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Current time:0:00Total duration:10:16

Evidence that DNA is genetic material 2

Video transcript

- [Voiceover] We spoke about the experiments of Friedrich Miescher and Wilhelm Roux and how their experiments helped show that genetic material is made up of either protein or nucleic acid or perhaps both. The next logical question to ask is, "Well, which one is it? "Is it protein? "Is it nucleic acid? "Or perhaps it's both?" This question was answered with the experiment of two scientists, Alfred Hershey and Martha Chase. In their famous Hershey-Chase experiment, that was published in 1952, they showed that it's nucleic acid that's genetic material, and not protein. Hershey and Chase worked with bacteriophages. A bacteriophage is a virus that specifically infects bacterial cells. It can also be referred to simply as a "phage." What is a bacteriophage? Well, it has nucleic acid, which can be either DNA or RNA. In this video, I'm just gonna refer to all nucleic acid as DNA, but keep in mind that, when I say that, it can also mean RNA, because in some viruses, the nucleic acid is RNA. That nucleic acid, or DNA, is surrounded by a protein coat. How do bacteriophages infect bacterial cells? Well, they get kind of close to the bacterial cell and sort of sit on top of it and inject their DNA into the bacterial cell. Then, their DNA gets integrated into the bacterial cells' DNA, which I'm gonna draw right here. Here's the bacterial cell's DNA. The virus's DNA gets somehow integrated, and now this bacterial cell's going to produce a whole bunch of viruses. This is some background information. Now let's talk about Hershey and Chase's actual experiment. Hershey and Chase took some phages and they put these pages in a medium, which means a broth that has a lot of nutrients so that these phages can now multiply and reproduce and make a lot more of themselves. But they wanted to label the protein code of the new generation of viruses. How were they gonna do that? They made sure that all of the amino acids that were in this broth had in them radioactive sulfur. They were labeled with sulfur-35, which is one of the radioactive isotopes of sulfur. In this way, they can keep track of where the protein's going and what's happening with the protein. The reason that they chose sulfur for this part of the experiment is because they wanted to label the protein, in particular, and sulfur is found in amino acids, which means that finding proteins with sulfur is not found in DNA. So this is a good way to make sure that they label the protein but not the DNA. Now, when this phage, or the phages that are put into the medium, reproduce, they're gonna take nutrients from this broth. Among those nutrients are amino acids, and they're going to incorporate those labeled amino acids into their protein coats. So they produced a generation of viruses that have radioactively-labeled protein coats. Now they allow this generation of viruses to infect a bacterial cell. You can see, they climb on top of the bacterial cell and inject their DNA into the cell. Remember, their DNA gets incorporated into the DNA of the bacterial cell. The bacterial cell will produce a whole bunch of viruses. Now, take note that the original protein coats, of course, remain outside of the cell. Hershey and Chase now wanted to separate the protein coats from the bacterial cells. So they centrifuged this mixture to get rid of the protein coats. Let's get rid of them. Notice how the phages inside this bacterial cell, they just have a regular protein coat that's not radioactively-labeled with s-35. That's why they're drawn in green. Now what they did was, they took these bacterial cells, lysed them, which means they made them burst, and they analyzed the viruses. They saw that the viruses were not at all radioactively-labeled. So they concluded that the protein coat must have remained outside the cell, outside the bacterial cell. If the protein coat remained outside of the bacterial cell, then it must be that that is not genetic material, because, in order for this bacterial cell to have produced viruses, it had to have contained the genetic material of the virus. If the protein coat remained outside, it must be that that is not the genetic material. Let's talk about the second part of Hershey and Chase's experiment. Again, they took a phage, or a couple of phages, and put it in a medium with a lot of nutrients so that it can reproduce and make lots of viruses. But this time, they wanted to label not the protein coat but the nucleic acid inside. Again, I'm just gonna refer to nucleic acid as DNA, but keep in mind that it can also be RNA. They wanna label the DNA, and how are they gonna do that? They made sure that all of the nucleotides that were in the broth ... and the viruses are gonna need nucleotides to make their DNA. All of them were radioactively labeled with phosphorus-32. P-32 is radioactive isotope of phosphorus, and this is how they're gonna label the DNA. The reason they chose phosphorus-32 is because phosphorus is found in DNA, in nucleotides, but it is not found in amino acids. So it will not get integrated into the protein coat. It's a great way to differentiate between the two. They put them in the broth in a lab and to reproduce, and they produce a generation of viruses that have this radioactively-labeled DNA, which I drew in that magenta. Of course, the protein coat is in green because it's not labeled in any way. They allowed these viruses, these phages, to infect the bacterial cell. The protein coat remains outside and it injects the DNA into the bacterial cell, and the bacterial cell is going to produce a whole bunch of phages. The protein coats, of course, remain outside. Now they're gonna centrifuge this mixture of protein coats and bacterial cells because they want to get rid of the protein coats. So they spin it in a centrifuge and the protein coats, which are less than (mumbles), will end up in a supernatant and the bacterial cells, which are heavier, end up in the pellet. We're gonna get rid ... Sorry about that. We're gonna get rid of these protein coats. Now Hershey and Chase lysed these bacterial cells to make them burst and they analyze the viruses inside. The viruses have a lot of nucleotides with this p-32, so there's a lot of radioactively-labeled DNA. Maybe not all the DNA had radioactively-labeled phosphorus, because some of the nutrients came from the bacterial cell, but a fair number of the viruses had radioactively-labeled DNA inside of them. They concluded that, since the DNA entered the cell, it must be DNA that's genetic material. Or, it should really be more specific. It's nucleic acid that's genetic material. In order for the bacterial cell to have produced viruses, it had to have inside of it genetic material. Since there's this radioactively-labeled DNA within the viruses, they concluded that nucleic acid is genetic material. We mentioned that Hershey and Chase published the findings of their experiment in 1952. A very, very short while later, in 1953, James Watson and Francis Crick published their famous paper in which they actually identified the structure of DNA, or nucleic acids. They put together a tremendous amount of research that was happening during their time and before their time and they identified the structure of DNA. They told us that DNA is a double-stranded helix with a sugar-phosphate backbone, the sugar in this case being deoxyribose. Then what you see on the inside are kind of what looks like the rungs of a ladder. Those are nitrogen bases. There are four of them, adenine, thymine, guanine and cytosine. Adenine and thymine pair up with each other and guanine and cytosine pair up with each other. Let's just recap the four experiments that we discussed. We spoke about Friedrich Miescher. Miescher was the first one to isolate and identify nucleic acids. Then we spoke about Wilhelm Roux. Roux's experiments helped show that it was the material in the nucleus that was genetic material. But still, at that point, people weren't quite sure if it's protein or DNA. Many people thought it was protein, because proteins are more complex than nucleic acids. Then we spoke about Hershey and Chase and how they helped prove that is was nucleic acid that's genetic material and not protein. Then Watson and Crick actually identified the structure of nucleic acids.