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But there are more complicated explanations. And we don't know, some of those more complicated explanations could very well be true. But from this, we have a very quick and easy graphical representation of how related different species could be and where they share common ancestors. So a bald eagle and an alligator, ba... | Phylogenetic trees Evolution Khan Academy.mp3 |
We've already talked about the process from going from DNA to messenger RNA, and we call that process transcription, and this occurs in the nucleus. And then that messenger RNA makes its way outside of the nucleus, and it attaches to a ribosome, and then it is translated into a protein. It is translated into a protein.... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And so you could say that this part right over here, this is happening as this is being facilitated by a ribosome, or it's happening at a ribosome. With that high-level overview, I now want to think a little bit in more detail about how this actually happens, or the structure of things where this happens inside of a ce... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So this right over here, this over here is the nucleus. Actually, let me draw it like this. And instead of just drawing the nucleus with one single line, I'm going to draw it with two lines, because it's actually a double bilipid membrane. So this is one bilipid layer right over here, and then this is another one right... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So this is one bilipid layer right over here, and then this is another one right over here. And I'm obviously not drawing it to scale. I'm drawing it so that you can get a sense of things. So each of these lines that I'm drawing, if I were to zoom in on this, so if I were to zoom in on each of these lines, so let's zoo... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So each of these lines that I'm drawing, if I were to zoom in on this, so if I were to zoom in on each of these lines, so let's zoom in. And if I got a box like that, you would see a bilipid layer. So a bilipid layer looks like this. You have the circle as a hydrophilic end, and those lines are the fatty hydrophobic en... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
You have the circle as a hydrophilic end, and those lines are the fatty hydrophobic ends. So that's our bilipid layer. So that's each of these lines that I have drawn. Each of them are a bilipid layer. So the question is, well, how does the mRNA, obviously you have all this transcription going on. You have the DNA. You... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Each of them are a bilipid layer. So the question is, well, how does the mRNA, obviously you have all this transcription going on. You have the DNA. You have the mRNA. It's all in here, this big jumble of chromatin inside the nucleus. How does it make its way outside of this double bilipid layer? And the way it makes i... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
You have the mRNA. It's all in here, this big jumble of chromatin inside the nucleus. How does it make its way outside of this double bilipid layer? And the way it makes its way out is through nuclear pores. So a nuclear pore is essentially a tunnel, and there are thousands of these, is a tunnel through this bilipid la... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And the way it makes its way out is through nuclear pores. So a nuclear pore is essentially a tunnel, and there are thousands of these, is a tunnel through this bilipid layer. And the tunnel is made up of a bunch of proteins. So this right over here, and this is kind of a cross section of it, but you could almost imagi... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So this right over here, and this is kind of a cross section of it, but you could almost imagine it, if you're thinking of it in three dimensions, you would imagine a tunnel, a tunnel made out of proteins that goes through this double bilipid membrane. And so the mRNA can make its way out and get to a free ribosome and... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So let me draw the entire cell right over here. So this is the cell. This right over here is the cytosol of the cell. And you might be sometimes confused with the term cytosol and cytoplasm. Cytosol is all the fluid between the organelles. Cytoplasm is everything that's inside the cell. So it's the cytosol and the orga... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And you might be sometimes confused with the term cytosol and cytoplasm. Cytosol is all the fluid between the organelles. Cytoplasm is everything that's inside the cell. So it's the cytosol and the organelles and the stuff inside the organelles is the cytoplasm. So cytoplasm is everything inside of the cell. Cytosol is... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So it's the cytosol and the organelles and the stuff inside the organelles is the cytoplasm. So cytoplasm is everything inside of the cell. Cytosol is just the fluid that's between the organelles. So anyway, the free ribosome over here, this translation is good for proteins used within the cell itself. The proteins can... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So anyway, the free ribosome over here, this translation is good for proteins used within the cell itself. The proteins can then float around the cytosol and used in whichever way is appropriate. But how do you get protein outside of the cell or even inside the cellular membrane? Not within the cell, but embedded in th... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Not within the cell, but embedded in the cell membrane or outside of the cell itself? And we know that cells communicate in all sorts of different ways. And they produce proteins for other cells or for use in the bloodstream or whatever it might be. And that's what we're going to focus on in this video. So contiguous w... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And that's what we're going to focus on in this video. So contiguous with this what's called the perinuclear space right over here, so the space between these two membranes. So you have this perinuclear space between the inner and outer nuclear membrane. Let me just label that. That's the inner nuclear membrane. That's... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Let me just label that. That's the inner nuclear membrane. That's the outer nuclear membrane. You could continue this outer nuclear membrane, and you get into these kind of flaps and folds and bulges. And this right over here is considered a separate organelle. So you get this thing that looks like this. And I'll just ... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
You could continue this outer nuclear membrane, and you get into these kind of flaps and folds and bulges. And this right over here is considered a separate organelle. So you get this thing that looks like this. And I'll just do it the best that I can draw it. And this right over here is called the endoplasmic reticulu... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And I'll just do it the best that I can draw it. And this right over here is called the endoplasmic reticulum. So this right here is endoplasmic reticulum, which I've always thought would be a good name for a band. And the endoplasmic reticulum is key for starting to produce and then later on package proteins that are ... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And the endoplasmic reticulum is key for starting to produce and then later on package proteins that are used for that are either embedded in the cellular membrane or used outside of the cell itself. So how does that happen? Well, the endoplasmic reticulum really has two regions. It has the rough endoplasmic reticulum.... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
It has the rough endoplasmic reticulum. And the rough endoplasmic reticulum has a bunch of ribosomes. So that's a free ribosome right over here. This is an attached ribosome. These are ribosomes that are attached to the membrane of the endoplasmic reticulum. So this region where you have attached ribosomes right over h... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
This is an attached ribosome. These are ribosomes that are attached to the membrane of the endoplasmic reticulum. So this region where you have attached ribosomes right over here, that is the rough endoplasmic reticulum. I'll call it the rough ER for short, perhaps an even better name for a band. And then there's anoth... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
I'll call it the rough ER for short, perhaps an even better name for a band. And then there's another region, which is a smooth endoplasmic reticulum. And the role that this plays in protein synthesis, or at least getting proteins ready for the outside of the cell, is you can have messenger RNA. Let me do that in that ... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Let me do that in that lighter green color. You can have messenger RNA find one of these ribosomes associated with the rough endoplasmic reticulum. And as the protein is translated, it won't be translated inside the cytosol. It'll be translated on the other side of the rough endoplasmic reticulum. Or you could say on t... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
It'll be translated on the other side of the rough endoplasmic reticulum. Or you could say on the inside of it, on the lumen, or in the lumen of the rough endoplasmic reticulum. Let me draw that a little bit better. So let's say that this right over here is the membrane of the endoplasmic reticulum. And then as a prote... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So let's say that this right over here is the membrane of the endoplasmic reticulum. And then as a protein, or as an mRNA is being translated into protein, the ribosome can attach. And let's say that this right over here is the mRNA that is being translated. Let's say it's going in that direction right over here. Here ... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Let's say it's going in that direction right over here. Here is the membrane of the ER. So ER membrane. This right over here, and actually the way I've drawn it right over here, this is just one bilipid layer. So let me just, I could draw it like this. I could do it like this. And this is actually, this bilipid layer i... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
This right over here, and actually the way I've drawn it right over here, this is just one bilipid layer. So let me just, I could draw it like this. I could do it like this. And this is actually, this bilipid layer is continuous. It's continuous with the outer nuclear membrane. So let me just make it like that so you g... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And this is actually, this bilipid layer is continuous. It's continuous with the outer nuclear membrane. So let me just make it like that so you get the picture. And then at some point in the translation process, the protein can be spit out on the inside. As it's being translated, it can be spit out on the inside of th... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And then at some point in the translation process, the protein can be spit out on the inside. As it's being translated, it can be spit out on the inside of the endoplasmic reticulum. So this is the lumen. This is the ER lumen right over here. So we're inside the endoplasmic reticulum here. Here we're outside. Here we'r... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
This is the ER lumen right over here. So we're inside the endoplasmic reticulum here. Here we're outside. Here we're outside in the cytosol. So that way you get the protein now inside the ER, inside the endoplasmic reticulum. And it can travel through it. And at some point, it can bud off. | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Here we're outside in the cytosol. So that way you get the protein now inside the ER, inside the endoplasmic reticulum. And it can travel through it. And at some point, it can bud off. So let's say, imagine the protein is right over here. And the smooth endoplasmic reticulum has many functions. And I won't get into all... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And at some point, it can bud off. So let's say, imagine the protein is right over here. And the smooth endoplasmic reticulum has many functions. And I won't get into all of the depth of how it's involved. But at some point, that protein can bud off. So let me draw a budding off protein. So let's say this is the membra... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And I won't get into all of the depth of how it's involved. But at some point, that protein can bud off. So let me draw a budding off protein. So let's say this is the membrane of the endoplasmic reticulum. And a protein, let's say, ends up right over here. And then it can bud out. So it could go from that to that. | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So let's say this is the membrane of the endoplasmic reticulum. And a protein, let's say, ends up right over here. And then it can bud out. So it could go from that to that. I think you see where this is going. And then it could go to something like this. It could go from this to something like this. | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So it could go from that to that. I think you see where this is going. And then it could go to something like this. It could go from this to something like this. Now it has budded out. And when you have a protein, or really you have anything that's being transported around a cell with its own little mini membrane, we c... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
It could go from this to something like this. Now it has budded out. And when you have a protein, or really you have anything that's being transported around a cell with its own little mini membrane, we call this a vesicle. So now it'll bundle up. And now it is a vesicle. Now this vesicle can then, let me draw some of ... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So now it'll bundle up. And now it is a vesicle. Now this vesicle can then, let me draw some of these vesicles holding some proteins. So let me draw that. It can then go to the Golgi apparatus, which I'll draw in blue right over here as best as I can. So the Golgi apparatus. This is not, obviously, there could be bette... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
So let me draw that. It can then go to the Golgi apparatus, which I'll draw in blue right over here as best as I can. So the Golgi apparatus. This is not, obviously, there could be better drawings of something like this. And then they can essentially do the reverse process. And they can attach themselves to the Golgi, ... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
This is not, obviously, there could be better drawings of something like this. And then they can essentially do the reverse process. And they can attach themselves to the Golgi, oftentimes the Golgi body, named after Mr. Golgi, who discovered this. And then the proteins, once they get into the inside of the Golgi body,... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
And then the proteins, once they get into the inside of the Golgi body, then they essentially go into a maturation process so that they're ready for transport outside of the cell or maybe to be embedded into the cellular membrane. So this right over here is the Golgi body, or a Golgi body, or a Golgi apparatus. Apparat... | Endoplasmic reticulum and Golgi bodies Biology Khan Academy.mp3 |
Now the cell cycle is not a sort of thing that occurs in a very unchecked manner. There's actually a lot of regulation in play here. In fact, there are two key places that we have extensive regulation of the cell cycle. The first checkpoint is right here between the G1 and the S phase. So we regulate before we get to t... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
The first checkpoint is right here between the G1 and the S phase. So we regulate before we get to the point of DNA replication. The other major checkpoint is right here between G2 and the step where we jump right to mitosis. And there are a couple of proteins that regulate this process. Two main ones are called cyclin... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
And there are a couple of proteins that regulate this process. Two main ones are called cyclin-dependent kinases. So cyclin-dependent kinases, which as you may recall, a kinase is something that adds a phosphate group. So I'll put in parentheses, it'll plus a phosphate group. And it'll add a phosphate group on other en... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
So I'll put in parentheses, it'll plus a phosphate group. And it'll add a phosphate group on other enzymes or proteins to either activate or inactivate them. And these cyclin-dependent kinases will work together with a protein you might be able to guess the name of, cyclins. Because what else would these kinases depend... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
Because what else would these kinases depend on? So an important thing to notice is that these cyclin-dependent kinases or CDKs are always present. All the different types are always present in a cell, but their default form or their default function is for them to be inactive. And so they need to be activated by these... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
And so they need to be activated by these cyclin proteins. And the point of regulation here is that specific cyclins, so I'll write specific with just spec, so specific cyclins are made at specific times. And again, the reason why they're both so important is that when you have a cyclin-dependent kinase, it is only act... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
It's at this point again that this guy is active. And the CDK is the business end of this complex. So that's the reason why in G1 you'll see the production of cyclins D and cyclin E. And from there you'll see CDK2 bound to your cyclin E, and at the same time you'll also have your CDK4 bound to your cyclin D. These acti... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
So I'll draw just a little reaction over here where we add a phosphate group on our RB protein. So when RB is phosphorylated, it can't inhibit DNA replication like it usually is supposed to do. The phosphate group renders it inactive. And this is sort of the setup we have as we go further on in our cell cycle. In the S... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
And this is sort of the setup we have as we go further on in our cell cycle. In the S phase we have cyclin A produced. Cyclin A will complex again with CDK2 most directly to activate DNA replication. So it helps to activate DNA replication. And in a similar way we have cyclin B only produced in the G2 phase because the... | Cell cycle control Cells MCAT Khan Academy (2).mp3 |
And we're gonna focus on a conceptual level. I'm not gonna go into all of the, I guess you could say biochemical details. Really just give you the conceptual idea of what happens. So right over here, this could be a fragment of DNA. I have what I have, this is eight base pairs depicted. And just to be clear, we talked ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So right over here, this could be a fragment of DNA. I have what I have, this is eight base pairs depicted. And just to be clear, we talked about this in the introductory video to DNA. DNA is much more than a handful of base pairs. A DNA molecule can be tens of millions of base pairs long. So for example, this might be... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
DNA is much more than a handful of base pairs. A DNA molecule can be tens of millions of base pairs long. So for example, this might be a section of a much longer molecule. So the much longer strand of DNA, and even there I'm probably not giving justice to it, but this might just be this very, very small section, right... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So the much longer strand of DNA, and even there I'm probably not giving justice to it, but this might just be this very, very small section, right, let me just in a different color, this little section right over here zoomed in. So once again, it might be part of a molecule that has not seven or eight base pairs, but ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So just like that. So let's understand what a molecular basis of heredity would need to do. Well first of all, it would need to be replicable. Or something, we would need to be able to replicate it. As a cell divides, the two new cells would want to have the same genetic material. So how does DNA replicate? And this pr... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Or something, we would need to be able to replicate it. As a cell divides, the two new cells would want to have the same genetic material. So how does DNA replicate? And this process is called replication. So let me, replication, and we covered this in the introduction video as well, but it's nice to see the different ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And this process is called replication. So let me, replication, and we covered this in the introduction video as well, but it's nice to see the different processes next to each other. And replication, well you can imagine taking either, splitting these two sides of the ladder, and actually let's do that. So let me copy... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So let me copy and paste. So if I take that side right over there, and so let me copy and then paste it. And then there we go. A little bit of it is dropping below the video, but I think that serves the purpose. And then, and then let's copy and paste the other side. So let me select that. And then I copy, and then I p... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
A little bit of it is dropping below the video, but I think that serves the purpose. And then, and then let's copy and paste the other side. So let me select that. And then I copy, and then I paste. And it's just like that. And so you can imagine, if you were to split these, these, I guess you could call them two sides... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And then I copy, and then I paste. And it's just like that. And so you can imagine, if you were to split these, these, I guess you could call them two sides of the ladder, then either side could be used to construct the other side, and then you would have two strands, two identical strands of the DNA. And so let's see ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And so let's see what that actually looks like. So let me get my pen tool out now. Let me deselect this. Get the pen tool out. It's a new tool I'm using, so let me make sure I'm doing it right. All right, so from this side, from this left side, or at least what we are looking at is the left side, you can then construct... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Get the pen tool out. It's a new tool I'm using, so let me make sure I'm doing it right. All right, so from this side, from this left side, or at least what we are looking at is the left side, you can then construct another right side based on this information. A always pairs with T, if we're talking about DNA. So aden... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
A always pairs with T, if we're talking about DNA. So adenine pairs with thymine, just like that. Thymine pairs with adenine. Let me do that a little bit neater. Thymine pairs with adenine. Guanine pairs with cytosine. That's cytosine. | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Let me do that a little bit neater. Thymine pairs with adenine. Guanine pairs with cytosine. That's cytosine. Cytosine pairs with guanine, falling a little bit down here. And just like that, I was able to construct a new right-hand side using that left-hand side. So maybe I'll do the new, the new sugar phosphate backbo... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
That's cytosine. Cytosine pairs with guanine, falling a little bit down here. And just like that, I was able to construct a new right-hand side using that left-hand side. So maybe I'll do the new, the new sugar phosphate backbone in yellow. And we can do the same thing here using the original right-hand side. So using ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So maybe I'll do the new, the new sugar phosphate backbone in yellow. And we can do the same thing here using the original right-hand side. So using the original right-hand side, once again, the Ts pair with the As, thymine, let me do that in that adenine's color. So we have an adenine and thymine, adenine and thymine,... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So we have an adenine and thymine, adenine and thymine, adenine and thymine. Thymine pairs with adenine, so thymine, adenine. Thymine, adenine. Guanine pairs with cytosine. Guanine, guanine, and then cytosine pairs with guanine. So cytosine, just like that. And so you can take half of each of this ladder and then you c... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Guanine pairs with cytosine. Guanine, guanine, and then cytosine pairs with guanine. So cytosine, just like that. And so you can take half of each of this ladder and then you could use it to construct the other half, and what you've essentially done is you've replicated the actual DNA. And this is actually a kind of co... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And so you can take half of each of this ladder and then you could use it to construct the other half, and what you've essentially done is you've replicated the actual DNA. And this is actually a kind of conceptual level of how replication is done before a cell divides and replicates, and then the entire cell duplicate... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So the next thing you're probably thinking about, okay, well, you know, it's nice to be able to replicate yourself, but that's kind of useless if that information can't be used to define the organism in some way, to express what's actually happening. And so let's think about how the genes in this DNA molecule are actua... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Expression. And actually, that actually warrants a little bit of a detour, because you hear sometimes the words DNA and chromosome and gene used somewhat interchangeably, and they are clearly related, but it's worth knowing what is what. So when you're talking about DNA, you're talking literally about this molecule her... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Now, when you have a DNA molecule and it's packaged together with other molecules and proteins and kind of given a broader structure, then you're talking about a chromosome. And when you're talking about a gene, you're talking about a section of DNA that's used to express a certain trait, or actually, used to code for ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
It might define, it could, this information for one gene, it could define a protein. This one might, this section right over here could do, could be used to define another gene. And genes could be anywhere from several thousand base pairs long all the way up into the millions. And as we'll see, the way that a gene is e... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And as we'll see, the way that a gene is expressed, the way we get from the information for that section of DNA into a protein, which is really how it's expressed, is through a related molecule to DNA, and that is RNA. And actually, let me write this down. RNA, RNA. So RNA stands for ribonucleic acid. Ribonucleic acid,... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So RNA stands for ribonucleic acid. Ribonucleic acid, let me write that down. Ribo, ribonucleic, nucleic acid. Nucleic acid. And so you might remember that DNA is deoxyribonucleic acid so the sugar backbone in RNA is a very similar molecule. Well, now it's got its oxy, it's not deoxyribonucleic acid, it's ribonucleic a... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Nucleic acid. And so you might remember that DNA is deoxyribonucleic acid so the sugar backbone in RNA is a very similar molecule. Well, now it's got its oxy, it's not deoxyribonucleic acid, it's ribonucleic acid. The R, let me make it clear where the RNA comes from. The R is right over there, then you have the nucleic... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
The R, let me make it clear where the RNA comes from. The R is right over there, then you have the nucleic, that's the N, it's found, or, well, it's nucleic, and then it's A, acid, same reason why we called the DNA nucleic acid. So you have this RNA. So what role does this play as we are trying to express the informati... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So what role does this play as we are trying to express the information in this DNA? Well, the DNA, especially if we're talking about cells with nuclei, the DNA sits there, but that information has to, for the most part, get outside of the nucleus in order to be expressed. And one of the functions that RNA plays is to ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So the step that you go from DNA to mRNA, messenger RNA, is called transcription. Let me write that down. Transcription. Transcription. Transcription. And what happens in transcription, let's go back to looking at one side of this, one side of this DNA molecule. So let's say you have that right over there. | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Transcription. Transcription. And what happens in transcription, let's go back to looking at one side of this, one side of this DNA molecule. So let's say you have that right over there. Let me copy and paste it. So there we go. Actually, I didn't want to do that. | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So let's say you have that right over there. Let me copy and paste it. So there we go. Actually, I didn't want to do that. I wanted the other side. So actually, I think I'm on the wrong, let me go back here. And so let me copy and then let me paste. | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Actually, I didn't want to do that. I wanted the other side. So actually, I think I'm on the wrong, let me go back here. And so let me copy and then let me paste. There we go. So let's say you have, let's say that you have part of this DNA molecule, or you have one half of it, just like we did when we replicated it, bu... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And so let me copy and then let me paste. There we go. So let's say you have, let's say that you have part of this DNA molecule, or you have one half of it, just like we did when we replicated it, but now we're not just trying to duplicate the DNA molecule, we're actually trying to create a corresponding mRNA molecule,... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So this might be part of a gene that, actually, whoops, let me make sure I'm using the right tool. This might be part of a gene that is, you know, this section of our DNA molecule right over there. And so transcription is a very similar conceptual idea where we're now going to construct a strand of RNA, and specificall... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And so it's very similar, except for when we're talking about RNA, adenine, instead of pairing with thymine, is now going to pair with uracil. So let me write this down. So now you're gonna have adenine pairs not with thymine, but uracil. DNA has uracil instead of the thymine, but you're still going to have cytosine an... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
DNA has uracil instead of the thymine, but you're still going to have cytosine and guanine pairing. So for the RNA, and in this case the mRNA, that's going to leave the nucleus, A is going to pair with U. U for uracil. So uracil, uracil, that's the base we're talking about. Let me write it down. Uracil, uracil. Thymine... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Let me write it down. Uracil, uracil. Thymine is still going to pair with adenine. It's still going to pair with adenine. Just like that. Guanine's gonna pair with cytosine. Guanine, and cytosine, and cytosine's going to pair. | DNA replication and RNA transcription and translation Khan Academy.mp3 |
It's still going to pair with adenine. Just like that. Guanine's gonna pair with cytosine. Guanine, and cytosine, and cytosine's going to pair. Cytosine's going to pair with guanine. And so when you do that, now these two characters can detach, and now you have a single strand of RNA, and in this case messenger RNA, th... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Guanine, and cytosine, and cytosine's going to pair. Cytosine's going to pair with guanine. And so when you do that, now these two characters can detach, and now you have a single strand of RNA, and in this case messenger RNA, that has all the information on that section of DNA. And so now that thing can leave the nucl... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And so now that thing can leave the nucleus, go attach to a ribosome, and we'll talk more about that in future videos, exactly how that's happened. And then this code can be used to actually code for proteins. Now how does that happen? And that process is called translation. So translation, translation, which is really... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And that process is called translation. So translation, translation, which is really taking this base pair sequence and turning it into an amino acid sequence. Proteins are made up of sequences of amino acids. So translation. So let's take our mRNA, or this little section of our mRNA, and actually let me do it, let me ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So translation. So let's take our mRNA, or this little section of our mRNA, and actually let me do it, let me draw it like this. Let me draw it like this. And let's see, I have, it is UAC, so it's gonna be UAC, then UU, then ACG, okay? And then we have an A, let me make sure I change to the right color. We have an A th... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And let's see, I have, it is UAC, so it's gonna be UAC, then UU, then ACG, okay? And then we have an A, let me make sure I change to the right color. We have an A there, and then we have this UUA, CG, all right? Now let me put a C right over there. I'm just taking this and I'm writing it horizontally. I have a C here, ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
Now let me put a C right over there. I'm just taking this and I'm writing it horizontally. I have a C here, not a G, it's a C. And then finally I have a G. And of course it'll keep going on and on and on. And what happens is, each sequence of three, and you have to be very careful where it starts, and so this is in som... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And what happens is, each sequence of three, and you have to be very careful where it starts, and so this is in some ways a delicate and surprising, but at the same time surprisingly robust process, every three of these bases code for a specific amino acid. And so three bases together, so these bases right over here, I... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
And we actually haven't drawn the next codon after that because we need three bases to get to the next codon. And how many possible codons do you have? Well, you have one of four bases and you have them in three different places, so you have four times four times four possible codon words, I guess you could say, and fo... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So you have 64 possible codons, which is good because you have 20 possible amino acids. So this is overkill and allows codons to be used for other purposes as well. And they also, you might have more than one codon coding for the same amino acid. So you have 64 possible codons, they need to code for 20 amino acids. And... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
So you have 64 possible codons, they need to code for 20 amino acids. And so this codon right over here, with the ribosome, and we'll talk more about how that happens, can code for, can code for, say, could code for amino acid one. So let me just write it here. This is amino acid one. And actually this amino acid is br... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
This is amino acid one. And actually this amino acid is brought to here. They're actually matched together by another type of RNA. This is mRNA we're talking about right over here. This is mRNA, but there's another type of RNA called tRNA that essentially brings these two characters together. So the tRNA, and I'm just ... | DNA replication and RNA transcription and translation Khan Academy.mp3 |
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