Class 12 Biology (India)
- DNA replication and RNA transcription and translation
- Central dogma of molecular biology
- Intro to gene expression (central dogma)
- Central dogma
- Overview of transcription
- Transcription and mRNA processing
- Post-transcriptional regulation
- Eukaryotic pre-mRNA processing
- Transcription and RNA processing
Created by Tracy Kim Kovach.
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- At4:13Tracy says that the poly-A tail helps to promote termination of transcription by the RNA polymerase. I thought that the addition of the poly-A tail was done as a post-transcriptional modification via polyadenylate polymerase. Is there a poly-adenosine segment on the coding strand or how exactly does this work?(11 votes)
- The way she words it can be confusing. The poly-A tail is not present AT transcription, rather, as transcription finishes, the poly-A tail is immediately added. The tail can actually be added at multiple termination locations resulting in different protein products. I think this is why she said it is involved in termination. To be clear though, the poly-A tail IS NOT present at transcription, and is added quickly to 'lock-in' the termination location.(14 votes)
- from whr does the 5'cap comes?and of wht is it made of?(2 votes)
- The 5' cap refers to a methyl guanosine that is linked to the 5' end of the mRNA via a tri-phosphate bridge.(10 votes)
- I could be wrong, but I think the note at5:49is an unnecessary correction? Cytidine is actually just cytosine attached to a ribose ring, and in this case CDAR does work on cytidine, as recorded in editing of apolipoprotein B. (Source: http://www.ncbi.nlm.nih.gov/pubmed/11863358?dopt=Abstract&holding=f1000,f1000m,isrctn)(7 votes)
- do promoters always come before operators?(4 votes)
- I would say yes. If a promoter were to be after the operator, then a repressor (which binds to the operator) would have no effect on transcription because it would not block the RNA polymerase.(5 votes)
- 3:02"And capping at the 5' end, converts this end of the mRNA to a 3' end by a 5' to 5' linkage "
Does that mean that the 5' cap changes that end to 3'?
If that is the case, does the poly A tail change that end from 3' to 5'?(5 votes)
- The end that is capped is always referred to as the 5' end, even though the unusual 5'-5' bond between the meG and the transcript proper does make the cap sort of look like a 3' end.
The tail does not change the end from 3' to 5', it merely extends the 3' end.(2 votes)
- What is the value of an intron? What is the overall purpose/function, if not transcribed?(2 votes)
- Think of an intron as a way to differentiate different protein translations along a given chromosome. So if there is a sequence of ...AACGTCCG..., and the intron is CG a certain protein would arise that is different than if the intron were GTC. This allows for different protein configurations.
I think of it as similar function to a frame shift on DNA replication.
So even though only the exons get translated it is necessary for the formation of a specific mRNA (and hence a protein). (though there is evidence that there are other functions of the intron thought that is beyond the scope of this video)
hope that helps(4 votes)
- so repressors can bind to both operators and silencers. The difference is when they bind to operators, DNA Polymerase can bind but cannot progress, but when repressors bind to silencers, DNA Polymerase cannot bind at all, right?(2 votes)
- The edit was incorrect. The instructor was correct in calling it "cytidine" according to Wikipedia:
The editing involves cytidine deaminase that deaminates a cytidine base into a uridine base.(2 votes)
- how does the A tail promote translation?? I thought the 5 cap promoted it my telling the cell where to start the translation.
So does that mean the A tail tells it where to stop?(2 votes)
- At4:15she says that the poly-A tail helps with termination of transcription but how does this work if the poly-A tail is added once transcription is already done?(1 vote)
- She worded it weirdly in this video, but you're right. It is added after transcription is over, but it 'promotes' the end of transcription because the addition and presence of the poly-A tail on an mRNA strand tells you that transcription is over and that it is time to go to the cytoplasm or ER for translation.(2 votes)
Voiceover: Let's talk about post-transcriptional regulation which is regulation basically once DNA has been transcribed into mRNA and I've drawn out this little schematic for you here and it kind of just shows you how a DNA strand has a corresponding RNA strand and then the mRNA strand afterwards and I'll sort of explain what all the different colors and words mean in just a little bit. So once DNA is transcribed by RNA polymerase into the corresponding RNA strand, this RNA strand needs to get what I call a tidy little haircut and then don some protective outerwear before it can leave the comfort of the nucleus for its big debut into the cytoplasm in the form of a fully processed messenger RNA or mRNA strand. Now keep in mind that this form of regulation occurs in eukaryotes only and this modification also helps to stabilize the mRNA to protect it from premature degradation before it gets translated into a protein. So as you can see here, DNA gets transcribed one to one, base for base, into RNA and you can see here that there are sections of the RNA that ultimately make it into the finished mRNA, these short segments, which are turned exons, and they are the sequences that code for the ultimate protein product. And then there are short non-coding segments of RNA that get cut or spliced out and this would be the haircut that I alluded to earlier and so these are called introns, and this is accomplished by a large molecular entity called the spliceosome. So the spliceosome binds on either side of an intron, loops the intron into a circle, and then cleaves it off, and then ligates the two cut ends of the exposing exons together, kind of cinches them together. And an easy way to remember which sequences are exons, which ones are introns, is that exons exits the nucleus and introns stay in the nucleus so exons kind of stands for exit. Now even though the mRNA has gotten its nice little haircut, it's not quite ready to leave the nucleus just yet. It has to grab what is called a 5' prime cap and a 3' prime poly-A tail. Now what are those things that I just mentioned? So a 5' prime cap refers to changes at the 5' prime end of the mRNA and remember that this is the phosphate end of the nucleotide bases in the mRNA and some people like to remember this as F for five prime and for fosphate so that's how you can kind of keep the two ends straight. And capping at the 5' prime end converts this end of the mRNA to a 3' prime end by a 5' prime to 5' prime linkage which basically just protects the mRNA from exonucleases which degrade foreign RNA. The cap also promotes ribosomal binding for translation and also helps the regulation of nuclear export of the mRNA. Now the poly-A tail, that goes on the other end, the 3' prime end of the mRNA which has the terminal hydroxyl group and so what do I mean when I say poly-A tail? Well, poly-A tail refers to polyadenylation in which multiple adenosine monophosphates or basically adeonine bases are added to act as a buffer for exonucleases in order to increase the half life of mRNA and again, protect it from degradation. And so the purpose of the poly-A tail is really very similar to the 5' prime cap which is basically to protect from degradation, help with promoting translation, and regulating nuclear export. The poly-A tail also does one more thing and it kind of just helps with transcription termination for the RNA polymerase that's transcribing the messenger RNA. Polyadenylation is catalyzed by an enzyme called polyadenylate polymerase which as the adenosine monophosphates using adenosine triphosphate as the substrate and the poly-A tail is built until it's about 250 or so nucleotides long. So overall the 5' prime cap and the poly-A tail help to stabilize the mRNA for translation. That's the key point to take home from here. So once the mRNA has donned its cap and tail and had its introns spliced out, its now ready to exit the nucleus to be translated into a protein. Now additionally, there's one more type of RNA regulation called RNA editing, which is a process that results in seqeuence variation in the RNA molecule and is catalyzed by various enzymes. RNA editing is relatively rare and these events may include insertion, deletion, and base substitution of nucleotides within the edited RNA molecule. Now one of these enzymes is called adenosine deaminase acting on RNA or ADAR enzymes which convert specific adenosine residues to inosine in an mRNA molecule by hydrolytic deamination. Another type of editing is called cytosine deaminase acting on RNA, or CDAR, which involves deamination of cytosine to uridine by cytidine deaminase. RNA editing is currently being extensively studied in relation to infectious diseases because the editing process alters viral enzymes and their function so kind of an exciting, new emerging concept in post-transcriptional regulation.