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Current time:0:00Total duration:10:25

Transcription and mRNA processing

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Video transcript

what we're going to do in this video is a little bit of a deep dive on transcription and just as 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 at 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 and 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 and 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 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 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 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 part 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 go too 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 separates the strand 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 can only add more nucleotides on the 3-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 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 is forming that it is interacting with you call that the template strand because that's forming that's acting 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 its 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 is going to be complementary to this one over here just as this come this nucleotide was complementary to that one over there and you can see it 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 are the RNA you would 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 ends 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 find a a similar nitrogenous base and that is your cell but your cell plays the role of thymine so you're essentially coding the same information so once again 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 would even write you know put looks in quotations I even do the quote things with my fingers when I said that is that you know 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 it's fascinating how these things interact and every time you're studying biology and someone like me is going to give you these nice clean narratives of how the these these enzymes interact with the different macromolecules like the DNA or the RNA should always remember it this is amazing this these are these molecules interacting with each other bouncing into each other it's happening incredibly fast inside of the cell you should be in awe of this 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 what does this thing actually stop and it stops once once we so this this RNA polymerase is going to keep going on and then is blue where we we've labeled this a terminator so let me right 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 and more particularly that it somehow creates something structurally that the the polymerase just let's go one mechanism that's depicted right over here is that the the the mRNA that's coated and this is typical at 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 there's hairpin along with the things around the hairpin essentially 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 how the Terminator can act it might be sequences that parts of the polymerase complex recognize and it forms it makes a conformation change so that the RNA polymerase let's go now if we're talking about a if we're talking about a prokaryote we're done we would have formed this will 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 we're talk about a eukaryote 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 modified guanine right over here which is going to help in the translation process as the ribosomes attached on to it and then you have this poly a tail and it's called a poly a tail because it has a bunch of ad means at the end right over here and these not only help in the translation process it helps it helps make sure that the information is more robust at the ends of the MRNA don't in some way become or makes it less likely that they're going to become damaged now the other thing that needs to be processed and this is one of those fascinating things in evolutionary biology is that we will have in this M&R in this mRNA sequence you're going to have parts of the sequence which we currently consider to be nonsense sequence nonsense sequences and we call them introns and I'm going to put it in quotes because in general and evolution it's it's seldom that things have absolutely no purpose but these are not coding for the protein that is going to be coded by our initial gene and so these are actually processed out they are spliced out and I'm not going to go into all the details of the actors that cause the splicing but as part of this eukaryotic processing you add the you add the cap you add the tail and then you splice out the introns and then once you've spliced out the introns all you have left are the exons so you have that is going to be connected to that it's going to be connected to that and so this is what you have resulted this is in a you carry out your mRNA and that's what we saw that's what we saw right over here that can then let me underline that in a color you can see right over here which then migrates out of the nucleus to a ribosome where it can be translated