Okay. So now we're going to talk about translation elongation. So when we left off before, we were at the AUG start codon. The initiator tRNA had come in, the AUG start codon had been identified, and the ribosome came, ready to get started. The initiator tRNA is sitting in the ribosome and it's ready. But what happens is when it gets ready, it just pauses for a second, the ribosome is released, and then it elongates. And whenever it's released, it goes. It goes quickly. It's booking it down that mRNA, translating really as fast as that little ribosome can go. Just go, go, go, go, go, go.
So what do we need to know to talk about the elongation step of translation? Well, the first thing we need to know is that ribosomes have sites in them, which we've talked about in other videos. They have the A site, the P site, and the E site, and they have different functions, and we'll talk about which elongation steps in each function. And then there's a second group of proteins called elongation factors. Two really important ones are EF-Tu and EF-G, and these factors, they get their energy from GTP, so they come in, they break down GTP, they get their energy, and what they do is they associate with the ribosome, they associate with the tRNA, they associate with the mRNAs, and they help this process. So they're just helper proteins that are like, hey ribosome, I see you're translating, you're going pretty fast, do you need any help? And the ribosome says, yes, and so they come in and help. Help.
So let's go through each of the steps. So we already have the initiator methionine, it's already on. The initiator tRNA has done its job, and it's gone. So now, what's the next tRNA doing? So the next tRNA is binding to an elongation factor, EF-Tu, which is bound to GTP, that's where it gets its energy, and that tRNA enters into the A site of the ribosome. When that tRNA enters into that A site, we get GTP is hydrolyzed, and that means that it turns into GDP, that extra P is released. And the EF-Tu is also released. So we had the tRNA coming in with the elongation factor, the elongation factor helped it get in, then once the tRNA was there, the the elongation factor didn't need to help it anymore, so it hydrolyzed its GTP and left. Then the tRNA moves to the P site, and this is where the amino acid is attached to the polypeptide chain that's growing. It entered with EF-Tu, EF-Tu left, it moved to the P site, it attaches its amino acid, and then whenever it's in the P site, the other elongation factor comes in and binds. EF-G says, hey, do you need help getting out of the P site? And the tRNA said, yeah. Can you help me get out of this P site? And so the EF-G says, sure. Let me hydrolyze this GDP or GTP, and then I'll get you to the E. So that's what it does. The EF-G comes to the P site, it hydrolyzes its GTP, and that energy moves the tRNA to the E site. So these elongation factors are binding to these tRNAs that are helping the tRNA to move from site to site. So the EF-Tu helps it get into the A, GTP hydrolysis releases it, it moves to the P site. When it's in the P site, it needs to get to the E site, so the EF-G comes in. EF-G releases its energy, and then it moves tRNA to the E site. So here we have an example. Here's our ribosome with our sites. We have our tRNA sitting here in the two different sites. This one's going to be the P site because of the new polypeptide chain. And these EF factors are going to come in, they're going to bind to the tRNA at any kind of step and help it get to the next one. So for EF-Tu, it's helping it get into the A, for EF-G, it's helping get to the E. So with that, let's move on to termination.
