- DNA questions
- Eukaryotic gene transcription: Going from DNA to mRNA
- Molecular structure of DNA
- Antiparallel structure of DNA strands
- Telomeres and single copy DNA vs repetitive DNA
- Leading and lagging strands in DNA replication
- Transcription and mRNA processing
- Speed and precision of DNA replication
- Translation (mRNA to protein)
- Differences in translation between prokaryotes and eukaryotes
- DNA repair 1
- DNA repair 2
- Semi conservative replication
- Protein modifications
- Jacob Monod lac operon
- DNA structure and function
Created by Efrat Bruck.
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- What is the function of the shine-delgarno region and why is it only in prokaryotic cells?(13 votes)
- The Shine-Dalgarno sequence helps recruit the ribosome to the mRNA to initiate protein synthesis by aligning the ribosome with the start codon.
It's not used in eukaryotes because the initiation of translation is far more complicated in eukaryotes than prokaryotes.(35 votes)
- Can translation also occur in the rough endoplasmic reticulum because doesn't it also contain ribosomes?(4 votes)
- Yes, translation can occur anywhere ribosomes are located. There are free ribosomes that are present in the cytoplasm and ribosomes that are in the rough endoplasmic reticulum.(14 votes)
- but what if friendly bacteria that produce vitamins are lysed by some bacteriophage? This would produce an unneeded immune response since:
1) The bacteria that was killed was beneficial and the fission rate is faster
2) Bacteriophages do not infect eukaryotic cells so they are to us eukaryotes a harmless virus.(5 votes)
- Even if the lysed bacteria were beneficial to us, the resulting debris needs to be cleaned up. This might be assisted by the immune response so generated.(6 votes)
- What about splicing? Does it happen before all of this??(3 votes)
- Splicing of introns from eukaryotic mRNA is initiated only after the process of capping has been completed. Splicing might get completed before or after tailing occurs.(7 votes)
- When she says the ribosome binds to the 5'-cap, does that really mean the small subunit binds? I thought that the large subunit doesn't bind until the small subunit + tRNA with methionine have found the start codon. Please clarify! Thanks! :)(6 votes)
- I learned that replication or transcription only occur 3' to 5', that means ribosomes can only add new nitrogenous bases on the 3' end. But then how come ribosome attach to 5' end and continue towards 3' end? If it is anti-shia parallel than doesn't it mean that it is adding on the 5' side of the new mRna??(2 votes)
- You're mixing up a few things here. First off, mRNA is transcribed in the 5' to 3' direction. This means that RNA polymerase reads the template DNA strand in the 3' to 5' direction in order to build the mRNA strand in the 5' to 3' direction.
Nitrogenous bases are one of the three groups that make up a nucleotide (along with a pentose sugar and a phosphate group) and do not need to be added on during transcription (nucleotides do). Ribosomes do not add nucleotides to the growing mRNA molecule, RNA polymerase does. Ribosomes are involved in translation, converting the mRNA into a polypeptide chain.(6 votes)
- So if the prokaryotes have the Shine-Dalgarno sequence on their mRNA, where would the Kozak sequence be on the eukaryote's mRNA?(3 votes)
- Since chloroplasts and mitochondria evolved from prokaryotes, do they translate/transcribe without the 5' cap and poly-a tail and use Fmet instead of met?(2 votes)
- Why you said non-coding region after 5'cap and after stop codon? There shouldn't be any introns (non coding regions) on mRNA after processing.(2 votes)
- [Voiceover] Let's talk about some of the differences between how translation happens in prokaryotic cells and how it happens in eukaryotic cells. And I want to focus mainly on the mRNA just before it's ready to be translated. So let's start with our prokaryotic mRNA and let's look at our five prime side first. So we have this yellow part right here, and that's the noncoding region. And it's called the noncoding region because the ribosome is not actually going to read that part. So that particular sequence of amino acid is not that important. And then after the noncoding region we have the Shine-Dalgarno sequence. And the Shine-Delgarno sequence is the site that the ribosome's going to recognize and bind to. So let's just throw a ribosome right over here. This is where the prokaryotic ribosome is going to bind. And then after the Shine-Delgarno sequence, we have another noncoding region. Just gonna abbreviate it NCR. And then we have our start codon, which is typically AUG, so that tells us to start. And so the ribosome's going to start translating, it's going to read this entire section, put together the corresponding polypeptide chain, until it hits the stop codon, which tells it to stop translating. And then we have another noncoding region. Let's look at our eukaryotic mRNA. And so it's pretty similar, but you can see there are some differences. So we'll start with our five prime side first. So you see this red nucleotide right over here. That's the five prime cap. And the five prime cap is simply a guanine nucleotide. So I'm gonna draw a G inside, Guanine, and it's going to have a methyl group somewhere on the molecule. So I'm gonna draw a methyl group. And the bond between this guanine and the nucleotide right near it is a bond that's different than the bond that you'd typically find between two nucleotides. And so that's really all the five prime cap is. And the five prime cap is actually the ribosomal binding site in eukaryotes. So that means that in eukaryotes, the ribosome's going to recognize this particular part and bind to it. So after the five prime cap, we have this other noncoding region which the ribosome's not going to translate. And then the ribosome is going to hit the start codon again. AUG tells it to start, and it's gonna start translating, so it's going to translate this entire section until it hits the stop codon. And then we have another noncoding region. And then we hit something that looks different than what we've seen in the prokaryotic mRNA, so this section with blue nucleotides, and that's called the poly-A tail. And the poly-A tail is a bunch of nucleotides that are all A's, or adenines, so I'm gonna draw A's inside all of these nucleotides. And the poly-A tail is actually pretty long, so it's typically anywhere between 100 and 250 nucleotides long. So that's pretty long. So I didn't exactly draw it to scale. And the purpose of both the five prime cap, and the poly-A tail is to prevent this mRNA from being degraded by enzymes. So it acts as kind of a signal that does not allow enzymes to break it down or degrade it. And so you might be wondering, well, what about prokaryotic mRNA? How come they don't have anything similar to prevent them from being degraded. And the brief answer to that question is that in prokaryotic cells, transcription, that's an R, and translation, both happen in the same place. So prokaryotic cells don't exactly have a nucleus. They have this cytosol and transcription and translation are happening in the same place. And not only are they happening in the same place, but they can actually be happening at the same time. So you can have a piece of mRNA that's being formed, and while it's being formed, a ribosome will attach to it and being to translate it. But, in eukaryotic cells, things are a little bit different. So transcription... happens in the nucleus, and translation happens in the cytoplasm where there are ribosomes. And so the mRNA, after it's made, has to travel, from the nucleus to the cytoplasm to where the ribosomes are. And so because it's traveling this relatively large distance, it's going to encounter a lot of different things, including enzymes that might break it down. And so it needs this extra protection to prevent it from being damaged in any way. There's one more difference I want to talk about in how translation happens in prokaryotes and eukaryotes and that is what the first amino acid in the polypeptide chain will be. So in prokaryotic cells, the first amino acid in the chain is always formylmethionine. And formylmethionine is simply the amino acid methionine, but with a formyl group attached. And in case you don't remember what a formyl group looks like, it looks like that. In eukaryotic cells, the first amino acid in all the polypeptide chains is simply methionine. And it's interesting to note that formylmethionine actually acts as an alarm system in the human body. So if you had some bacterial cells in your body that were damaged in any way, there would be these formylmethionines floating around, and that tells your body that there are bacteria around, and it's going to trigger an immune response.