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And these things are pushing each other apart. The two centrosomes might be pushing apart to get to the opposite ends of the actual cell. But they're bringing, there's all sorts of interesting mechanisms that are bringing along these microtubules, bringing the chromosomes. Once again, splitting the homologous pairs. An... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
Once again, splitting the homologous pairs. And how they split is random. You know, this pink one could have been on the right side. This orange one could have been on the left side. Or vice versa. Once again, this adds more kind of variation amongst the gametes. So even all of the resulting gametes that get produced, ... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
This orange one could have been on the left side. Or vice versa. Once again, this adds more kind of variation amongst the gametes. So even all of the resulting gametes that get produced, they all will have different genetic information. So this is anaphase one. You're pulling these apart. And then you can imagine what ... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
So even all of the resulting gametes that get produced, they all will have different genetic information. So this is anaphase one. You're pulling these apart. And then you can imagine what happens in telophase one. So telophase one. Telophase. Telophase one. | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
And then you can imagine what happens in telophase one. So telophase one. Telophase. Telophase one. Telophase one. And this is fairly analogous to what happens in mitosis in telophase. So now you have your cytokinesis is beginning. | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
Telophase one. Telophase one. And this is fairly analogous to what happens in mitosis in telophase. So now you have your cytokinesis is beginning. So, and actually it might even begin earlier. In mitosis it happens as early as anaphase. At least the cytokinesis is starting. | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
So now you have your cytokinesis is beginning. So, and actually it might even begin earlier. In mitosis it happens as early as anaphase. At least the cytokinesis is starting. But you're starting to see that. These two things, the homologous pairs are fully split apart and they're at opposite ends. And actually they cou... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
At least the cytokinesis is starting. But you're starting to see that. These two things, the homologous pairs are fully split apart and they're at opposite ends. And actually they could begin to, they can begin to unravel into their chromatin state. So this one began to unravel into its chromatin, into its chromatin st... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
And actually they could begin to, they can begin to unravel into their chromatin state. So this one began to unravel into its chromatin, into its chromatin state. It has a little bit of the magenta. Whoops, it has a little bit of the magenta right over here. This is unraveling as well. This is unraveling like that. Onc... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
Whoops, it has a little bit of the magenta right over here. This is unraveling as well. This is unraveling like that. Once again into its chromatin state. The cellular, and let me do the other ones as well. So this is, this one right over here beginning to unravel. This one over here beginning to unravel. | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
Once again into its chromatin state. The cellular, and let me do the other ones as well. So this is, this one right over here beginning to unravel. This one over here beginning to unravel. It's got a little bit of orange on it. It's got a little bit of orange on it. The nuclear membrane begins to form again. | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
This one over here beginning to unravel. It's got a little bit of orange on it. It's got a little bit of orange on it. The nuclear membrane begins to form again. The nuclear membrane begins to form again. In some ways it's reversing what happened in prophase one where the nuclear membrane disappeared and the chromosome... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
The nuclear membrane begins to form again. The nuclear membrane begins to form again. In some ways it's reversing what happened in prophase one where the nuclear membrane disappeared and the chromosomes condensed. And let me draw, let me draw, this is centrosomes which are outside of the nuclear membrane just like that... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
And let me draw, let me draw, this is centrosomes which are outside of the nuclear membrane just like that. And the microtubules are also dissolving. The microtubules are also dissolving. And you have your cytokinesis. So your cytokinesis, so these separate, these separate into two cells. So once again when we did the ... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
And you have your cytokinesis. So your cytokinesis, so these separate, these separate into two cells. So once again when we did the overview of meiosis we said look, the first phase of meiosis you go from a diploid, a diploid germ cell to two haploid cells. And these aren't quite our end product yet. This right over he... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
And these aren't quite our end product yet. This right over here, what we have just gone through, what we have just gone through, all of this combined that we have just gone through, this is meiosis one. And in the next video we're gonna go through meiosis two. Whoops, I didn't mean to do that. This is, so let's see. A... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
Whoops, I didn't mean to do that. This is, so let's see. All of this is meiosis one. Let me write that in a different color and bold. So this is all meiosis, meiosis one here. And you can see each of these cells now have a haploid number. They now have the haploid, haploid number of two chromosomes each. | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
Let me write that in a different color and bold. So this is all meiosis, meiosis one here. And you can see each of these cells now have a haploid number. They now have the haploid, haploid number of two chromosomes each. Now each of those two chromosomes do have two sister chromatids. And as we'll see meiosis two, whic... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
They now have the haploid, haploid number of two chromosomes each. Now each of those two chromosomes do have two sister chromatids. And as we'll see meiosis two, which is very similar to mitosis, is going to split up the sister chromatids from each of these chromosomes, which gives us two daughter chromosomes. So we're... | Phases of meiosis I Cells MCAT Khan Academy.mp3 |
And so the next question is, well, what gives them that shape? What allows them to form that kind of cubic rectangular prism shape? And the answer is, it's the cell wall. So that's the cell wall. So let's make sure we can orient ourselves properly in this picture. So clearly, if I didn't have this cutout, all I would b... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
So that's the cell wall. So let's make sure we can orient ourselves properly in this picture. So clearly, if I didn't have this cutout, all I would be seeing is the outside. All I would be seeing is the cell wall, but we've cut it out, and we can see the different layers. We have the cell wall on the outside. Right bel... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
All I would be seeing is the cell wall, but we've cut it out, and we can see the different layers. We have the cell wall on the outside. Right below that, we have the cellular membrane, or the plasma membrane. So that's the cell, cellular membrane, cellular membrane, right under that. And then under that, the cellular ... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
So that's the cell, cellular membrane, cellular membrane, right under that. And then under that, the cellular membrane is containing the cytoplasm, and inside of the cytoplasm, we have all sorts of things. This big thing that is taking up a lot of the volume inside of this plant cell, that's a vacuole, which we have de... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
Vacuole. And it's a combination of this internal pressure, things like the vacuole, and just frankly, the pressure from all of the fluid inside the cell pushing outwards, plus the cell wall kind of holding it all in. That's what gives plants their structure. That's why a plant is able to grow and be upright. So that's ... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
That's why a plant is able to grow and be upright. So that's my drawing of a plant. I actually have a plant in my room that I'm looking at right now, and it's able to grow and be upright. And so you have the cell wall, you have the cellular membrane, you have the other organelles, I have some chloroplasts here, key for... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
And so you have the cell wall, you have the cellular membrane, you have the other organelles, I have some chloroplasts here, key for photosynthesis, have our good friend's mitochondria, we have our nuclear membrane, I should say this yellow thing is the inner nuclear membrane, has the DNA inside, then you have the endo... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
And over here, it might be a little bit surprising to you, because when I've always imagined a wall, a cell wall, I imagined something like a brick wall, something that's impenetrable. But this drawing shows us something different. And just to be clear what's going on here, so this is our cellular membrane. Sorry, I wr... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
Sorry, I wrote cellular membrane. So right over here, I have my lipid bilayer. And then right on top of that, I have the cell wall. But you see, it isn't just a thick, like a brick wall, something that's impenetrable. You see, you have all of these polysaccharide fibers running across it. So you have things like cellul... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
But you see, it isn't just a thick, like a brick wall, something that's impenetrable. You see, you have all of these polysaccharide fibers running across it. So you have things like cellulose, which we saw is a polymer of glucose arranged in a certain way. Hemicellulose, which has different types of monomers associated... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
Hemicellulose, which has different types of monomers associated with it. We have pectin, which is another polysaccharide. And all of these things, you've actually probably eaten, if not today, probably in the last week. When we talk about fiber in your diet, you're talking about things like the cellulose and the pectin... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
When we talk about fiber in your diet, you're talking about things like the cellulose and the pectin, things that your body can't digest. But when you eat a plant, you're getting it because you're eating their cell walls. And it does cool things, like slows the absorption of glucose in your intestines. It absorbs water... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
It absorbs water, so I guess you could say things pass a little bit easier. But just to see, the key thing here is this isn't a wall. This actually allows, or it is a wall. It's officially the cell wall. But it's not a thick, it's not an impenetrable wall like you might associate the wall of the room that you're in. Yo... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
It's officially the cell wall. But it's not a thick, it's not an impenetrable wall like you might associate the wall of the room that you're in. You can see that it has space for small molecules to flow. And it's really more like a mesh or like a fabric. And so the cellular membrane actually has access to the fluid and... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
And it's really more like a mesh or like a fabric. And so the cellular membrane actually has access to the fluid and to the molecules that are between the cells. And so just to be clear what we're looking at, this layer right here, that's the cellular membrane. That's the lipid bilayer. This right over here, this is th... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
That's the lipid bilayer. This right over here, this is the cell wall. I'll do that in a different color. That is the cell wall. And then right above the cell wall, that's the space between the cells, which we call the middle lamella. So the space between the cells we call the middle lamella. So this also is, this righ... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
That is the cell wall. And then right above the cell wall, that's the space between the cells, which we call the middle lamella. So the space between the cells we call the middle lamella. So this also is, this right over here is also the middle lamella. So all of that is interesting, but you might say, okay, well, how ... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
So this also is, this right over here is also the middle lamella. So all of that is interesting, but you might say, okay, well, how hard is a cell? I get that it's a mesh, but clearly the cells are able, or the plants are able to stand upright. Is that because the cell wall provides all of that rigidity? And the answer... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
Is that because the cell wall provides all of that rigidity? And the answer is kind of. The cell wall is like this mesh. It helps these cells have their shape. But if you stop watering a plant, you're going to see it kind of wilt over. And that's because part of its ability to stand up is from the internal pressure of ... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
It helps these cells have their shape. But if you stop watering a plant, you're going to see it kind of wilt over. And that's because part of its ability to stand up is from the internal pressure of the cells, but also part of its shape is the actual cell wall. Now, some of you might say, well, I've seen plants that ar... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
Now, some of you might say, well, I've seen plants that are much, much more rigid than this plant you've just drawn. What about things like trees? What about wood? Wood seems very rigid, in fact, so rigid that we can build actual walls out of wood. And the answer there is these more mature plants, actually, once the ce... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
Wood seems very rigid, in fact, so rigid that we can build actual walls out of wood. And the answer there is these more mature plants, actually, once the cell has stopped growing and you have your cell wall, more layers of cellulose and other molecules can be built to form what's called a secondary cell wall. So this c... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
And so when you look at wood, what gives wood its structure, even if you were to take out all of the water, even if you were to dehydrate the wood, it's still going to have its rigidity because the cellulose layers and the other molecules, they're so thick that it's able to have its rigid form. Now, the last thing I wa... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
These are plasmodesmata. Plasmo, these are plasmodesmata. And to get a better understanding of what they're like, imagine this is one cell, so I'll write here cell one, and let's say this is cell two, cell two right over here, and I have a cross section. You see the plasmodesmata are these tunnels that form between not... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
You see the plasmodesmata are these tunnels that form between not just the membrane and the cell wall, and the plasmodesmata, it forms between the two cells. And so you can actually have a flow of cytosol and small molecules directly, directly between these two cells. And if you want to get a little bit more involved i... | Plant cell walls Structure of a cell Biology Khan Academy.mp3 |
For example, someone might have told you, hey, you walk kind of like your dad, or your smile is kind of like your mom, or your eyes are like one of your uncles or your grandparents. And so there's always been this notion of inherited traits. But it wasn't until the 1800s that that started to be studied in a more scient... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
But even then, even Mendel, who was starting to understand the mechanisms of, or he was trying to understand how inheritance happens, and he even could start to breed certain types of things, even he didn't know exactly what was the molecular basis for inheritance. And the answer to that question wasn't figured out unt... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
But it was really the structure of DNA that made people say, hey, that looks like the molecule that's storing the information. And just to be clear, DNA wasn't discovered in 1953. DNA was discovered in the mid-1800s. It was this kind of, this molecule that was inside of nuclei, of cells, and for some time, people said,... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
It was this kind of, this molecule that was inside of nuclei, of cells, and for some time, people said, oh, maybe this could be a molecular basis of inheritance. You know, you could imagine what you would need to be a molecular basis of inheritance. It would have to be a molecule or a series of molecules that could con... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
But it wasn't until 1953, when this double helix structure of DNA was established that people said, hey, this looks like our molecule. So first, let's just talk about the structure here, and then actually we'll talk about where this name, DNA, deoxyribonucleic acid, comes from. And then we'll talk a little bit about wh... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
We might go in-depth on the expression of information in future videos. So this structure right over here, and this is a visual depiction of a DNA molecule, you can view this as kind of a twisted ladder. It has these two, I guess you could say, sides of the ladder that are twisted. That is one side right over there, an... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
That is one side right over there, and then it is another side. There is another side right over here. And in between those two sides, or connecting those two sides of that twisted ladder, you have these rungs. And these rungs are actually where the information, the genetic information is, I guess you could say, stored... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And these rungs are actually where the information, the genetic information is, I guess you could say, stored in some way. Because these rungs, it's a sequence of different bases. And when I say bases, you might say, wait, this says acid, why are you saying bases right over here? Well, the word deoxyribonucleic acid co... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Well, the word deoxyribonucleic acid comes from the fact that this backbone is made up of a combination of sugar and phosphate. And the sugar that makes up the backbone is deoxyribose, so that's essentially the D in DNA. And then the phosphate group is acidic, and that's where you get the acid part of it. And nucleic i... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And nucleic is, hey, this was found in nuclei of cells. It is nucleic acid, deoxyribonucleic acid. But it's not, it also, it is actually mildly acidic all in total, but for every acid, it actually also has a base. And that base, those bases form the rung of the ladders. And actually, each rung is a pair of bases. And a... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And that base, those bases form the rung of the ladders. And actually, each rung is a pair of bases. And as I said, that's where the information is actually stored. Well, what am I talking about? Well, let me talk about the four different bases that make up the rungs of a DNA molecule. So you have adenine. And so, for ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Well, what am I talking about? Well, let me talk about the four different bases that make up the rungs of a DNA molecule. So you have adenine. And so, for example, this part right over here, this section of that rung might be adenine. Maybe this right over here is adenine. This right over here. Remember, each of these ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And so, for example, this part right over here, this section of that rung might be adenine. Maybe this right over here is adenine. This right over here. Remember, each of these rungs are made up by, it's a pair of bases. And that might be adenine. Maybe this is adenine. And I could stop there. | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Remember, each of these rungs are made up by, it's a pair of bases. And that might be adenine. Maybe this is adenine. And I could stop there. I'll do a little more adenine. Maybe that's adenine right over there. And adenine always pairs with the base thymine. | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And I could stop there. I'll do a little more adenine. Maybe that's adenine right over there. And adenine always pairs with the base thymine. So let me write that down. So adenine pairs with thymine. Thymine. | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And adenine always pairs with the base thymine. So let me write that down. So adenine pairs with thymine. Thymine. So if that's an adenine there, then this is going to be a thymine. If this is an adenine, then this is going to be a thymine. Or if I drew the thymine first, well, I'll say, okay, it's going to pair with t... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Thymine. So if that's an adenine there, then this is going to be a thymine. If this is an adenine, then this is going to be a thymine. Or if I drew the thymine first, well, I'll say, okay, it's going to pair with the adenine. So this is going to be a thymine right over here. This is going to be a thymine. If I were to ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Or if I drew the thymine first, well, I'll say, okay, it's going to pair with the adenine. So this is going to be a thymine right over here. This is going to be a thymine. If I were to draw this, this would be a thymine right over here. Now, the other two bases, you have cytosine, which pairs with guanine, or guanine t... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
If I were to draw this, this would be a thymine right over here. Now, the other two bases, you have cytosine, which pairs with guanine, or guanine that pairs with cytosine. So guanine. And we're not going to go into the molecular structure of these bases just yet, although these are good names to know because they show... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And we're not going to go into the molecular structure of these bases just yet, although these are good names to know because they show up a lot and they really form kind of the code, your genetic code. So guanine pairs with cytosine. Guanine and cytosine. Cytosine. So actually, if this is, let's say there's some cytos... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Cytosine. So actually, if this is, let's say there's some cytosine there, let's say cytosine right over here, maybe this is cytosine, maybe this is cytosine, maybe this is cytosine, this is cytosine, and maybe this is cytosine, then it always pairs with the guanine. If we're talking about, so let's see, this is guanine... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Actually, just to make it a little bit more complete, let me just color in the rungs here as best as I can. So those are guanine, so they're gonna pair with cytosine, pair with cytosine, pair with cytosine. And when it's drawn this way, you might start to see how this is essentially a code, the order of which the bases... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
It is because that information, to a large degree, is encoded genetically. It affects a lot of what makes you you, and actually not even just within a species, but also across species. Humans have more genetic material in common with other humans than they do with, say, a plant, but all living creatures as we know them... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
This is the basis by which they are passing down their actual traits. Now, you might be saying, well, how much genetic information does a human being have? And the number will either disappoint you or you might find it mind-boggling. The human genome, and every species has a different number of base pairs, to a large d... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
The human genome, and every species has a different number of base pairs, to a large degree correlated with how complex they are, although not always, but the human genome has six million, sorry, not six million, six billion. Six million would be disappointing. Even billion might be disappointing. Six billion base pair... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Six billion base pairs. Six billion base pairs. And when you have your full complement of chromosomes, and this is in most of the cells in your body, outside of your sex cells, the sperm or the egg cells, this is going to be spread over 46 chromosomes. 46 chromosomes, or I guess you could say 23 pair of chromosomes. So... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
46 chromosomes, or I guess you could say 23 pair of chromosomes. So if you divide six billion by 46, you get a little over, on average, 100 million, I think it's 100 and something million base pairs per chromosome. And some chromosomes are longer, actually some of the longest are over 200 million, and some might be sho... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
That's just on average. Now, this number might, to some of you, might be exciting. You're like, oh, I thought I was a simple creature. I didn't know I was this complex. Six billion, that's a lot of base pairs. That feels like a lot of information. For others of you, it might not feel so great. | DNA Biomolecules MCAT Khan Academy (4).mp3 |
I didn't know I was this complex. Six billion, that's a lot of base pairs. That feels like a lot of information. For others of you, it might not feel so great. You might say, hey, wait, I could store this much information on a modern thumb drive or on a hard disk. I thought I was more unique than that. And of course, w... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
For others of you, it might not feel so great. You might say, hey, wait, I could store this much information on a modern thumb drive or on a hard disk. I thought I was more unique than that. And of course, we all are special and unique, but you might say, oh, six billion base pairs, I thought I was infinitely complex a... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And of course, we all are special and unique, but you might say, oh, six billion base pairs, I thought I was infinitely complex and whatever else, and there's some arguments for that along some other directions. But this is the approximate length, I guess you could say, the approximate size of the actual human genome. ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
We have about 20 base pairs depicted here. Imagine if you had about 200 million of these base pairs, and then you were to take this thing and you were to kind of coil it up into that thing is a chromosome. Is a chromosome. And you're saying, wait, I have that much information in most of the cells of my body? This thing... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And you're saying, wait, I have that much information in most of the cells of my body? This thing must be incredibly compact. And if you said that, I would say, yes, you are correct. This, the radius, the radius of the DNA molecule is on the order of one nanometer. One nanometer, which is a billionth of a meter. So you... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
This, the radius, the radius of the DNA molecule is on the order of one nanometer. One nanometer, which is a billionth of a meter. So you can start to assess kind of the scale of this thing. This is a very dense way to actually store information. But just to have an appreciation of, and you might have seen it when I wa... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
This is a very dense way to actually store information. But just to have an appreciation of, and you might have seen it when I was coloring in, on why the structure lends itself to being able to replicate the information or even to be able to translate or express the information, let's think about if you were to take t... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
So you essentially have half of the ladder. And so if you only have half of the ladder, you're able to construct the other half of the ladder. Let's take an example. Let's say, and I'll just use the first letter to abbreviate for each of these bases. So let's say you have some, so let's say this is one of the, this is ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Let's say, and I'll just use the first letter to abbreviate for each of these bases. So let's say you have some, so let's say this is one of the, this is the sugar phosphate backbone right over here. So this could be one of the sides. And let's say there's some adenine, actually, let me do them in the right color. So y... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And let's say there's some adenine, actually, let me do them in the right color. So you've got some adenine, adenine, maybe some adenine right over here. Maybe there's an adenine there. Maybe you have some thymine, thymine, maybe thymine right over here. Then you have some, you have some guanine, guanine, guanine. And ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
Maybe you have some thymine, thymine, maybe thymine right over here. Then you have some, you have some guanine, guanine, guanine. And then let's say you have some cytosine and you have some cytosine. So with just half of this ladder, I guess you could say, you're able to construct the other half. And that's actually ho... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
So with just half of this ladder, I guess you could say, you're able to construct the other half. And that's actually how DNA replicates. This ladder splits and then each of those two halves of that ladder are able to construct versions of the other half, or versions of the other half are able to be constructed on top ... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
So how does that happen? Well, it's based on how these bases pair. Adenine always pairs with thymine if we're talking about DNA. So if you have an A there, you're gonna have a T on this end, T on this end. T's right all over here, T right over there. If you have a T on that end, you're gonna have an A right over there,... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
So if you have an A there, you're gonna have a T on this end, T on this end. T's right all over here, T right over there. If you have a T on that end, you're gonna have an A right over there, A, A. If you have a G, a guanine on this side, you're gonna have a cytosine on the other side. Cytosine, cytosine, cytosine. And... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
If you have a G, a guanine on this side, you're gonna have a cytosine on the other side. Cytosine, cytosine, cytosine. And if you have a cytosine, you're gonna have a guanine on the other side. And so hopefully that gives you an appreciation of how DNA can replicate itself. And as we'll see also, how this information c... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
And so hopefully that gives you an appreciation of how DNA can replicate itself. And as we'll see also, how this information can be translated to other forms of either related molecules, but eventually to proteins. And just to kind of round out this video, to get a real visual sense of what the DNA molecule looks like,... | DNA Biomolecules MCAT Khan Academy (4).mp3 |
In the previous video, we talked about the importance of glucose as a simple sugar. We talked about its molecular structure. What I want to do in this video is study how glucose can be, how we can use it as a building block for more complex sugars and more complex carbohydrates. So this right over here, I've copy and p... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
So this right over here, I've copy and pasted two glucose molecules. We can number their carbons. This is one, two, three, four, five, six. One, two, three, four, five, six. We have them in their cyclic form. And what we're going to do is explore what would happen if this oxygen right over here, I'll highlight it in th... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
One, two, three, four, five, six. We have them in their cyclic form. And what we're going to do is explore what would happen if this oxygen right over here, I'll highlight it in this magenta color, were to use one of its lone pairs, one of its lone pairs to do what's in organic chemistry referred to as a nucleophilic a... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
And the reason why that could happen is this number one carbon right over here, it's attached to two oxygens. Oxygens are very electronegative. They like to hog electrons when they're in a covalent bond. So that's going to give this carbon a partially positive charge. And this oxygen is very electronegative. It's going... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
So that's going to give this carbon a partially positive charge. And this oxygen is very electronegative. It's going to hog the electrons from this hydrogen and the number four carbon on the right-hand glucose molecule. So it's going to have a partially negative charge. And so it is going to be nucleophilic. It's going... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
So it's going to have a partially negative charge. And so it is going to be nucleophilic. It's going to be attracted to, I guess you could say, the carbon nucleus, to the partially positive charge right over here. And so as it does that, it's going to use a lone pair to form a bond. It's going to share it with the carb... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
And so as it does that, it's going to use a lone pair to form a bond. It's going to share it with the carbon. And then the carbon can let go of another bond. So it could let go of both of these electrons in that bond. Now, you could say, maybe that just goes back to the oxygen and it forms a hydroxide anion. Or we coul... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
So it could let go of both of these electrons in that bond. Now, you could say, maybe that just goes back to the oxygen and it forms a hydroxide anion. Or we could imagine, well, maybe it'll be used. Maybe it forms a hydroxide anion first. Or maybe that bond immediately goes and picks up a hydrogen ion out of the solut... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
Maybe it forms a hydroxide anion first. Or maybe that bond immediately goes and picks up a hydrogen ion out of the solution, from a hydronium ion sitting someplace. So this can be used to form a bond with this hydrogen ion. Which is really, this is just a proton here. You take an electron away from hydrogen, it's just ... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
Which is really, this is just a proton here. You take an electron away from hydrogen, it's just going to be a proton. Well, what's that going to do? Well, that's going to link these two glucose molecules. And it's going to link it just like this. And it's important to keep track of our molecules here. So this oxygen is... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
Well, that's going to link these two glucose molecules. And it's going to link it just like this. And it's important to keep track of our molecules here. So this oxygen is now going to be this oxygen. It is now going to be that oxygen. This bond between the number four carbon on the right-hand side of that oxygen is th... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
So this oxygen is now going to be this oxygen. It is now going to be that oxygen. This bond between the number four carbon on the right-hand side of that oxygen is this bond right over here. This, when we took this electron pair to form, this bond with the number one carbon, that is, let me do it in that magenta color,... | Dehydration synthesis or a condensation reaction Biology Khan Academy.mp3 |
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