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

Extranuclear inheritance 2

Explore the fascinating world of extranuclear inheritance with Carl Correns' experiments on the 4 o'clock plant. Discover how leaf color, determined by chloroplast DNA, exhibits maternal inheritance. Dive into the endosymbiotic theory explaining why mitochondria and chloroplasts have their own DNA. It's a journey through genetics beyond the nucleus! Created by Efrat Bruck.

Want to join the conversation?

Video transcript

- There was a scientist by the name of Carl Correns. And he was a somewhat of a contemporary of Gregor Mendel. He lived from 1864... to 1933. And Gregor Mendel lived from 1822 to 1884. So they were contemporaries, but Correns was younger than Mendel, and we will soon see that Carl Correns helped discover some things that do not fall to the category of Mendelian genetics. So Carl Correns did a lot of experiments with this plant called the 4 o'clock plant. That's how we know it. Which is what you're looking at, I know it looks a little bit like a tree, but it's supposed to be a plant. And actually the scientific name for the 4 o'clock plant is mirabilis jalapa, not exactly sure how to pronounce that, but anyway. So he did a lot of experiments with the 4 o'clock plant and one very interesting thing about this plant is that you can have within the same plant leaves that are a lot of different colors. So let's say that this is the main branch of our plant. And we're going to say that the leaves that come off anywhere on this branch are going to be white, so there are our white leaves. And we're gonna say that any leaves that come off of this branch are going to be green, so those are our green leaves. And then the leaves that come off of this branch are variegated. And that means that they have a pattern of green and white mixed. So there are our variegated leaves. And why is it that there are all these different colors within the plant? So let's take a look at some of the cells. Let's take a look first at a cell that's from a leaf that's green. So this is in a leaf that's green, so that's our cell. Here's our nucleus. And in case you're wondering why I'm drawing this cell as a square it's because plant cells have this cell wall that give it a more rigid shape and make it a bit more square-ish, closer to a square than a circle. But anyway, that's our cell, and this cell will have chloroplasts in it which I'm drawing as these green little circles. And remember chloroplasts have their own DNA, sometimes referred to as cpDNA. And chloroplast DNA has in it basically the stuff that the chloroplasts needs to carry out photosynthesis. And one of the genes in the chloroplast DNA is a gene that makes chlorophyll which is a pigment that's involved in photosynthesis, and chlorophyll is what makes the leaf green. It's a pigment that turns the leaf green or if you wanna be more specific, chlorophyll absorbs all of the colors in sunlight except for green, so green is really reflected, but anyway. The point is that chlorophyll makes the leaf green. So now let's take a look at a cell that comes from a white leaf. So this cell comes from a white leaf, so again we'll draw our nucleus. And then it has chloroplasts, but the chloroplasts in this cell, the DNA in those chloroplasts have a mutation. So the cpDNA has a mutation that does not allow it to produce chlorophyll, or it allows it to produce on a very, very tiny amount of chlorophyll 'cuz it needs chlorophyll to survive, but it's not enough that the leaf will be green. So I'm just gonna write very, very tiny amount of chlorophyll... That's why it appears white. And then the variegated leaves... Well, they have some cells that have regular chloroplasts that make chlorophyll and then they have some cells with the mutated chloroplasts or chloroplasts with mutated DNA that do not make chlorophyll, or rather make a very small amount. And then they actually have a third type of cell... which maybe you're guessing it already, have both types of chloroplasts. They have chloroplasts that do make chlorophyll, and then they have chloroplasts that have mutated DNA and do not make much chlorophyll. And what Correns noticed when he crossed a whole bunch of these plants together, he noticed that the progeny had nothing to do with the sperm cell or the pollen cell. It had only to do with the egg cell. So wherever he would take the seed from, this is where the egg cell's located, if he took it from a branch that only had white leaves, all the progeny had only white leaves. It didn't matter what the pollen cell was. And the same if he took a seed from a branch that had only green leaves, so all the progeny had all green leaves no matter where the pollen cell came from. And this is because the trait that we're looking at, the color of the leaf, well that's determined by the DNA in the chloroplast. And the chloroplast exhibits maternal inheritance. It is going to be inherited only through the egg cell, or through the maternal line, or another way to say this it exhibits extranuclear inheritance... because a chloroplast has DNA that's outside of the nucleus. Let's take a closer look at what Carl Correns did. So we have this chart to help us out. In the first column we have the egg cell of the female, that's the seed. And then we're gonna cross it with what we have in the second column which is the pollen cell, that's the male gamete. And they all look the same because it doesn't make any difference in our case. And then we have in the third column our zygote, or the result. So let's look at our first row. So we have this egg cell that came from a branch that had leaves that were only white. So it came from a flower plant with only white leaves. And when we cross it with a pollen cell, no matter what that pollen cell is, no matter where it came from, we always get the same result. We will always get a plant with only white leaves. Let's look at our second row. So we have this egg cell that came from a branch that had only green leaves. And again, no matter what we cross it with, no matter where the pollen cell came from- a white leaf, a variegated leaf, a green leaf, or a flowering green leaf- so we always get the same result. We get a plant that has only green leaves. It gets a little bit more interesting when we look at the variegated, at the egg cells from the variegated parts of the plant because there are three different types of egg cells we could have. We could have one that resembles the cell you see if it came from a flower with only white leaves. Then we have another egg cell that looks like that. It looks like the egg cell you'd find in a plant that had only green leaves. And then we have this third type of interesting cell that has the combination of the quote unquote normal chloroplasts that are green, and then it has some chloroplasts that have that mutation that don't allow it to make chlorophyll or make a very small amount of chlorophyll, so it's kind of mixed. Anyway, let's look at egg type one, we cross it with a pollen cell, we always get the same result. You get a plant with only white leaves. And then we look at egg cell type two, whatever we cross it with, we get only green leaves. And then if the egg cell type three, so the zygote of course will have both types of chloroplast, but remember this zygote is going to divide further and it's gonna divide into, if it divides, you know, randomly, it'll divide into three different types of cells. Some of the cells will look like this with the chloroplasts with the mutated DNA, some of them are going to look like that with the regular chloroplasts, and then some of them are going to be mixed. And then this will give you a variegated plant. Some of the leaves are going to have that mixed pattern, you might have some leaves that are white, you might have some leaves that are green. And so the bottom line, take home message, is, as I explained before, because the particular trait we're looking at, leaf color, because the gene for that trait is in the chloroplast, it exhibits maternal inheritance. Maternal inheritance is a type of extranuclear inheritance. I'm just gonna write that in parentheses. Because this inheritance has to with DNA that's outside of the nucleus, but anyway. This exhibits maternal inheritance because it has nothing to do, this particular trait is not passed down through the male, well it's only passed down through the female. And that is, as we explained before, because the chloroplasts are coming only from the egg cell. The sperm cell does not contribute any chloroplasts to the zygote, it only contributes DNA that's in the nucleus. So therefore, the leaf color of this 4 o'clock plant exhibits maternal inheritance. And the same concept would apply to the mitochondria. So we explained mitochondria also has its own DNA, and so if a person were to have a disease that had to do with the DNA inside of the mitochondria, we would know that that person got it from his or her mother and not from his or her father because the mitochondria also exhibits maternal inheritance. There is one more thing about extranuclear inheritance that I want to mention. And that is, why is it that mitochondria and chloroplasts have their own DNA? Is there something that can explain that? And there is... the endosymbiotic theory... seeks to explain why mitochondria and chloroplasts have their own DNA. And this theory tells us that mitochondria and chloroplasts were once independent prokaryotes. So they lived independently, so of course if they're independent, they need to have their own DNA. But eventually, they joined what I'm going to call an ancestral... eukaryotic... cell. And in case you're wondering like, when this happened, we'll say about one and a half billion years ago. So what happened is that the mitochondria and chloroplasts joined an ancenstral eukaryotic cell, and I'm gonna call it "ancestral" because it's not exactly a eukaryotic cell that we'd see today, but it's a cell that would eventually become a eukaryotic cell. We could also call it a host cell because it's gonna host mitochondria and chloroplasts. So let's put them inside. So now they're living in this host cell. And why do you think they would wanna do this? So they're going to live together in symbiosis. And symbiosis is when organisms live together and each one kind of gives the other something and everybody gains something. So an example of this that you might've heard of is in our intestine, in our gut, we have bacteria e. coli, and at first glance it might seem like it's not such a good thing, but it's actually a really good thing because we give the e. coli a warm and cozy place to live, they get some nutrients from us, and in exchange, they make for us vitamin k which is something very useful for us. So that's an example of symbiosis. Each one of the organisms kind of tries to give something and everybody's happy. So what's going on in the host cell? So the host cell gives the mitochondria and chloroplasts a nice place to live and gives them nutrients, and in exchange, the chloroplast makes glucose through photosynthesis, and then the mitochondria takes that glucose and produces ATP. And then that ATP is used as energy, the mitochondria uses some of the ATP, the chloroplasts uses some of the ATP, and of course the host cell uses some of the ATP. So everybody's happy, everybody's getting something. And then eventually this cell, you know, evolved in many different ways, and of course it became the eukaryotic cell that we know of is today, but you know not exactly the way it was because chloroplasts are not found in all eukaryotic cells. It's basically found in plants as in algae, but that's the basic idea. So let's go back to the term endosymbiotic theory. So endo just means "inside" because the mitochondria and chloroplasts began to live inside a host cell. And symbiotic simply refers to symbiosis. I'm just gonna draw an arrow down here, refers to symbiosis because each part, the mitochondria and chloroplast and host cell, everybody's getting something. And so this would explain why mitochondria and chloroplasts have their own DNA because once upon a long time ago, they were independent prokaryotes, and they lived on their own.