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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 partially negative charge at th... | Molecular structure of DNA Macromolecules Biology Khan Academy (3).mp3 |
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 partial positive charge of ... | Molecular structure of DNA Macromolecules Biology Khan Academy (3).mp3 |
What we're going to do in this video is give ourselves a little bit of a tour of eukaryotic cells. And the first place to start is just to remind ourselves what it means for a cell to be eukaryotic. It means that inside the cell, there are membrane-bound organelles. Now what does that mean? Well, you could view it as s... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
Now what does that mean? Well, you could view it as sub-compartments within the cell, membrane-bound organelles. And in this video in particular, we're going to highlight some of these membrane-bound organelles that make the cells eukaryotic. So let's just start with some of the ingredients that we know is true of all ... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
So let's just start with some of the ingredients that we know is true of all cells. So you'll have your cellular membrane here, a little bit big so that we have a lot of space to draw things in. So this is our cellular membrane. I'll do a nice shading so you appreciate that it'll actually be three-dimensional. We see s... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
I'll do a nice shading so you appreciate that it'll actually be three-dimensional. We see so many slices of cells that sometimes we forget that they are more spherical or that they have three-dimensional shape to them. They're not all spherical. They can have different shapes. Now all cells, and there are some exceptio... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
They can have different shapes. Now all cells, and there are some exceptions that we've talked about in previous videos, I should say most cells will have some genetic information in them in the form of DNA. So that is our DNA right over there. Now one of the key characteristics of a eukaryotic cell is that that geneti... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
Now one of the key characteristics of a eukaryotic cell is that that genetic information is going to be inside a membrane-bound organelle. And that membrane-bound organelle or the membrane that binds or that surrounds the DNA here, that is the nuclear membrane. So let me draw the nuclear membrane right over here. And I... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And I'll put some shading in to appreciate that that also is going to be in three dimensions around the DNA. And so that is the first membrane-bound organelle that we're going to discuss, the nucleus. Now the nucleus, it turns out, is connected to another membrane-bound organelle. And we're gonna study this in future v... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And we're gonna study this in future videos. What right here, I'm drawing holes or pores in the nuclear membrane. And those pores connect to something, it's a very fancy word, called the endoplasmic reticulum. And the endoplasmic reticulum is essentially these layers of these membranes. So I'm gonna do my best job at t... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And the endoplasmic reticulum is essentially these layers of these membranes. So I'm gonna do my best job at trying to draw an endoplasmic reticulum. Imagine extending from these pores, going into a space that has these, really these layered membranes that have a lot of surface area. And I'm not gonna go all the way ar... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And I'm not gonna go all the way around this nucleus, but in many cells, it will go around all the way around the nucleus. And this right over here, and this is just a rough diagram, that is our endoplasmic, endoplasmic, not splasmic, endoplasmic, endoplasmic reticulum, which I've mentioned in previous videos would be ... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
That mRNA will make its way out of that nuclear membrane through one of these pores, and then make its way to a ribosome that is attached to the membrane of the endoplasmic reticulum. And so that's a ribosome there. I'm gonna do a bunch of ribosomes. And so, as we've talked about in previous videos, the ribosomes are r... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And so, as we've talked about in previous videos, the ribosomes are really where you take that genetic information from that mRNA, and then you translate it into a protein. So the ribosomes are the protein synthesis, so let me label that. So this right over here is a ribosome. And some ribosomes might be attached to th... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And some ribosomes might be attached to the endoplasmic reticulum. Some of them might just be floating out here in the cytoplasm, so that would be a free ribosome. Free ribosome. And even from the point of view of the endoplasmic reticulum, the parts of the endoplasmic reticulum where you have ribosomes attached, this ... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And even from the point of view of the endoplasmic reticulum, the parts of the endoplasmic reticulum where you have ribosomes attached, this is known as rough endoplasmic reticulum. It's the ribosomes that are making them rough. It looks that way in a microscope. So I'll say rough ER for endoplasmic reticulum for short... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
So I'll say rough ER for endoplasmic reticulum for short. And then you also have parts of the endoplasmic reticulum where you do not have ribosomes attached, and because that looks smooth through our microscope, it has been called, you can imagine, smooth endoplasmic reticulum. There are things known as Golgi bodies. O... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
Once again, another fascinating name. Gotta love these names in biology. That look kind of like an endoplasmic reticulum, but detached from the nuclear membrane. So let's say it's something like that. That's my best drawing there. That's a Golgi body. And these are really good at packaging molecules, even proteins that... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
So let's say it's something like that. That's my best drawing there. That's a Golgi body. And these are really good at packaging molecules, even proteins that might have just been produced, and packaging them so that they can be used outside of the cell, for example. So, and we'll go into detail in other videos where a... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And these are really good at packaging molecules, even proteins that might have just been produced, and packaging them so that they can be used outside of the cell, for example. So, and we'll go into detail in other videos where a protein might go to the Golgi body, get a little envelope around it, get some little proc... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
And so I'll do this mitochondria in magenta because that's a nice, powerful color. So mitochondria, and I love mitochondria because it's fascinating how they even came to be. Mitochondria actually have their own DNA, and all of your mitochondrial DNA comes from your mother, and so that's actually very interesting for t... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
But mitochondria, this is where your, I'm gonna say, let's see what we can see inside of this. This is where your ATP is produced. This is your mitochondria. It's really the powerhouse of the cell. What's interesting about mitochondria is evolutionary biologists believe that the ancestors of mitochondria, because mitoc... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
It's really the powerhouse of the cell. What's interesting about mitochondria is evolutionary biologists believe that the ancestors of mitochondria, because mitochondria have their own DNA, they might have been independent organisms, independent cells, and at some point in our evolutionary past, they started living in ... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
We don't have them, because we don't have photosynthesis, but this is a chloroplast, and if you could see inside, you could see the little thylakoid stacks right over here. You could see the little thylakoids if you could see inside, and so this right over here is a chloroplast, chloroplast, and this would be plants an... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
Now, there's also some other membrane-bound organelles that are maybe less famous than the mitochondria or the chloroplast or, for sure, the nucleus, and that might be something like a vacuole, and in plants, vacuoles tend to be very big. I could draw it, you know, this is three-dimensional, so I'll draw it on top of s... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
It's viewed as a kind of a storage compartment, but it can also contain enzymes that help digest things, that help break things down so that they can be used in some way, so that is a vacuole, and they don't just exist in plants. They can also exist in animal cells, but in plant cells, they tend to be, they can be very... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
Now, I'll leave you there. These aren't all of the structures in eukaryotic cells, but these are enough of the structures so that you can appreciate that there are a lot of membrane-bound organelles in eukaryotic cells, and to be clear, even if I were to show all of the membrane-bound structures, that's not all the com... | Organelles in eukaryotic cells Cells High school biology Khan Academy.mp3 |
So that special case is called a retrovirus. So first, let's zoom in and take a look at some unique things about the retrovirus that make it different from other viruses. So first of all, it is an enveloped, single-stranded RNA virus. And inside of this envelope, it also carries three special proteins. And right now, j... | Retroviruses Cells MCAT Khan Academy.mp3 |
And inside of this envelope, it also carries three special proteins. And right now, just be aware that there are three special proteins. I'll talk about them more when we get to each step where they're important. So as you know, enveloped viruses can enter in one of two ways, either through tricking receptors, receptor... | Retroviruses Cells MCAT Khan Academy.mp3 |
So as you know, enveloped viruses can enter in one of two ways, either through tricking receptors, receptor-mediated endocytosis, or through direct fusion. And it just so happens that in our example, and we're talking here about the retrovirus HIV, this retrovirus will enter the cell with direct fusion. So now that thi... | Retroviruses Cells MCAT Khan Academy.mp3 |
Oh, we forgot about the protein, so let me redraw those in right now. So these are the proteins that were originally inside of that capsid. So everything inside of that coat is released. And this is where the first special step occurs. So we're gonna say that this red protein is reverse transcriptase. So reverse transc... | Retroviruses Cells MCAT Khan Academy.mp3 |
And this is where the first special step occurs. So we're gonna say that this red protein is reverse transcriptase. So reverse transcriptase will hop on to the RNA, and it reverse transcribes the RNA, which means that, so it reads from 5' to 3' end, and you will form complementary DNA, shown here in pink. And the reaso... | Retroviruses Cells MCAT Khan Academy.mp3 |
And the reason it's called reverse transcription is usually you take DNA to make RNA, but in this case, you take RNA to make DNA. And because this is the complementary DNA strand, we call this cDNA, complementary. And then reverse transcriptase will work again on this same RNA to make another cDNA strand. Because it's ... | Retroviruses Cells MCAT Khan Academy.mp3 |
Because it's the same exact code, it can recombine with the other cDNA strand to make a double-stranded DNA. And so now what happens is you have integrase coming along. And let's make integrase blue. So integrase comes along, clips off each of the 3' ends. So now these are slightly shorter on each end. And sorry, this ... | Retroviruses Cells MCAT Khan Academy.mp3 |
So integrase comes along, clips off each of the 3' ends. So now these are slightly shorter on each end. And sorry, this is a bit hard to see because this strand's 3' end is over here. And while the first one is actually clearly labeled as this is the 3' end. And by clipping off those 3' sections, they form these sticky... | Retroviruses Cells MCAT Khan Academy.mp3 |
And while the first one is actually clearly labeled as this is the 3' end. And by clipping off those 3' sections, they form these sticky ends because unpaired DNA wants to be paired, and integrase has suddenly removed that part. And you might be wondering what happens to this RNA, and what happens is that it actually g... | Retroviruses Cells MCAT Khan Academy.mp3 |
So that's no longer there. And integrase does exactly what it says. It will follow this path and integrate this HIV DNA into the host's DNA. And one thing I just want to very quickly mention is that if I had drawn this to be super accurate, this would need to have a nucleus around it because the HIV retrovirus infects ... | Retroviruses Cells MCAT Khan Academy.mp3 |
And one thing I just want to very quickly mention is that if I had drawn this to be super accurate, this would need to have a nucleus around it because the HIV retrovirus infects human eukaryotic cells, which have a nucleus. So it actually will travel through the nuclear membrane to get to the genome. And here, integra... | Retroviruses Cells MCAT Khan Academy.mp3 |
So just imagine this is all double-stranded, but just for simple drawing's sake, this will just be one line. So this is viral DNA, and this is called the provirus stage. So you can see that this is similar to the lysogenic cycle that we talked about before. But unlike the regular lysogenic cycle, it's not dormant or la... | Retroviruses Cells MCAT Khan Academy.mp3 |
But unlike the regular lysogenic cycle, it's not dormant or latent. It actually does not have that repressor gene, that typical lysogenic viruses have. So it is actively transcribed whenever the host's DNA is transcribed. So since the host cell thinks this is normal DNA, it will make RNA. And I just wanted to call this... | Retroviruses Cells MCAT Khan Academy.mp3 |
So since the host cell thinks this is normal DNA, it will make RNA. And I just wanted to call this viral mRNA so you have an idea that the cell cannot tell that this mRNA shouldn't have happened. So this mRNA exits the nucleus, and these viral RNAs are now in the cytosol. Again, once this viral mRNA exits the nucleus a... | Retroviruses Cells MCAT Khan Academy.mp3 |
Again, once this viral mRNA exits the nucleus and goes into the cytoplasm, it's just like any other RNA, and some of these will be translated into proteins, like the capsid proteins, and of course the three proteins that we began with, which are the reverse transcriptase, the integrase, and actually the last one we hav... | Retroviruses Cells MCAT Khan Academy.mp3 |
And you can see that you now have all of the parts that can self-assemble into new viruses. So again, all of these viruses that are formed will have the RNA, the reverse transcriptase, the integrase, and the protease. So you'll notice that these are actually missing one thing. They're missing their envelope. And so the... | Retroviruses Cells MCAT Khan Academy.mp3 |
They're missing their envelope. And so they're called immature viruses. And unlike the typical lytic cycle, it doesn't just break open the membrane. In fact, it takes advantage of the membrane. And so these viruses will come along, and they will bud off. So this one will enter here, and this one will enter here. Oops, ... | Retroviruses Cells MCAT Khan Academy.mp3 |
In fact, it takes advantage of the membrane. And so these viruses will come along, and they will bud off. So this one will enter here, and this one will enter here. Oops, and that's missing a border, I just realized. So there you go. And they will bud off, and that will be their envelope. And sorry, they're missing the... | Retroviruses Cells MCAT Khan Academy.mp3 |
Oops, and that's missing a border, I just realized. So there you go. And they will bud off, and that will be their envelope. And sorry, they're missing the proteins, and I'll just draw them in again. So again, these are still immature, right? And before they go on to infect other cells, they have to mature somehow. So ... | Retroviruses Cells MCAT Khan Academy.mp3 |
And sorry, they're missing the proteins, and I'll just draw them in again. So again, these are still immature, right? And before they go on to infect other cells, they have to mature somehow. So what happens is that protease inside of here will cleave those other proteins to make sure that they're fully functional befo... | Retroviruses Cells MCAT Khan Academy.mp3 |
And we can look at the telltale signs that this is DNA. And in particular, we can look at the five-carbon sugar on its backbone. We see, and let's actually number the carbons. This is one prime, two prime, three prime, four prime, five prime. We can see on the two-prime carbon, we don't have an oxygen attached to it. W... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
This is one prime, two prime, three prime, four prime, five prime. We can see on the two-prime carbon, we don't have an oxygen attached to it. We don't have a hydroxyl group attached to it. And because of that, we know that this is not ribose. This is deoxyribose. This right over here is deoxyribose. And these two are ... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And because of that, we know that this is not ribose. This is deoxyribose. This right over here is deoxyribose. And these two are also deoxyribose. So that tells us that we have two strands of DNA, deoxyribonucleic acid. So let me write this down. This part of the chain, this is derived from a deoxyribose being attache... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And these two are also deoxyribose. So that tells us that we have two strands of DNA, deoxyribonucleic acid. So let me write this down. This part of the chain, this is derived from a deoxyribose being attached to phosphate groups in a nitrogenous base. So deoxyribose. So what would we have to do if we wanted, instead o... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
This part of the chain, this is derived from a deoxyribose being attached to phosphate groups in a nitrogenous base. So deoxyribose. So what would we have to do if we wanted, instead of viewing this as two strands of DNA in a double helix formation, well, how would we have to rearrange, how would we have to edit the le... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Well, to turn this into RNA, or to make it look like RNA, on the two prime carbon, well, we wanna turn the deoxyribose into just ribose, so we would wanna add a hydroxyl group right over here. So add a hydroxyl group over there. Actually, let me do that. Do the hydrogens in white. So add one hydroxyl group there. And I... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Do the hydrogens in white. So add one hydroxyl group there. And I wanna do it on all the sugars on the left strand's backbone. If I want this to be a single strand of RNA, and RNA tends to be single-stranded. So oxygen and then a hydrogen. And so this hydroxyl, adding this hydroxyl group, instead of just having another... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
If I want this to be a single strand of RNA, and RNA tends to be single-stranded. So oxygen and then a hydrogen. And so this hydroxyl, adding this hydroxyl group, instead of just having another hydrogen, just a hydrogen by itself over there, this tells us that this sugar is no longer deoxyribose. This is ribose. So now... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
This is ribose. So now we have ribose. We now have ribose in our backbone, which is a telltale sign that, well, at least now we have the backbone of RNA, ribonucleic acid, versus DNA, deoxyribonucleic acid. Now you might think we're done, but we're not quite done, because the nitrogenous bases on RNA are slightly diffe... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Now you might think we're done, but we're not quite done, because the nitrogenous bases on RNA are slightly different than the nitrogenous bases on DNA. On DNA, your nitrogenous bases are adenine, guanine, are adenine, guanine, and adenine and guanine are the two-ringed nitrogenous bases right over here. This is adenin... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And you also have cytosine. Cytosine, I'm gonna do these all in different colors. Cytosine and thymine. I'm getting to the punchline too fast. And this right over here is cytosine, and this is thymine. And cytosine and thymine are single-ringed nitrogenous bases. We call them pyrimidines, adenine and guanine. | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
I'm getting to the punchline too fast. And this right over here is cytosine, and this is thymine. And cytosine and thymine are single-ringed nitrogenous bases. We call them pyrimidines, adenine and guanine. We call them purines. This is a little bit of a review. In RNA, you still have adenine, you still have guanine, y... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
We call them pyrimidines, adenine and guanine. We call them purines. This is a little bit of a review. In RNA, you still have adenine, you still have guanine, you still have cytosine, but instead of thymine, you have a very close relative of thymine, and that is uracil. So the way that this is drawn right now, this nit... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
In RNA, you still have adenine, you still have guanine, you still have cytosine, but instead of thymine, you have a very close relative of thymine, and that is uracil. So the way that this is drawn right now, this nitrogenous base, remember when we started this video, it was double-stranded DNA, this nitrogenous base r... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
So if I just do this, if I just do this, and if I were to replace it with a hydrogen that is just implicitly bonded there, well now I'm dealing with uracil. So now I'm dealing with uracil. So you see that uracil and thymine are very close molecules, or they're very similar nitrogenous bases, and that's why they can pla... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And it's still the case. And so what uracil pairs with, it pairs still with adenine, the same thing that thymine pairs with, and everything else is of course still the same. Now an interesting question, an interesting question is why uracil? Why not thymine? Or you could say why thymine? Why not uracil? And based on wh... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Why not thymine? Or you could say why thymine? Why not uracil? And based on what I've read, it actually turns out that uracil is a little bit more error prone. It might be able to bond with other things when you're coding. It's a little bit less stable than thymine. And so uracil, uracil, uracil makes the RNA molecule,... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And based on what I've read, it actually turns out that uracil is a little bit more error prone. It might be able to bond with other things when you're coding. It's a little bit less stable than thymine. And so uracil, uracil, uracil makes the RNA molecule, or actually makes the machinery of information transfer, it ma... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And so uracil, uracil, uracil makes the RNA molecule, or actually makes the machinery of information transfer, it makes it less stable. It's a less stable, I guess, way to transfer information. And based on what I've read, in evolutionary history, RNA molecules, most people believe, predate DNA molecules. And then when... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And then when you, so in the early stages you had a lot of change, and so uracil molecules were just fine, and there was a lot of errors and whatever else, but then once you had, I guess, information needed to be a little bit more persistent and a little less error prone, well then thymine helped stabilize, thymine hel... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
You actually want them to be somewhat unstable. So it's an interesting question to think about. Why do we have uracil instead of thymine? Or why do we have thymine instead of uracil? But this is one of the telltale signs of, that we are now dealing with an RNA molecule. So now what we have on the left hand side, now al... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Or why do we have thymine instead of uracil? But this is one of the telltale signs of, that we are now dealing with an RNA molecule. So now what we have on the left hand side, now all of this business, actually let me do this in a different color, all of this business, this strand, this strand right over here, we can n... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And if we assume that this is happening during transcription, when a DNA molecule, where a single strand of DNA would want to replicate its information, then this over here would be mRNA, messenger, messenger RNA. And so what's going on here? Well, let's think about it. This one, the way it's, the RNA, the messenger RN... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
This one, the way it's, the RNA, the messenger RNA, the way it's oriented, we have, if we go, we have phosphate group, then we go to five prime carbon, four prime, three prime, then phosphate group, then five prime, four prime, three prime, then phosphate group. So this is oriented five prime on top, three prime on the... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
This is a five prime carbon, this is a three prime carbon. So we have phosphate, three prime, five prime, phosphate. So we have three prime is on top, and five prime is on the bottom. So if we wanted to think about what's happening, maybe using the symbols for the nitrogenous bases, we could say, all right, we have our... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
So if we wanted to think about what's happening, maybe using the symbols for the nitrogenous bases, we could say, all right, we have our mRNA molecule here, and this is its five prime end, and this is its three prime end. And then the first, the top nitrogenous base right over here, this is uracil. This is uracil. And ... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And then the second one over here, this is, sorry, over here, this is cytosine. So this is cytosine. This is cytosine right over here. And this is being transcribed from a DNA molecule, from this DNA molecule on the right-hand side. So this is DNA. And this DNA has an anti-parallel orientation. It's parallel, but it's ... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And this is being transcribed from a DNA molecule, from this DNA molecule on the right-hand side. So this is DNA. And this DNA has an anti-parallel orientation. It's parallel, but it's kind of flipped over. The sugars are pointed in a different direction. So this is going from, this is the three prime end, this is the ... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
It's parallel, but it's kind of flipped over. The sugars are pointed in a different direction. So this is going from, this is the three prime end, this is the five prime end. And we see that the uracil is hydrogen bonded to adenine. Adenine right over here. So adenine, and I'll draw dotted lines to show the hydrogen bo... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And we see that the uracil is hydrogen bonded to adenine. Adenine right over here. So adenine, and I'll draw dotted lines to show the hydrogen bonds. And that the cytosine is hydrogen bonded to guanine. To guanine. So this right over here, that is guanine. And actually I'll do the hydrogen bonds in white. | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And that the cytosine is hydrogen bonded to guanine. To guanine. So this right over here, that is guanine. And actually I'll do the hydrogen bonds in white. So, you know, they are, actually there's multiple hydrogen bonds going on here. But just to be clear, this is mRNA, and on the right we have DNA. And this could be... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And actually I'll do the hydrogen bonds in white. So, you know, they are, actually there's multiple hydrogen bonds going on here. But just to be clear, this is mRNA, and on the right we have DNA. And this could be happening during transcription. This could be happening during, I'm having trouble changing colors. This c... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And this could be happening during transcription. This could be happening during, I'm having trouble changing colors. This could be happening during transcription. Now what are the types of RNAs out there? We've talked about this in other videos. Well you have messenger RNA, which is an important role in taking informa... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Now what are the types of RNAs out there? We've talked about this in other videos. Well you have messenger RNA, which is an important role in taking information from DNA and getting it eventually translated with the help of tRNAs and ribosomes. And though I've just mentioned another type of RNA, and that's transfer RNA... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And though I've just mentioned another type of RNA, and that's transfer RNA. So transfer RNA, tRNA. tRNA. And in the video, the overview video on transcription and translation, we talk about how tRNA does this. But it brings amino acids, it has amino acids attached at one end, and then it has anticodons on the other en... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And in the video, the overview video on transcription and translation, we talk about how tRNA does this. But it brings amino acids, it has amino acids attached at one end, and then it has anticodons on the other end that essentially pair, that pair with codon fragment or codons on the mRNA, and then that allows it to c... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
So a lot of times when we think about DNA, we think about, okay, mRNA or RNA is an intermediary to be able to eventually translate it into proteins. And that is often the case, but sometimes you also just want the RNA itself. The RNA itself plays a role in the cell beyond just transmitting information. And that's an ex... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And that's an example here with tRNA. And you can see it's interesting configuration where the amino acid will attach roughly in that area up there. And then you see the anticodon, the anticodon right down here in the bottom right. And different tRNA molecules will attach to different amino acids and they'll have diffe... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And different tRNA molecules will attach to different amino acids and they'll have different anticodons here. So this is another use for RNA. And then others include ribosomal RNA. Ribosomal RNA. And they actually play a structural role in ribosomes, which is where translation occurs. And you also have things called mi... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
Ribosomal RNA. And they actually play a structural role in ribosomes, which is where translation occurs. And you also have things called microRNA. MicroRNA, which are short chains of RNA, which could be used to regulate the translation of other RNA molecules. So RNA, you know, DNA gets a lot of the attention, but RNA i... | Molecular structure of RNA Macromolecules Biology Khan Academy (2).mp3 |
And then you have a sugar. And then you have a phosphate group and then you have a sugar. And so I could draw the strand something like this. So phosphate and then we have a sugar. Whoops, let me just draw all the phosphates ahead of time. So you have the phosphates on that end and then you have the sugars. And you see... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
So phosphate and then we have a sugar. Whoops, let me just draw all the phosphates ahead of time. So you have the phosphates on that end and then you have the sugars. And you see the same thing on the other strand as well where we have phosphate, phosphate with a sugar, then another phosphate, then a sugar, then anothe... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
And you see the same thing on the other strand as well where we have phosphate, phosphate with a sugar, then another phosphate, then a sugar, then another phosphate. Let me circle the sugars as well. So you have a sugar there and then you have the sugar there as well. So on the other strand, it's also going to look lik... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
So on the other strand, it's also going to look like this. So let me draw the phosphates. I'm just abstracting them now. So the phosphate and then you have the sugars in between the phosphates. And what links them, you can think of them as the rungs on the ladder, these are the complementary nitrogenous bases. And the ... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
So the phosphate and then you have the sugars in between the phosphates. And what links them, you can think of them as the rungs on the ladder, these are the complementary nitrogenous bases. And the reason why we call them nitrogenous bases, I actually forgot to talk about in the last videos, is that these nitrogens ar... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
They have an extra lone pair, the nitrogens have an extra lone pair that can be used up under the right conditions to potentially sop up more hydrogen protons. Now, a lot of people ask, well, if you have these nitrogenous bases here, why is DNA called an acid? Why is it called an acid? Well, the first thing is that the... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
Well, the first thing is that the basic properties of the nitrogenous base are offset to a good degree based on the fact that they're able to hydrogen bond with each other. And that's what actually forms these rungs, the rungs of the ladder when these complementary nitrogenous bases form these hydrogen bonds with each ... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
Now, the reason why we tend to draw them deprotonated is when they're so acidic that if you put them in a neutral solution, they're going to be deprotonated. So this is the form that you're more likely to find it in the nucleus of an actual cell once it's actually already deprotonated. But in general, phosphate groups ... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
And if I were to draw kind of a more pure phosphate group, and I talked about this already in the last video, I would have it protonated, and so I wouldn't draw that negative charge like that. So that's just a review of last time. But let's actually, it doesn't mean just, since I already started abstracting it, let's a... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
So let's draw the nitrogenous bases a little bit. So I have thymine here, and I will do thymine in this green color. So this right over there is thymine. So this is attached to thymine. And the complementary nitrogenous base to thymine is adenine, which I will do. Let's see, I'm running out of colors here. Let's see, a... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
So this is attached to thymine. And the complementary nitrogenous base to thymine is adenine, which I will do. Let's see, I'm running out of colors here. Let's see, adenine. I'll do this in an orange color. It's got so many nitrogens on it. So actually, so let me, so it actually should include that hydrogen right over ... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
Let's see, adenine. I'll do this in an orange color. It's got so many nitrogens on it. So actually, so let me, so it actually should include that hydrogen right over there. So this right over here is adenine. And they form, they have these hydrogen bonds between them right over here because they have partially negative... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
So actually, so let me, so it actually should include that hydrogen right over there. So this right over here is adenine. And they form, they have these hydrogen bonds between them right over here because they have partially negative and positive charges on either end that are attracted to each other. And then we go to... | Antiparallel structure of DNA strands Biology Khan Academy.mp3 |
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