DNA replication and RNA transcription and translation | Khan Academy
- [Voiceover] We've already talked about how DNA's structure as this double helix, this twisted ladder, makes it suitable for being the molecular basis of heredity.
And what we wanna do in this video is get a better appreciation for why it is suitable, and the mechanism by which it is the molecular basis for heredity
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, and we talked about this in the introductory video to DNA, DNA is much more than, you know, a handful of base pairs. The 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, let me do this 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 might have 70 million base pairs. 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 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. 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, you can imagine taking either splitting these two sides of the ladder, and actually let's do that. So let me copy and paste, so if I take that side right over there, 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 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 things you could call them two sides of the ladder, that 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 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. Alright, so from this side, from this left side, or at least what we are looking at as 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 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, 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 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 T is paired with the A, let me do that in adenine's color
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.
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 can 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 the entire cell duplicates itself. So that's replication. So the next thing you're probably thinking about, "Okay, well 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 actually expressed. So I'll write this as "expression". And actually that 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 here that has this sugar phosphate base and it has the sequence of base pairs, it's got this double helix structure, and so this whole thing this could be a DNA molecule.
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 a certain type of protein.
So for example this could be, this whole thing could be a strand of DNA, but this part right over, let's say in orange I'll do it, this part in orange right over here could be one gene, it might define information for one gene, it could define a protein, this section right over here 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 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. Actually let me write this down. RNA. So RNA stands for ribonucleic acid. Ribonucleic acid, let me write that down. 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 come from, the R is right over there, then you have the nucleic, that's the n, and then it's a, acid. Same reason why we call the DNA nucleic acid. So you have this RNA. 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 nucleii, 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 be that messenger, that messenger between a certain section of DNA and kind of what goes on outside of the nucleus, so that that can be translated into an actual protein. So the step that you go from DNA to mRNA, messenger RNA, is called transcription. Let me write that down. And what happens in transcription, let's go back to looking at 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 wanna 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 part of this DNA molecule, or you have 1/2 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. At least for that section of, at least for that gene. So
this might be part of a gene Actually whoops, let me make sure I'm using the right tool
This might be part of a gene that is this section of our DNA molecule right over there.
And so transcription is a very similar conceputal idea, where we're now going to construct a strand of RNA and specifically mRNA 'cause it's going to take that information outside of the nucleus
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 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, that's the base we're talking about, let me write it down, uracil.
Thymine is still going to pair with adenine, just like that
Guanine is gonna pair with cytosine, and cytosine is 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 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.
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 draw it like this. Now let's see, I have it is U A C, so it's gonna be U A C then U U then A C G okay? And then we have an A, let me make sure I change it to the right color. We have an A there, and then we have this U U A, C G, alright, 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 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, these I guess you could say this three letter word or this three letter sequence, that's called a codon. And this is going to be the next codon. And we actually haven't drawn the next codon after that 'cause 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 four times four times four is 64. 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 that 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 amino acid 1. So let me just write it here, this is amino acid 1. 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 gonna, it's got some structure here, I'm not drawing it completely right, but it's going to match right over here, where maybe it has an A, a U, and a G right over here and on this end it was attached to this amino acid, and so it matches them together. And then they're gonna have another tRNA that might attach to amino acid 2, which I will do in purple, and that just happens to coincide with, so it can complement right over here, so it attaches in the right place, so it's A A U right over here, this tRNA. And so it'll construct the sequence of amino acids. And as you put these amino acids together, then you're actually constructing a protein. So protein is essentially a bunch, a sequence of these amino acids put together. And these proteins are essentially the molecules that run life for the most part. Obviously you know if you eat an animal it's going to be made up of fat and sugars and proteins, but the proteins are the things that actually do a lot of the whether they're enzymes, whether they're structural, the muscle is formed from proteins, these are the things, and I'm just drawing a small segment of them, they could be thousands or more of these amino acids long. And they kind of form these incredibly complex shapes and they have all of these functions. This is what's kind of doing the work of life. And this for the most part, and this is kind of how the information for life is stored.
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