Let's review some of the major terms that are important when we talk about translation. We've already discussed what a codon and an anticodon are. Remember that the codon is the three base sequence found in the messenger RNA and the anticodon is the complementary three base sequence found in the transfer RNA and these designate which amino acid is being added to the protein. The start codon is always AUG. and it is, it encodes methionine and it's always the beginning of the protein. Sometimes finished proteins are processed to remove methionine, but when they're built we always start with the methionine.
In bacteria it's often something called a formal methionine, and so if you see this it's referring to something specific to prokaryotic cells. A nonsense codon is also called a stop codon. This is one of those three codons that does not encode any amino acids.
And that's a signal to the ribosome that translation is finished and the protein is completed. And when the ribosome reaches a nonsense codon, the complex will fall apart. And that ribosome can then reform onto another messenger RNA and build another protein. The growing polypeptide is what's happening during the elongation process. The Polypeptide continually has amino acids added to it until that nonsense codon is reached.
We talked about transfer RNA being the RNA molecule that has the anticodon that brings the amino acid to the ribosome. And of course ribosomal RNA is a structural component of the ribosome itself. On the ribosome, the video talked about the P site, also called the peptidyl site, the A site, which is also called the aminoacyl or the acceptor site, and the E site, which I call the eject site, because that's where the uncharged transfer RNA goes right before it leaves the ribosome.
I say it gets ejected out of the ribosome. That's how I remember which position that it's in. Okay, let's go through the steps of building a protein on a ribosome.
So first, we're going to talk about initiation. One unfortunate similarity between transcription and translation is that we refer to initiation, elongation, and termination for both processes, but they're actually different processes. So it's important to be able to tell the difference between initiation in transcription and initiation in translation. So we're focusing on translation right now.
In initiation in translation, the large and small ribosomal subunit will come together at the start codon, that AUG, okay? along with the first anticodon containing transfer RNA, and it's been charged with methionine. And they all come together to form this complex, this initiation complex.
And what happens is the start codon and the anticodon on the transfer RNA begin in the P-site. So this is when the initiation happens with the first amino acid containing transfer RNA, contains methionine, will start in the P-site. So this is the initiation complex. Now it's ready to go and begin elongating and adding amino acids to a growing polypeptide chain. After this first initiation step, amino acids always enter the ribosome in the A site, the aminoacyl site.
So you can see here we have our first methionine has been added. It's still attached to the first transfer RNA. And now we're going to add the second codon is UUA. That's going to bind to the complementary AAU.
That's going to come here. They'll base pair. That will bring leucine right next to this methionine.
And then that allows the ribosome to form a peptide bond between methionine and leucine. So if we go to our next picture here, we see the peptide bond is being formed between methionine and leucine. And what's going to happen now is this.
ribosome is going to move this direction, the direction of the arrow. Okay, so it's translocating along the messenger RNA. And what's going to happen is this methionine-containing tRNA is going to let go of the methionine because the methionine is now attached to the leucine.
It's going to move into the E site or the eject site, right? And the leucine-containing tRNA is going to move to the P site. So everything moves down once. Now we have our original uncharged transfer RNAs in the eSite.
The amino acid and tRNA that was in the A site is moved to the P site. We formed a peptide bond between methionine and leucine. And our next codon is GGU. That base pairs with an anticodon CCA, and that's going to bring a glycine to our growing polypeptide chain. Okay?
And then this showing a phenylalanine here, probably that means, if we go back, that probably means that we have a UUU. coming up, which we do. So again, this ribosome is going to move along the messenger RNA. We're ejecting our original, actually, this is the methionine.
This was the methionine-containing tRNA. Now our methionine has a covalent bond, a peptide bond with leucine. We're going to form another one with glycine, and this is going to shift again. So phenylalanine will move to the A site. The glycine tRNA will move to the...
P-site and this particular leucine-containing tRNA will be uncharged, and it's going to get ejected. So again, here's our ejected tRNA, and we continue to move amino acids and their tRNAs into the A-site and then the P-site, and we're forming a growing polypeptide chain. As the ribosome moves down the messenger RNA, we're just going to keep adding amino acids.
Based on the three base sequences these codons that are there. Alright, so let's go to the next slide here again Here's our mRNA the ribosome has been cruising down. It's been adding amino acids depending on these sequences and What's going to happen now?
Well, we're going to hit a stop codon and remember there's no transfer RNA that's going to have a anti-codon that matches up to this because there is no amino acid associated with the stop codon, sometimes called a nonsense codon. And so what happens is that's a signal to the ribosome that it needs to fall apart, release the protein, and now all of these molecules are going to be recycled. The ribosome will reform on a new mRNA somewhere. This particular transfer RNA will go back and get charged by its aminoacyl-tRNA synthase. And our new protein is going to fold.
It might be assisted by molecules called chaperonins that assist proteins in folding, depending on how big it is. All right. This messenger RNA can encode many, many copies of this protein. If another ribosome binds to the start codon, it can synthesize many copies. And actually, one of the ways cells control how long a protein is around is they'll actually control how long that mRNA is. available to make new protein.
So if it's available for a really long time, that protein might be made continuously. If the mRNA degrades very quickly, then maybe only a few copies of that protein gets made because it needs to be around in lower numbers or it needs a shorter lifespan in the cell depending on what regulatory role it has in the cell's function. One of the really interesting things about translation and a major difference between prokaryotes and eukaryotes is that in eukaryotes, the mRNA is made in the nucleus, but it has to exit the nucleus, and then it gets processed in the cytoplasm of the cell.
So when we say processed, we mean proteins are being synthesized from it. Now in bacteria, though, it's a little bit different, because in bacteria, the process of transcribing DNA and translating the mRNA can all happen in the same place, and that leads to kind of an interesting phenomenon. Here's our DNA molecule in a bacteria.
And the DNA, you can see that what's happening here is along, this is a gene, 5'to 3'. We're making mRNAs, and we're making multiple mRNAs from the same gene all at the same time. And as we make the mRNAs, we already have ribosomes attached making proteins. Now, this can't happen in a eukaryotic cell because this process happens in a different location in the cell.
than this process. So this process would have to happen in the nucleus, and then this translation happens in the cytoplasm. But in bacteria, it can happen all at the same time.
So, you know, one of the reasons bacteria can reproduce every 20 minutes is because they can do everything simultaneously. When you look at a eukaryotic cell, their doubling rates are much, much, much slower. They have to go through the process of actually bringing the... mRNA out into the cytoplasm.
It has to be processed. The introns have to be removed. It's more complicated.
So bacteria really have an edge on eukaryotic cells in terms of speed because they can be doing transcription and translation. all at the same time. Very efficient. All right, I'm going to leave you with a really beautiful animation of the entire process of transcription and translation.