Extranuclear inheritance 1
- [Voiceover] Normally when we think about DNA, we think about the nucleus of a cell and that's because a cell's DNA is contained in its nucleus, but there are actually a few exceptions to this general rule. There are certain organelles that actually have their own DNA and two very famous examples of this are the mitochondria and chloroplasts. Mitochondria and chloroplasts have their own DNA, which I'm just going to scribble here in blue, and not only do they have their own DNA but they can actually replicate their DNA and replicate themselves independently of the nucleus of the cell in which they are. Let's just talk briefly about mitochondria. Mitochondria are these organelles found in eucariotic cells and they're sometimes referred to as the "powerhouse" of the cell because they break down glucose to make this high-energy molecule called ATP, and then the cell takes this ATP and uses it for all sorts of cellular processes. And the mitochondrial DNA, written like that "mtDNA", has about 37 genes in it. And these genes, most of them have to do with the cellular respiration that's going on in the mitochondria. Let's talk a bit about chloroplasts. Chloroplasts are these organelles that are found in plant cells. They are also found in algae cells. And chloroplasts are the site of photosynthesis. If we wanted to be more specific, you have these stacks called granum, well in singular, it's granum, plural is grana, and those granum are made up of these... That's an m over there. And those granum are made up of these little circles called thylakoids, and photosynthesis happens within these thylakoids. So during photosynthesis, sunlight is harnessed, of course, with a bunch of other steps to make glucose. This is where the concept of making its own food comes from. It's actually making glucose. It's making its own food. And then, that glucose goes to the mitochondria of that cell and gets broken down, make ATP, and then the cell uses that ATP for whatever it needs to do. The DNA in chloroplast, sometimes are in cpDNA, has about 100 genes and these genes, also, most of them have to do with proteins or things that are involved in photosynthesis. And the reason that this is interesting is, well, let's take a look at how sexual reproduction normally takes place. We have an egg cell and the nucleus of this egg cell has only half the amount of DNA that a normal cell in that organism would have. We call that "n" and then we have a sperm cell. Remember, the sperm cell is really much, much smaller than an egg cell, so this is in no way drawn to scale. And the sperm cell also has in its nucleus, only half the amount of DNA that cells in this organism normally have. That's also "n". But, then they fuse to make a zygote. And this zygote is 2n. It has the normal amount of DNA that a cell in this organism would have. Half of it comes from the egg cell and half of it comes from the sperm cell. And on this zygote is going to divide into two cells and those two cells, of course, divide further and this goes on and on until they are enough cells to put together an organism. But this egg cell, well, it's a fully developed cell and it not only has genetic information, but it has organelles in the cytoplasm. It has these mitochondria in its cytoplasm and those mitochondria have DNA in it, which I'm just going to scribble some blue inside, and these zygote also has those mitochondria, because you remember, the zygote is practically an egg cell with the only difference being that it's nucleus has the additional DNA of the sperm cell. And remember, the sperm cell does not donate anything to the egg cell except for half of the DNA in the nucleus. It does not give the zygote anything else. You have those zygote with those mitochondria, and of course, they have their DNA in it. And then when this zygote replicates itself, so it replicates the nucleus, but it also replicates the mitochondria in the cytoplasm and these cells will... I'm going to skip up the nucleus. I'm just drawing the mitochondria. So I have these mitochondria, but these mitochondria came only from the egg cell and none of those mitochondria came from the sperm cell. And so, this brings us to concept of maternal inheritance. And, maternal inheritance, well it's basically like exactly the way it sounds, it's inheritance that happens only from the maternal line or only from the egg cell. So right here, we're showing that the mitochondria that this organism will eventually have originates from the mitochondria that it came only from the egg cell and not from the sperm cell. And therefore, it exhibits maternal inheritance. So, both mitochondria and chloroplasts exhibit maternal inheritance because they are in the egg cell that eventually becomes the organism. And the maternal inheritance, it's interesting to note, is contrary to mendelian genetics. Maternal inheritance is contrary to mendelian genetics because mendelian genetics assumes that half of the DNA comes from the egg cell, half from the sperm cell, it does not take into account any sort of genetic information that comes from only one of the gametes, for example just from the egg cell, and in fact everything we just described here can be referred to as extranuclear inheritance. Extranuclear inheritance would refer to any genes that are passed on from structures that are not in the nucleus. Extranuclear meaning outside of the nucleus. Mitochondria and chloroplasts are outside of the nucleus. When they are inherited, we refer to it as extranuclear inheritance. Now that we've introduced extranuclear inheritance, let's actually take a look at one of the earlier experiments that helped to discover extranuclear inheritance.
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