Hi. In this video, we're going to be talking about translation. Okay. So translation is the process of turning mRNA into a protein. And so there are 3 steps: initiation, elongation, and termination. In this video, we're going to focus on initiation, the next one will focus on elongation, and then one of the last ones will talk about termination. When we think about translation initiation, we need to consider 2 ways that it's initiated. The first is how prokaryotic cells do it, and the second is how eukaryotic cells do it, because it's different, of course.
Prokaryotic cell translation initiation requires specific sequences. So, what are those sequences? Well, the main one is going to be the Shine-Dalgarno sequence. And this sequence is going to be upstream, right before the codon, the initiation codon, the one that's going to start translation. And what it is, is when the initiator tRNA, which we're going to talk about soon. So tRNA is going to be really responsible for recruiting the ribosome and all sorts of things. So the initiator tRNA, a really special one, binds to the start codon, but it knows where to bind because of the Shine-Dalgarno sequence. So the Shine-Dalgarno sequence is letting the tRNA know where it needs to go. Another factor that is required are proteins, of course, and in prokaryotic cells these are called initiation factors, you may see them as IF for short, and there's IF1, IF2, IF3, and these proteins just sort of settle everything in. They say initiation is going to start here, let me get everything oriented correctly. That's what those proteins do. So that's prokaryotic translation with the Shine-Dalgarno sequence.
Eukaryotic translation initiation requires many more proteins than prokaryotic. So, the first thing is that in eukaryotic translation, the mRNA isn't just. In prokaryotic cells, when the mRNA is transcribed, it's translated pretty immediately after. In eukaryotic cells, that's not the case. When the mRNA is transcribed, it then undergoes some processing before it ends up being translated. And so the mRNA for eukaryotic cells exists in the cytoplasm. And because it's just been sitting out there for a little while, it's gone through some processing, it's kind of folded in on itself and a bunch of proteins have found it. So already, it starts out more complicated because there are a ton of proteins, there are secondary structures where the mRNA is folded in and on itself. And so when translation initiation begins, there's already a lot of proteins there, and those proteins either need to be removed or reoriented so translation can occur. Some of the first things that happen in translation initiation for eukaryotic cells are initiation factors. Now in prokaryotic cells, those were called IF, but in eukaryotic cells, it's called EIF, so you add that E for eukaryotic. And instead of 1, 2, 3, in eukaryotic, it's 4, A, B, and G, so EIF4, EIF4A, EIF4B, EIF4G. And what these initiation factors do is they say, hey, that mRNA needs to be translated, so what do I need to do? I need to bind to the 5' prime cap. Remember that the RNA has been processed with that cap? So they come in, they bind to that cap, they say, hey, all you other proteins need to leave because we got to get translation started. So it kicks all these other proteins off, and it exposes the mRNA, and it says, okay, we're getting you ready for translation, so you don't need to be folded in, you don't need to be covered with proteins, so let us come in, clean you up, get you ready. So then, whenever those initiation proteins have come in, they've cleaned up the mRNA, they've gotten the proteins off, what happens is we get an initiation_COMPLEX, which comes in, attaches to the mRNA, and looks for where translation is going to start. And where translation starts is the AUG start codon. So we started out with that complex mRNA that was covered in proteins, the initiation factors came in, cleaned it up, the initiation complex bound to it and starts running across it looking for the AUG start codon, and, when it finds it, it can start initiating. Now there's one more sequence you need to know about for eukaryotic initiation, and that's the Kozak sequence. So the Kozak sequence is a consensus sequence. Remember, a consensus sequence isn't conserved, so it's not the exact same between different organisms, but it's fairly similar between different organisms.
