5.5 Transcription BIOL 334 - Radford University

Hinzugefügt: 2026-05-12

[00:00:00]hello everybody in this lecture we're going to talk about transcription so let's get right into it by the end of this lecture you should be able to explain protein synthesis AKA gene expression uh describe how it impacts phenotype and state why it is essential to cell survival uh you should be able to explain transcription uh and not as much translation in this one we'll talk more about that in the next lecture point out differences in the process uh between procaryotes and UK carots Define reverse transcription describe structures and functions of the three main RNA types that we're going to discuss and summarize splicing of mRNA in ukar so let's start out with just some basic explanation so protein synthesis um which is what is necessary to actually Express genes um is when we take genetic gentic information within a cell and we create some sort of Gene product to do that we have to um first copy the DNA over to RNA through a process called transcription uh and then those transcripts are brought to the ribosome as messenger rnas and they build proteins through a process of translation so to kind of summarize that the gene is in the DNA and when it needs to be expressed it will be transcribed to a messenger RNA and that messenger RNA will be translated to a protein so protein synthesis is Central to life and it is going to be how you get most of the physical and functional features of cells and an example of this that we've touched on multiple times throughout class is that many of the antibiotics that we use to fight infections work by interrupting protein synthesis now there are three steps to transcription uh which is the process of making rnas um in UK carots transcription is going to occur in the nucleus and in procaryotes it's going to just occur in the cytoplasm now there are going to be three steps that's initiation elongation termination so this is similar in terms of uh DNA replication so one thing to keep in mind is which strand is going to actually be transcribed so the Strand that is transcribed is called The anti-sense Strand uh the anti-sense Strand is going to be very similar to the Leaning Strand and DNA replication so transcripts are only made continuously they are not made discontinuously and so that's what we see here so this is an RNA polymerase adding the um mrnas or sorry adding the uh rntps to make an mRNA and it's doing so continuously as the Strand unwinds and so um when we think about DNA replication we don't use these terms sense and anti-sense but we do when we're thinking about transcription and so the anti-sense Strand is going to be the thre Prime to five Prime uh strand when you open up that when you make that replication fork or that transcription fork or transcription bubble that would be making here the antient Strand is going to be read in that three prime to five Prime Direction um this is the same way that all the p races read the old Strand and so the old strand is oriented three prime to five Prime and then the New Strand is going to be added five Prime to three prime so it's important that you understand that distinction so be sure to kind of look at this image and get what I mean when we're reading the Strand that means that we're reading the old Strand and we're going to say that okay this is an a then this is a t this is a g whatever um and then we're going to polymerize the New Strand in a five Prime to three prime Direction so that's the bottom strand here and in this case this bottom strand is going to be mRNA um now the sin strand is not transcribed at all that's the one down here and it's nearly an exact match of the transcript right so the only difference between this transcript and the sring is that the transcript is going to be made out of RNA bases the S trand is going to be made out of DNA bases and the uh transcript is going to have uracils while the S strand is going to have thyes so uh an initiation we have a formation of a transcription bubble very similar to the initiation of replication uh your Topo isomer where gyas is going to unwind the super coils of the DNA right and then uh helicase is going to open that up now RNA polymerase needs to find these specific sequences in the DNA that are known as promoter sequences and they are and bacteria in particular which is what I'm going to discuss here you have these Nega 35 regions and these -10 regions uh so the sigma factor is going to be part of your RNA polymerase and it's going to be looking for these different uh sequences that are going to be Upstream uh or yeah of the U transcription start site um so the sigma factor is going to actually bind to that 35 region of the promoter and those two regions of the promoter are pretty highly conserved which means that they don't there's not a lot of um change in those bases over time but and and there's usually going to be a lot of uh carryover between species that have the same genes and that's because those sequences are highly selective for the promoters and if they're not getting good contact with the promoter then they're not going to get made and then the Gene's not going to survive and if that Gene is necessary for life then the whole organism isn't going to survive uh you may see the term tab box or tatab box uh referring to that negative 10 region because you have this ta TAA uh otif so once polymerase is bound once it finds its proper promoter uh it's going to begin replicating the DNA uh or transcribing the DNA to uh RNA and it does this for all types of RNA but when we're talking about gene expression we're specifically talking about mras and so it's going to uh read The anti-sense Strand three prime to five Prime but it's going to make the New Strand five Prime to three prime so this is exactly the same as DNA replication there's nothing different here the only thing that we can consider being different at all is just the fact that we're making a transcript instead of a new DNA string um so AR polymerase is the same type of enzyme as primase and so they don't need any leaning bases to begin transcription they can just start adding those RNA bases right away and then The Binding process is you to A and G to C Ina in this case U has two hydrogen U to a has two hydrogen bonds and G to C has three So eventually transcription is going to stop though so Unlike DNA replication DNA replication doesn't stop until you go through the entire genome um where through the entire chromosome and transcription you stop when you actually get to the end of the Gene and so there are two mechanisms that we can briefly talk about one is called Road dependent termination uh and in this you get this protein called row which is like this hexamer and it travels behind the RNA polymerase and it's are wrapped around the transcript so you kind of see it it's following that RNA pimas down there and as it's coming up the uh DNA is kind of un uh or the DNA and RNA binds uh bonds are being broken those hydrogen bonds and RNA polymerase is proceeding in that 5 to3 Direction and eventually the uh polymerase is going to have this string of guanines and so if you remember guanines have three uh hydrogen bonds and so they they actually are a little bit harder to uh make and when this happens when you get a string of them you slow down the polymerase and so it begins to stall and so then that uh that row is going to actually catch up and bounce the uh RNA polymerase off of the DNA and so essentially what happens here is that when you have row dependent terminate uh termination the RNA polyas slows down and then the row bumps it off of the DNA then we have row independent uh termination in this case we are going to actually terminate based on um structure uh that's being formed by the RNA and so here there's this there's also a GC Rich region of the transcript that causes this formation of a hair pin so one of the cool things about RNA is that it's single stranded but it almost always has secondary structure uh capabilities and so you get these GC Rich hair pins so basic basically this single stranded RNA folds over on itself to make the secondary structure and then you get a Urich track which is going to mean that you make a bunch of uh A's right and so when you hit that at uh Rich region you're going to make a bunch of use and so this is going to be weaker hydrogen bonds because there's only two hydrogen bonds there and that hair pin is actually going to um put strain on that weak Urich tract and that Urich tract is then going to um cause the RNA Plum rase to to stall up and break loose from the DNA and when RNA pimas is bounced off that DNA transcription stops and so this is a little bit of a uh summary of what's going on you've got initiation where RNA plas buys the promoter DNA M WIS revealing your timet trand R plase is going to elongate in the five to3 Direction uh adding complimentary nucleotides and then you get termination you're going to hit a sequence that basically causes RNA Pumas to slow down and in doing so the RNA dissociates from the um or the DNA so generally when we think of transcription we go from DNA to RNA although there is some capability of reverse transcription U viruses like HIV have reverse transcript a also some cells can use this as well this is where RNA is used to make a template uh DNA or copy DNA which we call cdna and uh that enzyme that does that is reverse transcriptase so really what I want you to just know there is the ability to go from RNA to DNA which makes C DNA uh is known as reverse transcription and uses reverse transcript case so we have three major types of rnas that we'll talk about um here in gene expression that's mRNA or messenger RNA uh that's TRNA or transfer RNA and R RNA or ribosomal RNA and so the MRNA is going to have the codons that code for the amino acids and for the Stop codon that stops translation and so the the way to think of this is that the MRNA carries the message which is the code for making the protein so the TRNA contains an anti-codon and it's complimentary to the codon so kodon binds to anti-codon and the TRNA recognizes that kodon on the MRNA but it brings in an amino acid as well and so it brings that amino acid into the ribosome and it attaches it to the growing amino acid chain and the rrna uh has this 3D structure that combines with your ribosome um and they can they have catalytic function so they help actually process and make these uh you know these these ribosomes work properly so here's kind of a example of those like in a picture format you've got your mrnas with your codons you got your trnas that have codon an anti-codon on one end and a amino acid on the other and then you have your rrnas which are combined with your ribosomes uh ukar have a special uh process of splicing mrnas um and they have to be transported out of the nucleus before they can be translated and so in the splicing of rnas there are sequence es that the final transcript don't need and uh they are clipped out and so you've got two types you've got exons and introns so when you when you transcribe out your Gene you're going to have exons and introns and so uh the exons are segments of mRNA decoded to build a protein and then entrons are are intervening sequences of mRNA that are not decoded to build protein right so we keep the exons and we cut the introns and so Mr splicing um has the potential for having alternative splicing that gives you different combinations of exons so you can actually pack more information into the same sequence so you don't have to have uh independent genes for each of those and so here is an example where you have a eukariotic um mRNA where you have these introns and exons introns and exons and then the Bome comes in and it's going to cut out the introns and then you keep your exons exons one two and three they get joined together and now you can translate that all right so uh here you should be able to now explain protein synthesis explain transcription and to some extent translation although we're going to talk much more about translation in the next one uh Divine reverse transcription uh describe the structures and functions of the three types of rnas and summarize mRNA splicing new carry notes all right hope you enjoyed see you soon