Transcription in Prokaryotes - Video Tutorials & Practice Problems
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Prokaryotic Transcription
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Hi in this video we're gonna be talking about pro carry attic transcription. So pro periodic transcription is different or differs from eukaryotic transcription. They're similar in a lot of ways but there are important differences. So first let's talk about the first step of transcription which is always initiation. Now, pro periodic transcription initiation requires certain factors of course. One of these factors is called a promoter and this promoter is actually a D. N. A sequence and this live upstream of the transcription start site. Remember before the gene and it's A D N A sequence that signals for transcription start and how it signals is because it recruits proteins to that area that will trigger transcription initiation. So we call the promoter sequence in pro carry optics consensus sequences. This means that they're not the exact same. They're not conserved sequences, meaning that it's pretty much the same sequence across many different organisms. But consensus sequence just means that there's some they're not exact, they're not even close enough to be exact but they're pretty similar across many different pro periodic species. So promoters and pro carry optics are consensus sequence and this is one major difference because in eukaryotic transcription, that's not the case. Now, One of these sequences that acts as a promoter is called the print print now box. Or sometimes you don't see it as this but you see it as just the 10 base pair sequence. Um because it lies 10 base pairs upstream of the transcription start site. So this is the sequence here. You don't necessarily need to know that. But just know that the priv priv now box lies 10 base pairs upstream of the start site, it is a very common pro periodic promoter. Now, in addition to the mailbox, there's another 35 base pair consensus sequence that doesn't necessarily have a fancy name, but it's also present. So you have one at the 10 base pair and one at the 35 base pair upstream. And then occasionally you get another sequence that is 40-60 base pairs upstream. And I write notice here that I'm writing this as negative 40 -60. That suggests that it's upstream of the sequence. So here's the three potential promoters. These two are very commonly here and this one is mainly optional. It's occasionally there. Now you have these promoters. So what comes in I said, proteins come in and buy into the promoter. Well what this is called is called the RNA polymerase hollow enzyme. And the hollow enzyme just means that it has the region of the protein that's going to catalyze the reaction. So the catalytic core the enzymes doing its job. But it also has other factors as well. So the RNA plays Hollow enzyme for pro carry Oats has the core enzyme which is going to be doing the like transcription part but it also has an important factor called the sigma factor and the sigma factor is a short peptide sequence and this controls specificity. So RNA polymerase in pro carry Oats does transcribe all RNA and so in order to specify what type is needed, whether rival zones needed or whether this is being used for um protein synthesis. The part that controls that is the sigma factor. And the sigma factor is kind of just this exchangeable factor. So there's many different types of sigma factors and each one is specific for a different type of RNA. And so whenever the cell needs to transcribe the RNA it brings in the appropriate sigma factor if it needs to transcribe protein or transcribe RNA that will be used for protein. It brings in the appropriate sigma factor. And um so the sigma factor is important for specificity. What RNA is actually being transcribed. So if we look at this, here's the initiation of transcription. Remember we have a DNA double helix. We have the RNA factor or RNA polymerase and the sigma factor is inside and that can be exchanged changed depending on the RNA that needs to be transcribed. And although it's written here in really horrible yellow color, I don't know who made this image but the yellow represents the promoter. So you can see the RNA flim races coming. It's binding this promoter and this will initiate transcription. So after transcription is initiated, initiated. What happens is the transcript has to be a long elongated. So that occurs after initiation and the region that's being transcribed at the time is called the transcription bubble. So this is a short sequence of DNA. It's around 18 nucleotides long and this is what's actively being transcribed. So only about eight nucleotides are being transcribed at any given moment. But you need 10 more to fit the proteins that are being associated. The RNA polymerase, the sigma factor, the unwound DNA. So it creates this like bubble that's a transcription where transcription is occurring or about to occur or have just occurred. And we call that bubble the transcription bubble. Now the transcription bubble continues along the whole way. Right? It transcribes the D. N. A until it reaches a certain sequence and that sequence is where termination occurs. So um there's many different types of sequences and or the sequence is called the termination sequence also called the terminator. Depending on the book that you use, I like terminator just because it sounds real fancy. But essentially it's just a DNA sequence that says hey, stop here and um it's found upstream of where it's actually terminated. And the reason is because the termination sequence signals for proteins to come in and help terminate it. So it's not going to terminate it there. Right, Because that would get rid of the sequence and it needs that sequence it terminates it sort of downstream um of that sequence. So if you have a sequence here you have your termination sequence here and it'll actually terminate downstream of that. Now there's different types of proteins that come in to help terminate. There's something called the road dependent terminators. You may also see row written in the fancy way but essentially it's pronounced Ro and this terminates the present terminates the transcript and the presence of a row protein. Then of course there's row independent terminators and do it in the absence of the rope protein. And then there's this fancy type. These are the two most common. And then there's this fancy type called intrinsic termination and this happens much more rarely. But what happens is that you have an RNA with a bunch of Euro still in it and you're a cell is not you know in D. N. A. A binds of T. It doesn't bind with you. So if you have a transcript that has a bunch of your sales, you have a binding with you on the template strand And that bond is weak, it's definitely not as weak as the 80 pair. So A. Is you. It's a week. So what happens is that RNA Plymouth raise these. This transcript has a bunch of Euro sales. These are weak bonds and it can cause disassociation of that transcript and the RNA polymerase and result in transcript termination Now oftentimes that happens it's too early and it shouldn't be happening like that but sometimes it happens when it's supposed to. But again, like I said, the intrinsic termination is more rare than the road dependent or row independent terminators. Now. Finally let's talk about this last little bit and this is very important because this is a big difference between pro carry optic and eukaryotic transcription. So pro periodic transcription can result in policies. Tronic M. R. N. A. What policies Tronic M. R. N. A. Is It's it's actually an RNA that has a group of genes. So there's been there's D. N. A. There's a group of genes in order and all of it has been transcribed into RNA and a single strand instead of each RNA represents each gene. So you have a single R. N. A. And it has four or 5 10 15 50 genes on it. And this means that there is only one terminator and that was present at the end of the group of genes. So that RNA polymerase just kept transcribing, kept transcribing until it reached the terminator. And that happened to be after a few genes were already present. And so the pro periodic this doesn't happen to eukaryotic transcription but in pro carry optic transcription. That means that that RNA that's produced has to be processed further so that each gene is cut out individually processed into individual genes and individual proteins instead of just one long M. RNA transcript or policies Tronic M RNA that contains multiple genes. So here we have an example we have initiation which I showed you before you have your RNA plan raise your sigma factor binding to your promoter then you have your transcription bubble as it's being transcribed. Remember this is the five prime in and the three prime end is over here. And because this is because it's yeah three prime and five prime because it's doing the RNA strand is the template sequence. And then there's some type of termination sequence here in green. And so when the RNA polymerase actually reaches the sequence, it dis associates the sigma factor dis associates and now you have um M. RNA. And sometimes this M. RNA. If its policies tronic M. RNA can contain multiple genes on it. Here's there's three and each one will have to be processed so that they eventually become separate M. R. N. A. S. And separate proteins. So that is pro carry attic transcription. Let's now move on.
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Problem
Problem
Which of the following is not an example of a prokaryotic promoter sequence?
A
TATA Box
B
Pribnow Box
C
35bp consensus sequence
D
-40-60 upstream consensus sequence
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Problem
Problem
What is the purpose of a sigma factor in prokaryotic transcription?
A
The sigma factor serves as a transcription initiator sequence
B
The sigma factor controls the specificity of the polymerase
C
The sigma factor forms the transcription bubble
D
The sigma factor controls transcription termination
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Problem
Problem
Prokaryotic transcription can create polycistronic mRNA.
A
True
B
False
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