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Pyrimidines. Pyrimidines. We call these pyrimidines. And these particular, these two on the right, these two purines, this one up here, this is adenine. And we talk about how they pair in the overview video on DNA. But this one right over here is adenine, this nitrogenous base. This one over here is guanine. | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
And these particular, these two on the right, these two purines, this one up here, this is adenine. And we talk about how they pair in the overview video on DNA. But this one right over here is adenine, this nitrogenous base. This one over here is guanine. That is guanine. And then, over here, this single ring nitrogen... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
This one over here is guanine. That is guanine. And then, over here, this single ring nitrogenous base, which makes it a pyrimidine, this is thymine. This right over here is thymine. This is thymine. And then last but not least, if we're talking about DNA, when we go into RNA, we're also gonna talk about uracil. But wh... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
This right over here is thymine. This is thymine. And then last but not least, if we're talking about DNA, when we go into RNA, we're also gonna talk about uracil. But when we talk about DNA, this one over here is cytosine. Cytosine. And you can see the way it's structured, that thymine is attracted to adenine, it bond... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
But when we talk about DNA, this one over here is cytosine. Cytosine. And you can see the way it's structured, that thymine is attracted to adenine, it bonds with adenine, and cytosine bonds with guanine. So how are they bonding? Well, the way that these nitrogenous bases form the rungs of the ladder, how they're drawn... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
So how are they bonding? Well, the way that these nitrogenous bases form the rungs of the ladder, how they're drawn to each other, this is our good old friend hydrogen bonds. And this all comes out of the fact that nitrogen is quite electronegative, so when nitrogen is bound to a hydrogen, you're going to have a partia... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
Let me do this in green. You're gonna have a partial negative charge at the nitrogen and a partially positive charge at the hydrogen. And then oxygen, we've always talked about as being electronegative, so it has a partial negative charge. So the partial negative charge of this oxygen is gonna be attracted to the parti... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
So the partial negative charge of this oxygen is gonna be attracted to the partial positive charge of this hydrogen, and so you're going to have, you're going to have a hydrogen bond. And that's the same thing that's gonna happen between this hydrogen, which is going, its electrons are being hogged by this nitrogen, an... | Molecular structure of DNA Macromolecules Biology Khan Academy.mp3 |
This is the air. These are some air molecules. Maybe they're nitrogen molecules. They're fairly far apart. In fact, in reality, they would be even more far apart than this. And then over here you have water molecules. We've seen this many times. | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
They're fairly far apart. In fact, in reality, they would be even more far apart than this. And then over here you have water molecules. We've seen this many times. You have the oxygen atom, and it's bonded to two hydrogen atoms. And the oxygen atom likes to hog the electrons more. It's more electronegative, so you hav... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
We've seen this many times. You have the oxygen atom, and it's bonded to two hydrogen atoms. And the oxygen atom likes to hog the electrons more. It's more electronegative, so you have a partially negative charge at this end and partially positive ends at this end. And that attraction between the partially positive end... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
It's more electronegative, so you have a partially negative charge at this end and partially positive ends at this end. And that attraction between the partially positive ends and the partially negative ends, that's what gives water all sorts of neat properties. Those are the hydrogen bonds. Those are the hydrogen bond... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
Those are the hydrogen bonds that give water all sorts of neat properties and keep it in its liquid state at just standard temperature and pressure. Now what I wanna think about is the surface in particular. And if you look at the surface of water, it might look completely smooth, but if you were to zoom in on a molecu... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
But roughly speaking, let's just say that this is roughly the surface of the water. Now what's going on at the surface? Well, all of these molecules are interacting through hydrogen bonds. Let's say this molecule right over here, it has hydrogen bonds pulling on it upwards, up to this one, pulling in this way, pulling ... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
Let's say this molecule right over here, it has hydrogen bonds pulling on it upwards, up to this one, pulling in this way, pulling it downwards, pulling it in really, to some degree, almost every direction. And they all have their kinetic energy and they're bumping around, but they're flowing past each other. The hydro... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
The molecules are attracted to each other. But if you look at the molecules on the surface, if you look at the ones on the surface, sure, they might have stuff pulling down on them, they might have stuff pulling them to the side, but they don't have anything pulling on them from above. And because of this, you could im... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
And that causes a phenomenon known as surface tension. So you have stronger, you have kind of a deeper, and this is still just hydrogen bonds, but since they're not being pulled in other directions by upwards by the air, they're able to get a little bit more closely packed and a little bit tighter. And this we refer to... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
Surface tension. And you have probably observed surface tension many, many, many times in your life in the form of, say, a water droplet. A water droplet, it's able to have this roughly round shape because all the little water molecules on the surface of the water droplet, and here the surface might even be on the bott... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
So they're able to form this type of a shape. You might have seen it if you go to a pond or a stream sometimes, so you see some still water, and say, let's say, let me do this in blue. So let's say that this is the surface of the water right over here. You might have seen insects that are able to walk on the surface of... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
You might have seen insects that are able to walk on the surface of the water, and I'm not doing a great job at drawing the insects, they don't look exactly like that, but they can walk on the surface of the water. You might have seen, or you might have even tried to do something like put a paper clip on the water. And... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
Unless you were to push on it a little bit, and allow it to puncture the surface, and then it would actually sink, which is what you would expect, because it is actually denser. You'd even see this if you were to take a cup. If you were to take a cup, and you were to fill it all the way up to the rim, and then a little... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
It won't immediately overflow. If you're very careful, you'll see that you form a bulge here. And that bulge is because those individual water molecules are more attracted to each other than they are to the surrounding air. So that allows for something of a little bulge. Obviously, if you keep pouring water, at some po... | Surface tension States of matter and intermolecular forces Chemistry Khan Academy.mp3 |
What I want to do in this video is give an overview of the endomembrane system in eukaryotic cells. Endomembrane system. And at a very high level, the endomembrane system is all of the membranes that interact with each other inside of a cell. So what membranes are we talking about? Well, you can start off by talking ab... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
So what membranes are we talking about? Well, you can start off by talking about the cell membrane itself. And all of these membranes, these have bilayers of phospholipids. Sometimes my brain malfunctions and I call them bilipid layers. But these are bilayers of phospholipids. So if I were to zoom in right over here, i... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
Sometimes my brain malfunctions and I call them bilipid layers. But these are bilayers of phospholipids. So if I were to zoom in right over here, if I were to zoom in right over there, that line, it really is a bilayer, a bilayer of phospholipids. So it would look like this. So you have your hydrophilic heads pointing ... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
So it would look like this. So you have your hydrophilic heads pointing outwards and your hydrophobic tails pointing inwards. So hydrophilic heads pointing outwards, hydrophobic tails pointing inwards, and it keeps going. So you have, if we think of it from left to right, you have a layer of two, or you have a bilayer,... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
So you have, if we think of it from left to right, you have a layer of two, or you have a bilayer, I should say, of phospholipids. That's going to be true of the cellular membrane. That's going to be true of the outer nuclear membrane. Right over here, we drew this one on the video on the endoplasmic reticulum. And so ... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
Right over here, we drew this one on the video on the endoplasmic reticulum. And so over here, you see these two membranes. You might say, okay, is this a bilayer? No, this is actually two bilayers. So this membrane right over here has a phospholipid bilayer, and this membrane over here also has a phospholipid bilayer.... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
No, this is actually two bilayers. So this membrane right over here has a phospholipid bilayer, and this membrane over here also has a phospholipid bilayer. This, the one, let me do this in another color, this one that I'm starting to trace in magenta, that's the outer membrane of the nuclear envelope, and it's continu... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
And then the one that I'm highlighting in this purple color, this is the inner membrane of the nuclear envelope. And all of this is part of the endomembrane system. So I've already started talking about the endoplasmic reticulum, and we go into some depth on that on the video on the endoplasmic reticulum and the Golgi ... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
And the endoplasmic reticulum in particular can represent up to or even more than 50% of the membrane associated, the phospholipid membrane associated with the cell. And we've talked about what goes on in the lumen of the endoplasmic reticulum. So this area right over here, right over here, we've talked about what happ... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
Proteins can get synthesized, actually other molecules like lipids can get synthesized there. They can, and then they can go to the smooth ER, and then the place where they can exit from the smooth ER, and we saw that in the video on the endoplasmic reticulum, how they can kind of bud out, we call this area, it's often... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
Transitional, transitional, transitional, transitional ER is this place where these proteins are being budded off, and they're budding off in vesicles. So this is the transitional ER. And all vesicles are are little small compartments that have a membrane around it that things like a protein can be transported in. And ... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
And not, you know, I don't want to beat a dead horse here, but all of these lines that I'm drawing, even though I drew it as a single line, these are phospholipid bilayers. So the membrane might be different, the phospholipid bilayers might be different when we go from one piece of the membrane to another, but they all... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
And when I say being matured, there's a bunch of enzymes in here, there's a bunch of Golgi enzymes in here, that can do all sorts of things to the proteins, tag them. They can actually add saccharides to them so that they become glycoproteins. They can tag them so they can be used in the cellular membrane, or be used o... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
So for example, this protein right over here, it butted off as a vesicle, it makes its way to the Golgi apparatus. The membrane can then merge and dump the protein into the Golgi apparatus. From there it can be matured, it might turn into a glycoprotein, who knows what happens to it. And then it could butt off again, a... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
And then it could butt off again, and then this protein that's now butted off, it could go to be embedded into the cellular membrane, the protein could be excreted from the cell, or it could go to other parts of the cell. Now those aren't everything I've just talked about, those aren't the only parts of the endomembran... | Endomembrane system Structure of a cell Biology Khan Academy (2).mp3 |
We already have an overview video of DNA, and I encourage you to watch that first. But what I want to do in this video is dig a little bit deeper, actually get into the molecular structure of DNA. And just as a starting point, let's just remind ourselves what DNA stands for. I'm gonna write the different parts of the w... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
I'm gonna write the different parts of the word in different colors. So it stands for deoxyribonucleic, ribonucleic, ribonucleic acid, ribonucleic acid. So I'm just gonna put this on the side. And now let's actually look at the molecular structure and how it relates to this actual name, deoxyribonucleic acid. So DNA is... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And now let's actually look at the molecular structure and how it relates to this actual name, deoxyribonucleic acid. So DNA is just a general term for nucleic acid. And the term nucleic comes from the fact that it's found in the nucleus, it's found in the nucleus of eukaryotes. So that's where the nucleic comes from. ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So that's where the nucleic comes from. And we'll talk about in a second why it's called an acid, but I'll wait on that. And now each DNA molecule is made up of a chain of what we call nucleotides. So what we call nucleotides. So it's made up of nucleotides. So what does a nucleotide look like? Well, what I have right ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So what we call nucleotides. So it's made up of nucleotides. So what does a nucleotide look like? Well, what I have right over here is I have two strands, I've zoomed in two strands of DNA. So you could view this side right over here as one of the, I guess you could say the backbones of one side of the ladder. This is ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Well, what I have right over here is I have two strands, I've zoomed in two strands of DNA. So you could view this side right over here as one of the, I guess you could say the backbones of one side of the ladder. This is the other side of the ladder. And then each of these bridges, and I will talk about what molecules... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And then each of these bridges, and I will talk about what molecules these are, these are kind of the rungs of the ladder. And a nucleotide, let me separate off a nucleotide. So a nucleotide would, so what I am coordinating off, what I am coordinating off right over here could be considered a nucleotide. So that's one ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So that's one nucleotide, and then it's connected to another. It's connected to another nucleotide, another nucleotide right over here. And on the right-hand side, we have a nucleotide, we have a nucleotide right over there, and then, actually, I wanna do it, let me do it slightly different. We have a nucleotide right ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
We have a nucleotide right over here on the right side, and then right below that, we have another, we have another nucleotide, we have another nucleotide. So depicted here, we essentially have four nucleotides. These two are on this left side of the ladder, these two are on the right side of the ladder. Now let's thin... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Now let's think about the different pieces of that nucleotide. So the one thing that might jump out at you is we have these phosphate groups. So this is a phosphate group right over here, this is a phosphate group right over here. Each of these nucleotides have a phosphate group. So this is a phosphate group over here,... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Each of these nucleotides have a phosphate group. So this is a phosphate group over here, and this is a phosphate group over here. Now the phosphate groups are actually what make DNA, or actually what make nucleic acid an acid. And you might say, wait, wait, the way you've drawn it, Sal, you have a negative charge. Som... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And you might say, wait, wait, the way you've drawn it, Sal, you have a negative charge. Something with a negative charge would attract protons, it would sop up protons. How can you call this an acid? This actually looks more basic. And the reason why DNA is typically drawn with these negative charges here is that it's... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
This actually looks more basic. And the reason why DNA is typically drawn with these negative charges here is that it's so acidic, and if you put it into a neutral solution, it's actually going to lose its hydrogens. Actually the DNA, if we actually want to be formal about it, the DNA molecules would actually have its ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Let me get rid of the negative charge just on this one. Whoops, just on this phosphate group over here. So if you get rid of the negative charge, and if this was bounded, this is bonded to a hydrogen, this so badly wants to grab these electrons. So this oxygen can grab these electrons, and then this hydrogen will just ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So this oxygen can grab these electrons, and then this hydrogen will just be grabbed by another water molecule or something, or so the proton will be let go. That's why we call it an acid. So if it wasn't in a solution, it would have the hydrogens, but it would be very acidic. As soon as you put it into a neutral solut... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
As soon as you put it into a neutral solution, it's going to lose those hydrogens. So the phosphate groups are what make it an acid, but it's confusing sometimes, because usually when you see it depicted, you see it with these negative charges, and that's because it has already lost its hydrogen protons. You're actuall... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So each of these nucleotides, they have a phosphate group. Now the next thing you might notice, the next thing you might notice is, the next thing you might notice is this group right over here. It is a cycle, it is a ring, and it looks an awful lot like a sugar, and that's because it is a sugar. So this sugar is based... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So this sugar is based on, it's a five-carbon sugar. What I have depicted here, this sugar, this is ribose. So this sugar right over here is ribose. This is when it's just as a straight chain, and like many sugars, it can take a cyclical form. Actually, it can take many different cyclical forms, but the one that's most... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
This is when it's just as a straight chain, and like many sugars, it can take a cyclical form. Actually, it can take many different cyclical forms, but the one that's most typically described is when you have the, let me show you a number of the carbons, because carbon numbering is important when we talk about DNA. If ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
One prime, two prime, three prime, four prime, and five prime. That's the five prime carbon. And so you form the cyclical form of ribose, is if you have the oxygen, you have the oxygen right over here on the four prime carbon, it uses one of its lone pairs, it uses one of its lone pairs to form a bond, to form a bond w... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And I drew it that way, because it kind of does bend, the whole molecule's going to have to bend that way to form this structure. And then when it forms that bond, the carbon can let go of one of these double bonds, and then that can, then the oxygen, the oxygen can use that, the oxygen can use those electrons to go gr... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And this form, just to be clear of what we're talking about, this is the one prime carbon, one prime, two prime, three prime, four prime, and five, five prime carbon. And where we see this bond, this is a one prime carbon, it was part of a carbonyl, now it lets go of one of those double bonds so that this oxygen can fo... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
This oxygen is that oxygen right there, notice, this oxygen is bound to the four prime carbon, and now it's also bound to the one prime carbon. And it was also attached to a hydrogen, It was also attached to a hydrogen, so that hydrogen is there. But then that could get nabbed up by another passing water molecule to be... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And net-net, it grabs up a hydrogen proton right over here, and so it can lose a hydrogen proton right there. So it's not adding or losing net-net. And so you form this cyclical form. And the cyclical form right over here is very close to what we see in a DNA molecule. It's actually exactly what we would see in an RNA ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And the cyclical form right over here is very close to what we see in a DNA molecule. It's actually exactly what we would see in an RNA molecule, in ribonucleic acid. And so what do we think we're talking about when we say deoxyribonucleic acid? Well, you could start with, you have a ribose here, but if we got rid of o... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Well, you could start with, you have a ribose here, but if we got rid of one of the oxygen groups, and in particular, one of, well, actually, if we just got rid of one of the oxygens, we replace a hydroxyl with just a hydrogen, well, then you're gonna have deoxyribose. And you see that over here. This five-member ring,... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
It looks just like this. The hydrogens are implicit to the carbons. We've seen this multiple times. The carbons are at where these lines intersect, or I guess at the edges, or maybe, and also where these lines end right over there. But you see, this does not have an, this molecule, if we compare these two molecules, if... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
The carbons are at where these lines intersect, or I guess at the edges, or maybe, and also where these lines end right over there. But you see, this does not have an, this molecule, if we compare these two molecules, if we compare these two molecules over here, we see that this guy has an OH, and this guy implicitly j... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So this is deoxyribose. So deoxyribose doesn't have this oxygen. It does not have the oxygen on the two prime carbon. So this, if you get rid of that, this is deoxyribose. So let me circle that. So what we're, this thing right over here, this thing right over here, that is deoxyribose. Deoxy, or it's based on deoxyribo... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So this, if you get rid of that, this is deoxyribose. So let me circle that. So what we're, this thing right over here, this thing right over here, that is deoxyribose. Deoxy, or it's based on deoxyribose, I guess before it bonded to these other constituents, you could consider this deoxyribose. And so that's where the... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Deoxy, or it's based on deoxyribose, I guess before it bonded to these other constituents, you could consider this deoxyribose. And so that's where the deoxyribo comes from. And then the last piece of it, the last piece of it is this chunk right over here. And these we call nitrogenous bases. So nitrogenous, nitrogenou... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And these we call nitrogenous bases. So nitrogenous, nitrogenous, nitrogenous bases. And you can see we have different types of nitrogenous bases. This is a nitrogenous base. This right over here is a different nitrogenous base. This right over here is another different nitrogenous base. Notice this one only has one ri... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
This is a nitrogenous base. This right over here is a different nitrogenous base. This right over here is another different nitrogenous base. Notice this one only has one ring, this one has one ring. This one has two rings. This one over here has two rings. And we have different names for these nitrogenous bases. | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Notice this one only has one ring, this one has one ring. This one has two rings. This one over here has two rings. And we have different names for these nitrogenous bases. The ones with two rings, the general categorization, we call them purines. So nitrogenous bases, if you have two rings, if you have two rings, we c... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And we have different names for these nitrogenous bases. The ones with two rings, the general categorization, we call them purines. So nitrogenous bases, if you have two rings, if you have two rings, we call them purines. That's a general classification term. Let me make sure, purines. And if you have one ring, maybe I... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
That's a general classification term. Let me make sure, purines. And if you have one ring, maybe I'll just write it this way, one ring, one ring, we call these pyrimidines. Pyrimidines. Pyrimidines. We call these pyrimidines. And these particular, these two on the right, these two purines, this one up here, this is ade... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Pyrimidines. Pyrimidines. We call these pyrimidines. And these particular, these two on the right, these two purines, this one up here, this is adenine. And we talk about how they pair in the overview video on DNA. But this one right over here is adenine, this nitrogenous base. This one over here is guanine. | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
And these particular, these two on the right, these two purines, this one up here, this is adenine. And we talk about how they pair in the overview video on DNA. But this one right over here is adenine, this nitrogenous base. This one over here is guanine. That is guanine. And then, over here, over here, this single ri... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
This one over here is guanine. That is guanine. And then, over here, over here, this single ring, this single ring nitrogenous base, which makes it a pyrimidine, this is thymine. This right over here is thymine. This is thymine. And then last but not least, if we're talking about DNA, when we go into RNA, we're also go... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
This right over here is thymine. This is thymine. And then last but not least, if we're talking about DNA, when we go into RNA, we're also gonna talk about uracil. But when we talk about DNA, this one over here is cytosine. Cy, cytosine. And you can see the way it's structured, that thymine is attracted to adenine, it ... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
But when we talk about DNA, this one over here is cytosine. Cy, cytosine. And you can see the way it's structured, that thymine is attracted to adenine, it bonds with adenine, and cytosine bonds with guanine. So how are they bonding? Well, the way that these nitrogenous bases form the rungs of the ladder, how they're d... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So how are they bonding? Well, the way that these nitrogenous bases form the rungs of the ladder, how they're drawn to each other, this is our good old friend hydrogen bonds. And this all comes out of the fact that nitrogen is quite electronegative, so when nitrogen is bound to a hydrogen, you're going to have a partia... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
Let me do this in green. You're gonna have a partial negative charge at the nitrogen and a partially positive charge at the hydrogen. And then oxygen, we've always talked about as being electronegative, so it has a partial negative charge. So the partial negative charge of this oxygen is going to be attracted to the pa... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
So the partial negative charge of this oxygen is going to be attracted to the partial positive charge of this hydrogen, and so you're going to have, you're going to have a hydrogen bond. And the same thing is going to happen between this hydrogen, which is going, its electrons are being hogged by this nitrogen, and thi... | Molecular structure of DNA Macromolecules Biology Khan Academy (2).mp3 |
If it's colored in, that means that they exhibit the trait. In this case, it's colorblindness. So Bill exhibits colorblindness. His phenotype is colorblind, while Bonnie does not exhibit colorblindness. Colorblindness is an X-linked recessive trait. If Barbara is expecting another child, so this is Barbara right here, ... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
His phenotype is colorblind, while Bonnie does not exhibit colorblindness. Colorblindness is an X-linked recessive trait. If Barbara is expecting another child, so this is Barbara right here, what is the probability that it will be colorblind? So pause this video and see if you can figure that out on your own. All righ... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
So pause this video and see if you can figure that out on your own. All right, now let's work through this together. So they're asking us about their next child here. What is the probability that it is going to be colorblind? And to help us with that, we can try to figure out the genotypes of Tom and Barbara. So Tom is... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
What is the probability that it is going to be colorblind? And to help us with that, we can try to figure out the genotypes of Tom and Barbara. So Tom is pretty straightforward. He is male, we know that because there's a square there. So X, he has an X chromosome and he has a Y chromosome. And colorblindness is an X-li... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
He is male, we know that because there's a square there. So X, he has an X chromosome and he has a Y chromosome. And colorblindness is an X-linked recessive trait. And so let me just make clear what's going on. So I'll do lowercase c for colorblind, colorblind. And I could do a capital C for the dominant trait, which i... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
And so let me just make clear what's going on. So I'll do lowercase c for colorblind, colorblind. And I could do a capital C for the dominant trait, which is not colorblind. But since they look so similar, I'll just use a plus for not colorblind, not colorblind. And so Tom, his phenotype, he is colorblind. And he only ... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
But since they look so similar, I'll just use a plus for not colorblind, not colorblind. And so Tom, his phenotype, he is colorblind. And he only has one X chromosome where the colorblind, what the colorblind trait is linked to. And so that must have the recessive allele right over there. So this is Tom's genotype. But... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
And so that must have the recessive allele right over there. So this is Tom's genotype. But what about Barbara? Well, we know Barbara's going to have two X chromosomes because Barbara is female. And we know that both of them can't be lowercase c because then Barbara would exhibit colorblindness. But how can we figure o... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
Well, we know Barbara's going to have two X chromosomes because Barbara is female. And we know that both of them can't be lowercase c because then Barbara would exhibit colorblindness. But how can we figure out her actual genotype? Well, we could look at her parents. So Bill over here is going to have the same genotype... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
Well, we could look at her parents. So Bill over here is going to have the same genotype as Tom, at least with respect to colorblindness. He is male, so he has an X chromosome and a Y chromosome. And because he exhibits colorblindness, that X chromosome must have the recessive colorblind allele associated with it. Now ... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
And because he exhibits colorblindness, that X chromosome must have the recessive colorblind allele associated with it. Now Bonnie, we do not know. She will be XX, will have two X chromosomes. Like Barbara, we know that both of these can't have the recessive allele because then Bonnie would be filled in. She would exhi... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
Like Barbara, we know that both of these can't have the recessive allele because then Bonnie would be filled in. She would exhibit colorblindness. But we don't know whether she is a carrier or whether she isn't. But let's just think about where Barbara got her chromosomes from. One of her X chromosomes comes from her f... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
But let's just think about where Barbara got her chromosomes from. One of her X chromosomes comes from her father. And the other one comes from her mother. So if she got this X chromosome from her father, her father only has one X chromosome to give, the one that has the colorblind allele. So if this is from her father... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
So if she got this X chromosome from her father, her father only has one X chromosome to give, the one that has the colorblind allele. So if this is from her father, it must have the colorblind allele here. And we know that the one from her mother does not have the colorblind allele because if it was like this, then Ba... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
So we know that this must be a plus here. It is the dominant non-colorblind allele. And so now we know both of their genotypes. And we can use those to then figure out the possible outcomes for their offspring. So for example, Tom can contribute a X chromosome that has a colorblind allele or a Y chromosome. And Barbara... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
And we can use those to then figure out the possible outcomes for their offspring. So for example, Tom can contribute a X chromosome that has a colorblind allele or a Y chromosome. And Barbara, right over here, can contribute an X chromosome that has a colorblind allele or an X chromosome that has the non-colorblind al... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
Barbara is a carrier. And so let me just draw a little Punnett square here. And so we have four possible outcomes for their children, and they're all equally likely. So you can get the X chromosome from Barbara that has a colorblind allele and the X chromosome from Tom that has the colorblind allele. You could have the... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
So you can get the X chromosome from Barbara that has a colorblind allele and the X chromosome from Tom that has the colorblind allele. You could have the X chromosome from Barbara with the colorblind allele and the Y chromosome from Tom. You could have the non-colorblind X chromosome, or the X chromosome that does not... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
Or you could have the non-colorblind X chromosome and the Y chromosome from the father. So there's four equal scenarios. And so in how many of these scenarios is the offspring colorblind? Well, here we have a colorblind female. She has two of the recessive alleles, so that female will be colorblind. This is a female ca... | Pedigree for determining probability of exhibiting sex linked recessive trait Khan Academy.mp3 |
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