, here it was. This material that could leave one bacterium, enter another one and transform it. It was the genetic material. It was... Well nobody knew what it was. See, that's the thing. What was it? It was clearly the genetic material, but was it DNA, was it protein, or was it something nobody had figured out yet. But the race was on. Once you knew what the genetic material was, now the door was open to figuring out how it works, because you're not going to figure out how something works until you know what it is.
And so we come to 1944. In 1944, a team, Avery, MacCleod and McCarty. 1944, Avery and all his buds did an experiment. And what Avery, MacCleod and McCarty did was they were the ones that finally broke what the transforming principle was. Let's see how they did it. What they did was they took these bacteria, remember the rough and smooth bacteria. And in taking the rough and smooth bacteria, you can take these things and you can actually boil them up and remove chemicals from them. So they took bacteria very much like Griffith's bacteria, and they would remove chemicals. And the way they would do that is they would digest everything else.
So they went looking for the transforming principle. So you take a bacterium. You take that bacterium and digest everything except one thing. So leave let's just say some protein. So we'll do one gross preparation here, and leave all the proteins, because we know that transforming principle is a protein. We're not sure which one, so let's just leave all of the proteins. You know what they found? No transformation. Now let's try it again. Proteins, we know it's proteins. No, no transformation. And you know what they found? You know what they found. They found that when they left the nucleic acids, they got transformation. Over and over again they got transformation. They got to the point where they would purify DNA in its purist form and get transformation. Think about it. Is that overwhelming evidence? It is to me. But it wasn't to a lot of people.
Still, much of the scientific community was convinced that DNA could not be the transforming principle. DNA was too simple to carry those abilities that the genetic material needs. It had four nucleotides. It was too simple. It wasn't as complex, and nobody knew about its shape. Anyway, but it couldn't have been as complex as proteins.
1952, in 1952, some major things happened on this planet. I was born. That was a very important year to me, but probably a little more important was the work of Hershey and Chase. Whenever I say that I was born in 1952, it's the worst thing I can do to my students, because they all stop paying attention to figure out how old I am. So now that you've done that, let's move on and talk about what Hershey and Chase did. Hershey and Chase worked with a thing called a bacteriophage. Now what is a bacteriophage? Is that a form of bacteria? No, did you ever wonder if a bacterium could get sick? I know bacteria make you sick. If you've ever had pneumonia or strep throat, you've been infected by bacteria. Well bacteria, the good news is, can get sick. And they get sick and dead from a, I guess you would call it a parasitoid of some kind. It's something that enters the bacterial cell and literally destroys it so it can make its own young. It kind of sounds like something out of a movie about aliens doesn't it? How cool is this? It sounds just like that. And you know what? These things exist. The good news is bacteria get infected by them. The bad news is so do you.
What are they called? They're called viruses. Yes, that's how a virus works. Let's take a look at how a virus works. A virus that infects a bacterium is called a bacteriophage. And just so you know the lingo, we refer to them as phages. So when anyone walks up to you and starts talking about a phage, you know that's what they're talking about, something that infects a bacterium. Here's the thing about a bacteriophage. If you take a look at a bacteriophage, you'll notice that they have a lot of different shapes. This particular bacteriophage, which is called a T2 bacteriophage, has a shape like a rocket. But it's not its shape that intrigued Hershey and Chase. This baby is the perfect tool to solve the problem. Why? Well let me explain to you what these phages do, and then you'll see why it's the perfect tool.
Phages infect bacteria. We said that. Here's an E. coil. E. coli is a bacterium that's everywhere. It's all over your body. It's in your body. It's a common bacterium. We use it experimentally all of the time. What the phages do is a phage will land on a bacterial cell. And before you know it, that bacterial cell has a bunch of young ones inside of it, but they're not young bacteria. This whole idea of young and old doesn't really cut it with phages, but anyway. What happens is you get this reproduction. Now think about it. That phage must have somehow gotten its genetic material into that bacterium. It must have somehow, in the same way that rough is transformed by smooth, it must somehow be transforming the genetic material of this bacteria and making it make viruses, making it make phages, because the next step is kind of ugly for the bacteria. It bursts. And out swim new phages to go on and infect other bacteria.
Back to this. Why are these the perfect tool to study and learn which is the genetic material? And it has to do with what a phage is made out of. It's made out of two chemicals, and two chemicals only. Do you want to guess what they are? I'll give you a hint. One of them is DNA. What do you think the other one is? Protein. The outside of a phage is protein. So if it only has protein, and it only has DNA, and it's injecting something somehow, some transforming factor into that bacterium, it can only be one of two things, DNA or protein. So if we could figure out which one it's shooting in there, we got it, at least in bacteriophages. We got it. We got the genetic material.
But how can you follow a molecule? You know the answer to this one. You can label molecules, you know that. So let's label these, but wait. We've got to label it with something that one has and the other one doesn't. What do you know about proteins? You know that proteins have sulfur in them. And you know that DNA has phosphorous in it. So let's grow our phages up in a special medium. And let's take those phages and let's label them up. And let's put in this growing medium, let's put some radioactive sulfur, if you will. It's going to be a radioisotope. And what it's going to do is it's going to be S35. And it's going to get into what? It's going to get into the proteins. So as these phages grow in here. They're going to have S35 in their protein coat. Now let's also label the DNA with radioactive phosphorous. So let's do 32P. And now you know that this very same phage is going to have radioactive phosphorous in it.
Now I'm going to ask you a question. I want you to predict something. If we take a look at the way these phages infect, what do you expect? If this phage lands on here, if DNA is the genetic material, what do you expect from the young viruses or the young phages? So when they pop out of there, what do you expect them to have if DNA is the genetic material? And what do you expect them to have if protein is the genetic material, because both of them are not? Let's see what they found.
Here's what they found. What they found was that when that bacteria burst open, what they found was that the phages that came out had radioactive DNA. Indeed, when these young phages came out, they had incorporated the DNA of their parent phage. So the DNA had gone into them. The DNA had gone into the bacteria. The DNA was transmitted. The DNA was the genetic material. This was radioactive phosphorous. It was the P32.
Well, one last thing, where did the radioactive sulfur end up? It stayed right behind. It never was injected into the cell. The sulfur stayed behind. The phosphorous went in. DNA is the genetic material.
Molecular Genetics
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Avery, MacCleod and McCarty/Hershey and Chase: DNA Wins! Page [1 of 2]