Translation (mRNA to protein) | Biomolecules | MCAT | Khan Academy

So we already know that chromosomes are made up of really long strands of DNA all wound up into our into themselves. Something like I'm just kind of drawing it as a random long strand of DNA all wound up in itself. On that strand, you have sequences which we call genes. So that might be one gene right over there, this might be another gene, that might be a gene right over there. Each of those genes can code for specific polypeptides or specific proteins.

The key question is how do you go from the information encoded in these genes, encoded as sequences of DNA? How do you go from that? How do you go from the gene which is encoded in DNA to protein, which is made up of polypeptides which are made up of amino acids? This is often called the central dogma of biology. But we already saw in the video of transcription that the first step is to go from the gene to messenger RNA. The RNA, the messenger RNA, you can view it as a transcript; we have rewritten the information now as RNA.

Then, the next step which we were going to dive into in this video is going from that messenger RNA to protein. This process is called translation because we're literally translating that information into a polypeptide sequence. You can see a little bit visually here, and this is all review. We cover a lot of this in the video on transcription. The overview of video on transcription and translation is: if you look at a eukaryotic cell and the bacteria in a prokaryotic cell, it's analogous. You just don't have the nuclear membrane, and you're not going to do the processing step that I'm going to talk about a little bit.

We went in detail in the video on transcription. You start with the DNA; you have your RNA polymerase as the main actor that's able to transcribe the RNA from that. If we're talking about a eukaryotic cell, what you end up with wouldn't be called mRNA; we would call that pre-mRNA. Pre-mRNA then needs to be processed. The introns need to be taken out, and we add a cap and a tail here. If we're talking about a eukaryotic cell, we'll then formally call that mRNA, and then it can travel.

This is where we get into the translation step. It can travel to a ribosome, which is where it will be translated into a polypeptide sequence. You see the analogous thing happening here in this bacterial or this prokaryotic cell right over here, except you don't see the nuclear membrane because this is prokaryotic and you don't see that processing step. So you could just consider this straight; this is mRNA right over there.

So the questions are: how does this thing happen, and what even is a ribosome? Let's zoom in a little bit on a ribosome right over here. There's a couple of interesting actors. One, as you can imagine, is the ribosome itself. It is made up of proteins plus ribosomal RNA. In the video on transcription, we're already familiar with messenger RNA, and we often view RNA like DNA as primarily encoding information. It's acting as a transcript for a gene, but it doesn't have to only encode information; it can also provide a functional structural role, which it does in ribosomal RNA.

This big, you know, this looks like an oversized hamburger bun or something right over here. This is a super oversimplification of what a ribosome looks like, and I encourage you to do a web search for image searches for ribosomes. You can get a more appreciation of how beautiful these structures are and how intricate they actually are. This is the site, and you can broadly think of the ribosome as having this, you know, this is the top bun and the bottom bun, and it's going to travel along the mRNA from the five prime end to the three prime end, reading it and taking that information and turning it into a sequence of amino acids.

So how does that truly happen? Well, each of these three nucleotides—every three nucleotides—there you recall that a codon. So that's a codon. Let me do this in a color that is visible on both white and black. So this next three nucleotides is a codon; this is a codon; this is a codon. What's actually the information is actually encoded in the nitrogenous bases. So this first codon right over here, we see it's AUG. The nitrogenous bases are adenine, uracil, and guanine, and this codon codes for the amino acid methionine.

But this is also a good one to know. AUG, let me write it over here: AUG is known as the start codon. This is where the ribosome will initially attach to start translating that messenger RNA. The way that this drawing is that we are just starting to translate this messenger RNA.

So how does that actually happen? How do we get from these three-letter sequences to specific amino acids? Let's think about it. How many possible three-letter sequences are there? There are four possible nitrogenous bases, so there's four possible things that could be in the first place. There's four possible things that could be in the second place, and there's four possible things that could be in the third place. So there are 64 possible permutations—four times four times four permutations.

You could think of it as 64 different codons, different ways of arranging the A, the U, and the G. That's good because there are many amino acids, and this is actually overkill because there are actually 22 standard amino acids and 21 that are found in eukaryotic cells. So we have more than enough permutations to cover the different amino acids. You know, it's not hard to find tables that will actually show us what the different sequences actually code for.

You can see here that you can take the first letter, the second letter, and then the third letter, figure, look at the different sequences, and you can say: "Okay, look at that AUG, adenine, uracil, guanine, that codes for methionine." Alright, over here we could, and you could do that with any of you—say cytosine, uracil, uracil—that codes for leucine.

It's not just one amino acid per codon; here you have four codons that code for leucine. And so it turns out that sixty-one of the codons—let me write this down—sixty-one of the codons of the possible 64 code for amino acids. Three play a role that essentially tells the ribosome to stop—three codons are stop codons, and you can see them right over here: UAA, UAG, UGA. That's how the ribosome knows to stop translating.

So AUG, that's a start codon, and it codes for methionine. That lets you know that these polypeptide chains are going to start with methionine, and then these characters tell it where to stop. But how does the amino acid actually get? How do they all get tied up together to form this polypeptide, and how do they get matched up? How do they actually get matched up with the appropriate codon?

That's where we have another RNA-based actor, and this is tRNA. So tRNA—the T stands for transfer—there are a bunch of different tRNAs that each can bind to specific amino acids. On parts of those tRNAs, they have what are called anticodons that pair with the appropriate codon. So this tRNA—and that's not what it looks like—I'll show you in a second what it looks like. That's a tRNA molecule.

tRNA, at one end of the molecule, is binding to the appropriate amino acid, methionine right over here. Then, at the other end of the molecule—although it's in the middle of the tRNA actual chain—you have your anticodon, and your anticodon matches up to the appropriate codon. This is how, if they bump into each other the right way, the ribosome is going to facilitate it. The AUG is going to be associated with the methionine.

If we look at what tRNA actually looks like—and this is still just a visualization—this is a strand of tRNA. You get a sense of, okay, it's a sequence of RNA right over here. This is it—I guess you say think of its 2-dimensional structure—but then it wraps around itself to form this fairly complex molecule. The anticodon, which is right here, is kind of in the middle of the sequence; it forms the basis for this end of the molecule. That's the part that's going to pair with the codon on the mRNA, and then at the other end of the molecule, at the other end of the molecule is where you actually bind to the appropriate amino acid.

So I know what you're thinking: alright, I see that the ribosome knows where to start. It starts at the start codon. I see how the appropriate tRNA can bring the appropriate amino acid, but how does the chain actually form? You can view this in three steps and associate it with those three steps—or three sites—on the ribosome.

The three sites we call this the A site, A, then you're going to be able to see it if I write it in black—A—or yellow. Alright, let me write it in blue. So that is the A site. This is the P site, and this is the E site. I'll talk in a second about why we call them A, P, and E.

So the A site is where the appropriate tRNA initially binds, the tRNA that's bound to an amino acid. You can see we're starting the translation process. The next thing that's going to happen is another tRNA—the one that matches, that has an anticodon that matches the UAU—that's going to bond over here in the A site, and it's bringing the appropriate amino acid with it. It's bringing the tyrosine with it.

So why is that called the A site? Well, A stands for amino acyl. An easy way to remember it is that it's the tRNA. It's a place where the tRNA that's bound to an amino acid—just what amino acid is going to bind on the ribosome. Once that happens—once this character comes here—let me draw that once this character comes right over here, there's going to be a peptide bond that forms between the two amino acids.

The ribosome itself can move to the right, so this tRNA will then be in the E site, this tRNA will then be in the P site, and then the A site will be open for another amino acid-carrying tRNA. So what do P and E sites stand for? What do the P and E sites stand for?

Well, you can see a little bit more clearly right over here. The P site is where you have the polypeptide chain actually forming. One way to remember it is that that's where you have the polypeptide chain, and now you have a new A site where you can bring in a new amino acid. The ribosome is going to shift; once this is bound, the ribosome, the peptide bond forms, and then the ribosome can shift to the right.

When the ribosome shifts to the right, we're going to be in this position where the thing that was here, that was in the A site, is now going to be in the P site. The thing that was in the P site is now going to be in the E site. It's now ready to exit, and that's why it's called the E site—because that's the site from which you exit.

This is going to keep happening until we get to one of the stop codons. When you get to one of the stop codons, then the appropriate polypeptide is going to be released, and we will have created this thing that could either be a protein or part of a protein.

This is very exciting because this is happening in your cells as we speak. In fact, if you think about things like antibiotics, the way that they work is that the ribosomes in prokaryotes are different enough than ribosomes in plants and animals, or in eukaryotes, that we can find molecules that hurt the function of ribosomes in prokaryotes but don't do it to eukaryotes.

So if you have bacteria in your bloodstream and if you take the appropriate antibiotic, it could disrupt this translation process in the bacteria but not in your cells that you want to keep.