Transcription and mRNA processing | Biomolecules | MCAT | Khan Academy

What we're going to do in this video is a little bit of a deep dive on transcription. And just as a a bit of a review, we touch on it on the video on on replication, transcription and translation. Transcription in everyday language just means to rewrite something or to rewrite some information in another form.

And that's essentially what's happening here. Transcription is when we take the information encoded in a gene in DNA and encode essentially that same information in mRNA. So transcription, we are going from DNA, we are going from DNA to messenger, messenger RNA. And we're going to in this video focus on genes that code for proteins.

So this first step is the transcription, the DNA to messenger RNA. And then in a future video, we'll dig a little bit deeper into translation. We will translate that information into into an actual protein.

But these diagrams give a little bit of an overview of it. It's a little bit simpler in bacteria. You have the DNA just floating around in the cytosol. And so the transcription takes place. You you start with that DNA, that protein coding gene in that in the DNA. And then from that, you code the messenger RNA. You see that in that purple color right over here. And then that messenger RNA can be involved with the ribosome and that's the translation process to actually produce the polypeptide, to produce the protein.

In eukaryotic cells, and we're going to get into a little bit more depth in this video.

The transcription, the DNA to mRNA, that happens inside of the nucleus.

And there's essentially two steps here. You go from DNA to what we would call pre-mRNA, let me write that down, pre-mRNA, which would be, which is depicted right over there. And then it needs to be processed to turn into what we would call mRNA, which then can leave the nucleus to be translated into a protein.

So now that we have that overview, let's dig a little bit deeper into this.

And understand the different actors and understand if we're talking about a eukaryotic cell.

What type of processing might actually go on.

So right over here, we are going to start with the protein coding gene inside of the DNA, right over here. And we are going to the the primary actor that's not the DNA or the mRNA here is going to be RNA polymerase. It's used to create a sequence that will become a nucleotide sequence that will become the messenger RNA.

So this RNA polymerase, it needs to know where to start. And the way it knows where to start is it it attaches to a sequence of the DNA known as a promoter. And every gene is going to have a promoter associated with it, especially if we're talking about eukaryotic cells. Sometimes you might have a pro a a promoter associated with a collection of genes as well.

But in general, if you've got a gene, you're going to have a promoter. And so that's where the that's how the RNA polymerase knows to attach right over there.

And so once it attaches, well then it is able to separate the strands. It separates the strands. And it's pretty interesting because when we get went in deep into replication, you saw all of these actors, the helicase and whatever else, but this RNA polymerase complex is actually quite capable. Not only it it separates the strands, and then it's actually able to code for the RNA.

And it does that the same way that when we study DNA polymerase, it does it in only one direction. It it can only add more nucleotides on the three prime end. So it encodes from the five prime to the three prime direction. Notice this arrow here, we're extending it on the three prime end of the RNA.

And so as you can see here, when it does this, it's only it's only encoding one side of or it's only interacting, I guess you could say, or or coding complementary information to one side. But let's think about this a little bit. We could call the side that it is that is that it is forming that it is interacting with. You could call that the template strand because that's forming that side of the DNA is acting as the template for forming that RNA.

But if you think about the information that that RNA is actually going to encode, well, it's going to it it's going to contain the same information as the coding strand of DNA, is the other strand of DNA. Because these include these nucleotides right over here, this nucleotide, it's going to be complementary to this one over here, just as this comp this nucleotide was complementary to that one over there.

And you can see it in a little bit more depth if we actually were to add the nucleotides. So this is the template strand. If you have a thymine, well, on the on the RNA, you'd have the adenine.

And look, on the coding strand of DNA, the one up here, you would also have an adenine. And they are essentially the coding strand and the RNA essentially end up being the same sequence with the one difference is that you won't find the thymine in the RNA. Instead, you will find a similar you'll you'll find a a similar uh nitrogenous base, and that is uracil.

But uracil plays the role of thymine, so you're essentially coding the same information.

So once again, this this bottom strand is acting as a template, but it's going to be the resulting RNA that gets coded is essentially going to have the same information that we had in the coding strand.

And just to get an appreciation for what this looks like, and I I would even write, you know, I'd put looks in in quotation, I even did a little quote things with my fingers when I said that, is that, you know, it's it's hard to really visualize what these things look like, but you can see here that the RNA polymerase complex, and this is for a specific organism, can be very, very complex and involved and and it's it's fascinating how these things interact.

And you know, every time you're studying biology and someone like me is going to give you these nice clean narratives of how the the these these enzymes interact with the different macromolecules like the DNA or the RNA.

You should always remember it it this is amazing. This these are these molecules interacting with each other, bouncing into each other. Uh it's happening incredibly fast inside of the cell.

Uh you should be in awe of this. It it's it's happening in in in in all of your cells or as we as we speak.

So this is this is pretty incredible stuff.

So the next thing you have to want to think about, you know, this right over here, we are extending the RNA.

Well, when does this thing actually stop?

And it stops once once we so this this RNA polymerase is going to keep going on and then as blue, we're we've labeled this a terminator. So let me write. So this area is a terminator. And there's multiple ways that that signals to the RNA polymerase that, hey, it's it's time to stop. Or in more particularly, that it it somehow creates something structurally that the polymerase just lets go.

One mechanism that's depicted right over here is that the the the mRNA that's coded and this is typical this or this can happen in bacteria is that the mRNA that's coded forms a hairpin. So it has to have the right complementary base pairs, base pairs right over here to form this hairpin. But this hairpin along with the things around the hairpin, essentially, uh make it impair the the polymerase to keep on going. And so the complex kind of changes a little bit and so it lets go, or at least that's how people believe it.

There's other forms of of how the terminator can act. It might be uh sequences that parts of the polymerase complex um recognize and it forms it it makes a a confirmation change so that the RNA polymerase lets go.

Now, if we're talking about a if we're talking about a prokaryote, uh we're done. We would have formed this would be our messenger RNA, which then can go to a ribosome and then be translated into a protein.

But if we're talking about a eukaryote, well, then we have to do a little bit of processing.

If this is a prokaryote right over here, this would be our mRNA.

If this is a eukaryote, then this is our pre-mRNA, which now has to be processed.

And you might say, well, how is that going to be processed?

Well, there's a couple of things that are going to be done. Some things are going to be added at the beginning and the end of the mRNA, the five prime cap. This is a modified guanine, modified guanine right over here, which is going to help in the translation process as the ribosomes attach onto it.