So now that we've synthesized a strand of messenger RNA through transcription and transported it into the cytoplasm, we can make a new peptide strand. And this is the essence of translation. In translation, essentially, we're changing from the language of nucleic acids into the language of amino acids.
And translation involves a variety. types of RNA so we'll have the messenger RNA that we synthesized in the nucleus we'll have ribosomal RNA as part of the ribosomes and transfer RNA which will be ferrying amino acids back and forth and we'll have those amino acids as well so we want to look more closely at how this happens and that's the focus of this short video so from the learning objectives we're going to be identifying and describing the stages of protein synthesis specifically translation and In Saladin, the important pages are 118 to 125, and he's got a big helpful figure that I think you'll like on page 121. All right, now when we talked about transcription, we said that it produces a strand of messenger RNA, and we also talked about the fact that we have a triplet code, which means we have three nitrogen-containing bases in a group that code for a single amino acid. On the messenger RNA, a codon is a three-base sequence that codes for a particular or corresponds to a particular amino acid. So what we have here, this is a codon table. And this is organized in such a way so that you can correlate easily the three-base sequence with a particular amino acid.
And you can do this here, you identify the first letter of your codon and then the second letter here. So if your codon was ACG, let's say ACG, ACG, oops, ACG here, then this would correspond to threonine as the amino acid. So this is a handy thing and we want to come back to this idea of the codons in the three base sequence because this is actually kind of an important foundational concept here.
Now before we go here what I want you to see is that all 20 amino acids are represented here. Some codons actually code for something different. So we've got three stop codons and we've got a start codon which will always be AUG. Some amino acids have just one codon that correspond to them. but some amino acids like leucine have got multiple codons that correspond to them.
So this is kind of how we make the jump from the DNA to the messenger RNA to the amino acids using our triplet code. Now, when we talk about translation, we often talk about a cycle. events that occur and as we said we've got numerous parts that are sort of moving all at the same time and this is kind of an overview figure to give you an idea of what's happening and what I want to do is I'll look at each of these individual steps in more detail in the upcoming slides but if you look at translation what we have is here's our messenger RNA our strand of messenger RNA it's a single strand we have a ribosome and the ribosome is composed of two subunits, a large subunit and a small subunit. Here's our transfer RNA, and our transfer RNA is going to be associated with, always associated with, an amino acid.
Now notice the relationship here between the transfer RNA and the messenger RNA. They actually interact at those nitrogen-containing bases, and the way that they interact is in a complementary way. So we see this complementary association. between adenine and uracil, guanine and cytosine, again and again and again here. So let's take a closer look, right?
So where do we start? So when we start, we've got our messenger RNA, our single strand of messenger RNA that's been processed. So it's a mature strand of messenger RNA in the cytosol.
So all the introns have been removed and the exons have all been spliced together. And we've got a single strand of messenger RNA. This messenger RNA is recognized by the small ribosomal subunit that kind of slides along it until it recognizes a start codon, which has the sequence A, U, G. At this point, then, it sort of stabilizes the messenger RNA in this sort of configuration, I guess, is a way to think about it. And this in turn allows a transfer RNA, which will be bounded to a methionine, to recognize this start codon.
Now notice here that the sequence of nitrogen-containing bases on the transfer RNA is complementary to the start codon. So our start codon is AUG. The anticodon on the transfer RNA is UAC, and we see this again and again. So this is called our initiator transfer RNA because it recognizes the start codon, and the initiator amino acid will always be methionine.
Now what we see then is that because we've got our messenger RNA sort of stabilized with our small subunit, the large ribosomal subunit is recruited to come in and sort of... bond to the whole complex. When the large ribosomal subunit associates with the small ribosomal subunit, it forms a kind of a groove that the messenger RNA kind of fits into kind of neatly.
I always think of this as being like a zipper. And so the large and the small subunits would be kind of the place on the zipper, like the pole that you pull up. And the messenger RNA would be one side of the zipper with the teeth that kind of stick out. and the individual transfer RNAs with the bases that are complementary to these messenger RNA bases, right, that recognize that they would be like the other side of your zipper.
So here, now that we've got our initiator complex situated and the large subunit has been recruited to join the complex, we see that in the large subunit, It's oriented in such a way that there are three pockets. This is how Saladin describes them. With the initiator sequence here, our initiator transfer RNA with the methionine amino acid attached to it will occupy the center site, which is called the P site. P stands for peptidyl.
And this is where our protein is going to be formed. And Saladin suggests that you could use the word P for protein to help you remember what's happening there. On one side of our P site, we have the A site. A actually stands for aminoacyl, but Saladin suggests that you might remember this as being the acceptor site. So a new transfer RNA that's going to be attached to a different amino acid will come into the complex here at the acceptor site.
On the other side of the P site is an E site, and E stands for exit. After we formed a peptide bond between the amino acids that are situated in the P and the A sites, then the transfer RNA that will move to the E site won't have an amino acid. And so the E site is kind of the place where it can exit the complex. So we'll see how this works, you know, kind of in just a second as we go forward. All right.
So here now we've got our complex all situated, our large and our small subunits. We've got our initiator transfer RNA in our P site, our E site is empty, and our A site, which happens to be just as wide as one codon, is waiting for a transfer RNA and an amino acid to enter into it. Now in this particular example, the codon in the A site has the sequence UGC, so it's looking for a transfer RNA that will be complementary to that.
So the complementary transfer RNA has to have the sequence A, C, G. This author is showing this particular transfer RNA is going into the site, but notice that this anticodon is not complementary to this codon. So this transfer RNA and this amino acid actually won't fit. When we do have an amino acid that does have the correct anticodon, as we see here, We notice that it occupies the A site, and we can see that this is in fact complementary to it. The anticodon is ACG. Now this puts these amino acids in close proximity to each other, and the ribosome also acts like an enzyme, and so it catalyzes the formation of a peptide bond.
So what we see is that a peptide bond is formed, and now our growing peptide bond is going to be attached to the transfer RNA that's located now in the A site. The ribosome then slides down the piece of messenger RNA such that the initiator transfer RNA that had originally had a methionine attached to it is now in the E site. And our new transfer RNA that's got the growing peptide chain is in the P site and the A site is empty. Because this discharged transfer RNA is now in the E site, it quickly dissociates from the complex.
And now we're just ready to have another transfer RNA come in and occupy this A site. And we repeat the process again. We repeat the process until we get to a stop codon. And a stop codon, one of those sequences will be UAG. There's no corresponding anticodon on a piece of transfer RNA that's complementary to this.
So there's no transfer RNA that binds to the messenger RNA in the A site when the stop codon is there. Instead, Saladin says that there is a dissociation complex, or he has a name for it, a removal complex here, that causes the protein to dissociate from the overall complex. When this happens, protein synthesis stops, and then the entire ribosomal complex and the messenger RNA complex dissociates. it can reform and then form another protein again and again. And in fact, what we often see, I'm going to go back here.
Actually, I'm going to go forward. What we often see is that along a single strand of messenger RNA, we have multiple ribosomes that are sort of all operating in a line to translate the protein and produce protein synthesis. Okay.
Now, this is Saladin's figure. This is figure 4.10 on page 122. And I like it because I think it's a really nice summary. So we start up here with the DNA, our double-stranded DNA here.
Here he's showing you the sequence, the triplet base sequence on the DNA here. And here he's showing you the messenger RNA with the start and the stop codons. And he's got it all organized by codons. Here he's showing you the transfer RNAs that will complementary base pair.
Complementarily. base pair with the codons on the messenger RNA. And notice that they are actually complementary.
And here he's showing you the resulting amino acid sequence, or the peptide, that will be produced here from this particular piece of DNA. All right. So now I've actually got eight questions for you in Top Hat.
And they're set so that you can answer them again and again and again. And I want to ask you here, if... If you saw a question related to protein synthesis, could you answer it on a quiz or on an exam? Because I have a pretty strong feeling that there's probably going to be at least one. So thanks for watching.