Transcription and RNA processing
Molecular structure of RNA
- [Voiceover] We've already payed a lot of attention to the molecular structure of DNA. In fact right depicted in front of us, we have two strands of DNA forming a double helix, and we can look at the telltale signs that this is DNA. In particular, we can look at the five-carbon sugar on it's backbone. We see, and let's actually number the carbons. This is 1', 2', 3', 4', 5'. We can see on the 2' 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 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 and 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, how would we have to edit the left hand strand, if instead we wanted to imagine that the left hand strand is a messenger RNA being generated during transcription with a single strand of DNA here on the right? Well, to turn this into RNA, or to make it look like RNA, on the 2' carbon, well, we want to turn the deoxyribose into just ribose, so we would want to add a hydroxyl group right over here. So I add a hydroxyl group over there, actually do the hydrogens in white. So add one hydroxyl group there, and I want to do 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. So adding this hydroxyl group instead of just having another hydrogen, this tells us that this sugar is no long deoxyribose. This is ribose. So we now have ribose in our backbone, which is a telltale sign that 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 different than the nitrogenous bases on DNA. On DNA, your nitrogenous bases are Adenine, Guanine. Adenine and Guanine are the two ringed nitrogenous bases. Right over here, this is Adenine. This is Guanine. And you also have Cytosine I'm gonna do these all in different colors. Cytosine and Thymine. And this right over is Cytosine, and this is Thymine, Cytosine and Thymine are single ringed nitrogenous bases. We called 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, 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 right over here is Thymine, and it forms hydrogen bonds with Adenine right over here. If I want to turn it to Uracil, I just have to get rid of this methyl group right over here, so if I just do this and replace it with a Hydrogen, that is just implicitly bonded there, well, now I'm dealing with Uracil. So you see that Uracil and Thymine are very close molecules or very similar nitrogenous bases, and that's why they can play a very similar role. And it's still the case, what Uracil pairs with, it pairs with Adenine, the same thing Thymine pairs with. And everything else is, of course, still the same. An interesting question is why Uracil? Why not Thymine? Or you can 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 coating, it's a little less stable than Thymine. So Uracil makes the RNA molecule, or actually makes the machinery of information transfer, it makes it less stable. It's a less stable way to transfer information. Based on what I've read, in evolutionary history, RNA molecules, most people believe, predate DNA molecules. 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 information needed to be a little more persistent and a little less error prone, well then, Thymine helped stabilize things. There's also the view, "why has Uracil stuck around?" Well, RNA molecules, they have all of these roles in cells. Messenger RNA molecules are taking information from the DNA and getting it transcribed or getting it translated at the ribosome. But they shouldn't hang out forever. 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, all of this business, actually let me do this in a different color. all of this business, this strand right over here, we can now, the way it's drawn, we can now consider this an RNA molecule, and if we assume that this is happening during transcription where a single strand of DNA would want to replicate it's information, then this over here would be mRNA, messenger RNA, and so what's going on here? Well, let's think about it. The messenger RNA, the way it's oriented, if we go, we have phosphate group, then we go to 5' carbon, 4', 3', then phosphate group, then 5', 4', 3', then phosphate group, so this is oriented 5' on top, 3' on the bottom, while these DNA molecules are oriented the other way. This is a 5' carbon. This is a 3' carbon, so we have phosphate, 3', 5', phosphate, so we have 3' is on top, and 5' 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 mRNA molecule here, and this is it's 5' end, and this is it's 3' end, and then the top nitrogenous base over here, this is Uracil. And then the second one over here, this is Cytosine. This is Cytosine. This is Cytosine over here, and this is being transcribed from this DNA molecule on the right hand side, so this is DNA, and this DNA has an antiparallel orientation It's parallel, but it's kinda flipped over. The sugars are pointed in a different direction, so this is going from the 3' end. This is the 5' end. And we see that the Uracil is hydrogen bonded to Adenine. That is Adenine And I'll draw dotted lines to show the hydrogen bonds. And that the Cytosine is hydrogen bonded to Guanine. So this right over here, that is Guanine. Actually I'll do the hydrogen bonds in white. Actually there's multiple hydrogen bonds going on here, but just to be clear, this is mRNA, and on the right, we have DNA. 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 has an important role in taking information from DNA and getting it eventually translated with the help of tRNAs in ribosomes, and though I've just mentioned another type of RNA, and that's transfer RNA, so transfer RNA, tRNA. And in the overview video on transcription and translation, we talk about how tRNA does this, but it has amino acids attached on one end, and then it has anticodons on the other end that essentially pair with codons on the mRNA, and, then, thus allows it to construct proteins. And actually, this right over here is a visualization of a tRNA molecule. 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 example here with tRNA. And you can see it's an interesting configuration, where the amino acid will attach roughly in that area, and then you see the anticodon right down here in the bottom right, 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, and they actually play a structural role in ribosomes, which is where translation occurs. And you also have things called microRNA, which are short chains of RNA, which could be used to regulate the translation of other RNA molecules. So DNA gets a lot of the attention, but RNA is really, really, really important, and a lot of people believe that RNA came first, and there's potential that the first life or pseudo-life ever was just self-replicating RNA molecules, and that DNA eventually evolved from RNA, but RNA stuck around, because it's still very useful.
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