Hi. In this video, we're going to be talking about the discovery of transposable elements. So when we talk about the discovery of transposable elements, the important name to know is Barbara McClintock, and she was the one who first discovered them in the forties or in the fifties when she was studying this.
So first, what is a transposable element? Transposable elements are pretty much just small DNA segments that can jump or move throughout the genome. So they exist in the genome and they're just sitting there chilling out, and then they're like, oh, I don't want to be in this chromosome anymore, I want to go over there, so they just cut themselves out and move over there. And so, transposable elements are found in nearly every organism, including you, including humans, they exist in us as well.
And so how they were discovered is McClintock studied chromosomal breakage, and she did so in maize, which in case you're unfamiliar with what maize is, it's corn. And so what she found is when she was studying this chromosomal breakage, she found that chromosome 9, which is just one of the chromosomes in corn, it tended to break in the same exact spot a lot. And that was very weird, because chromosomal breakage is an event that happens, but it's supposed to be random. You know, something weird causes the chromosome to break. But in this case, this chromosome just broke all the time in the exact same spot, and she was like, well, this is very odd.
So she's trying to figure out what factors caused this breakage, and she found two of them. The first is called dissociation factor or ds, and this is a factor located where the break occurred often. The second factor she found was the activator factor, and the activator factor was unlinked, so it wasn't nearby ds, but it was still present in the genome, and it was responsible for control and breakage. So if ac wasn't there, the breakage never happened.
And so she also found that the ac element was impossible to map. So no matter how many times she tried to, you know, do all these recombinations and calculate the recombination frequency and do everything that we've talked about before in our mapping chapters, she couldn't do it. Every time she did it, it was in a new location, and so she thought the ac was potentially moving around, like this is very odd.
And so what she found was that the ds and the ac, were dependent on each other. So ds is a non-autonomous element, and this means that this element cannot move or cause this break without assistance. So if it's there, but ac isn't, it's not going to do anything. It needs ac to be able to cause this breakage or jump around the genome. The autonomous element is ac and this can move, it can act, it doesn't need anything else, it completely acts on its own accord. So if we have ac, ds can move, but if we don't have ac, ds cannot, meaning that ds only moves if ac is present, so this is non-autonomous, while ac is autonomous.
Now, she was studying corn. Right? And that and it wasn't exactly easy in the 1940s just to be able to sequence all genome. Right? It wasn't actually feasible, so you had to be able to have some kind of phenotype to realize what was going on. And so she noticed that these corn kernels had an unstable phenotype that would change after a development.
So what this looks like is this: So you have this white kernel, and this was sort of a normal phenotype. The other normal phenotype, which isn't on here, was just full purple. So these were the normal phenotypes, but what she noticed is that she ended up getting a bunch of abnormal phenotypes that were very different and what she called unstable, and they're kind of spotted. Right? They you can see that they maybe started out white, but then at some point, they switched. They were unstable. They switched to purple, and then you get these, like, purple tiny purple spots. You can get the majority of it being purple, and it sort of creates these swirls, and it's different for every corn kernel. And that was very odd, and she wasn't sure what was going on with the genotype. Because she hypothesized that the purple had this phenotype, or this genotype. Right? Where it was dominant, it could have been this or this. And she suspected that the white was essentially recessive. Right? Where you had to have this. But she was, like, what in the world is going on with these white and purple spots? Like, what is this genotype that's causing this unstable phenotype, and how is it being regulated so that it's so different in every single kernel?
And so she did a lot of experiments to prove this, but essentially what she came up with is that there'll be three elements. You have your ac, you have your ds, and you have your gene, and this encodes for the color. And when everything is just sort of sitting in its original locations, you get a purple kernel, Right? Because this gene is undisturbed. But what happens if you have ac that's going to stimulate ds to jump, When ds jumps into the c gene, that's going to inactivate it. And when this color gene is inactivated, it's white. Right? So the ds is here, and obviously this isn't going to work because it's now sitting straight at the middle of this gene, so you get a white kernel. But what happens with these spotted? Well, what happens is actually they start out white, so they start out like this. So this is kind of the first step. So when the kernel starts dividing, it has this ds element smack dab in the middle of this color gene. But because ac is still present, what happens is that ds can jump again, and when it does, it reverts back to the normal purple phenotype. And so you start out with a white kernel, and then the ds element jumps, and then those regions can become purple now. And so what the timing of that jump and at what point in development it is can depend on what this color is. Right? Because the color is going to depend on the exact timing of when that ds element jumps, and it's going to be very different for each kernel. And so that is how you get this unstable phenotype of these different colors coming from this really just one gene that could either be purple or white.
And so she did a lot of controls of this too. Right? Because if ac isn't present, right, if ac isn't present here, the ds is going to stay and it's always going to be purple. But if the DS the AC is lost here, what's going to happen is the ds is always going to be stuck here, and then it will always stay white. And so she did a lot of controls of, like, when the AC was present, what did ds do? When it was absent, what did it do? And she found that, you know, the ds element couldn't move without AC. So if the ds element was stuck in the gene and AC wasn't there, it's going to stay there, and it's going to continue to be white. But if the ac element is there, it can jump out and revert to purple and cause this unstable phenotype.
So that is sort of the discovery of these transposable elements and how they came about. So remember Barbara McClintock and the fact that she's studying these unstable phenotypes in corn. So, with that, let's now move on.
