If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

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

Evidence that DNA is genetic material 2

Video transcript

we spoke about the experiments of Friedrich Miescher and we'll hum Roux and how their experiments helped show that genetic material is made up of either protein or nucleic acid or perhaps both so the next logical question to ask is well which one is it is it protein is it nucleic acid or perhaps it's both so this question was answered with the experiment of two scientists alfred hershey and martha cheese and 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 so Hershey and chase worked with bacteria phages and a bacteriophage is a virus that specifically infect bacterial cells and it can also be referred to simply as a phage so what is a bacteria phage well it has nucleic acid which can be either DNA or RNA in this video I'm just going to 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 and that nucleic acid or DNA is surrounded by a protein coat and how to bacteria phages 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 going to draw right here so here's a bacterial cells DNA so the virus's DNA gets somehow integrated and now this bacterial cell is going to produce a whole bunch of viruses so this is some background information now let's talk about hershey and chases actual experiment / she and chase took some phages and they put these phages in a medium which means a broth that has a lot of nutrients so that these stages can now multiply and reproduce and make a lot more of themselves but they wanted to label the protein coat of the new generation of viruses and how are they going to do that so they made sure that all of the amino acids that were in this broth had in them radioactive sulfur so they were labeled with sulfur 35 which is one of the radioactive isotopes of sulfur and in this way they can kind of keep track of where the protein is going and what's happening with the protein and 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 they're found in 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 so now when this phage or the phages that are put into the medium reproduce they're going to take nutrients from this broth and 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 and now they allow this generation of viruses to infect a bacterial cell so you can see they climb on top of bacterial cell and inject their DNA into the cell so remember their DNA gets incorporated into the DNA of the bacterial cell and 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 and Hershey and chase now wanted to separate the protein codes from the bacterial cells so they centrifuge this mixture to kind of get rid of the protein coats so let's get rid of them and notice how the phage is 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 and so now what they did was they took these bacterial cells lyse to them which means they need them burst and they analyzed the viruses and they saw that the viruses and we're not at all radioactively-labeled so they concluded that the protein coat must have remained outside the cell that's how the bacterial cell and if the protein coat were made 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 contain the genetic material of 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 her she and chases experiment so again they took a phage or a couple of phages and put 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 and again I'm just going to refer to nucleic acid as DNA but keep in mind that it can also be RNA so they want to label the DNA and how are they going to do that so they made sure that all of the nucleotides that we're in the broth and the viruses are going to need nucleotides to make their DNA so all of them were radioactively labeled with phosphorus-32 so p32 is your radioactive isotope of phosphorus and this is how they're going to label the DNA and the reason they chose phosphorus-32 is because phosphorus is found in DNA nucleotides but it is not found in amino acids so it will not get integrated into the protein coat so it's a great way to differentiate between the two so they put them in the broth and allow them to produce any produce a generation of viruses that have this radioactively-labeled DNA which i drew in that magenta and of course the protein coat is in green because it's not labeled in any way so they allow these viruses these stages 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 and the protein coats of course remain outside and now they're going to 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 dense will end up in the supernatant and the bacterial cells which are heavier end up in the pellet so we're going to get rid sorry about that so we're going to get rid of these protein coats now Hershey and chase lysed these bacterial cells they make them burst and they analyze the viruses inside and the viruses have a lot of nucleotides with this p32 and so there's a lot of radioactive label DNA maybe not all the DNA had radioactively-labeled phosphorus because some of the nutrients came from bacterial cell but a fair number of the viruses had radioactively-labeled DNA inside of them and so they concluded that since the DNA entered the cell it must be DNA that's genetic material or 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 a genetic material and 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 and a very very short while eater in 1953 James Watson and Francis Crick published their famous paper in which they actually identified the structure of DNA or nucleic acids so they put together a tremendous amount of research that was happening during their time and before their time and they identify the structure of DNA and they told us that DNA is a double-stranded helix with a sugar phosphate backbone the sugar in this case being deoxyribose and then what you see on the inside kind of what looks like the rungs of a ladder those are nitrogen bases and there are four of them adenine thymine guanine and cytosine and adenine and thymine pair up with each other guanine and cytosine pair up with each other so let's just recap the four experiments that we discussed we spoke about Friedrich Miescher and we sure was the first one to isolate and identify nucleic acids then we spoke about will homeroom Andrews 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 and 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 it was nucleic acid that's genetic material and not protein and then Watson and Crick actually identified the structure of nucleic acids