Biology: Translation: Ribosomal and Transfer RNA

Nice Information

04/06/2011

~ Kristen7

pretty informative

Great Information

01/29/2010

~ Sally1

Upon sitting in AP BIology 2 in my school for a few weeks, I quickly realized my teacher had trouble teaching complex concepts.
She swore to us that A's were unimaginable in her class.
And we believed her after the first few tests.
Upon watching these videos I feel 100% better equipped to tackle transcription and translation.
He not only makes the concepts easy to learn, but easy to understand.
I would suggest this for anyone!
Keep up the good work Thinkwell!

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We have messenger RNA, this magic appearing molecule that is transcribed from DNA in the cytoplasm looking to make a protein. It's carrying a message. And make all sorts of analogies you want, but the bottom line here is that this is like some kind of spy novel. The DNA says, "All right, here's what I want you to do. I want you to go make a protein. Take this code out to the cytoplasm, out to the cytosol and have my people make the protein. But I've got to put it in a language that they can talk, so I'm going to transcribe it into RNA language."
And so the RNA goes out to the cytosol and now needs structures around it to help it out, because all it is, in essence, is a molecule. And when it comes down to it, molecules are useless unless they can interact, somehow, with other molecules.
So here we have this long chain of nucleotides and these nucleotides have to come up with a polypeptide. Help is on the way in the form of other RNAs. So we're going to put messenger RNA - our nice little edited structure with all its message - aside for a second and look at what's going to help it out. And what's going to help it out is two other forms of RNA and the first one I want to tell you about is transfer RNA. So let's get all the pieces in the puzzle together and then, later on, we'll actually put this secret-to-life puzzle together. So let's talk about tRNA.
tRNA, as you know, stands for transfer RNA and it's role is to transfer something. Now remember, what do you need to transfer? Well, what do you want to make? You want to make polypeptides. So what you want to do is somehow get those polypeptides over to the mRNA, to the messenger RNA, and bring them to that messenger RNA and let the mRNA worry about who goes where. But you need a transfer. You need something to carry those amino acids - all 20 of those amino acids.
Okay, so we have all 20 of these amino acids that need to carried or transferred by tRNA. So it's role is a carrier of amino acids.
You know, you may have heard something before: structure follows function. Did you ever hear that before? I know you've heard that before. And it's true at the molecular level as well as the organismal level. Let's look at the structure of a tRNA and see how it follows its function. What's the structure of tRNA? tRNA is made in the nucleus, just like mRNA. But tRNA is going to form some secondary structures. So after it's made, we're going to get some twisting of that tRNA. And if we can take a look at this transfer RNA, in an artist's rendition, what we see is that it's still a chain of ribonucleotides. So you can see ACCACGGC. There are some modified nucleotides, thus we have little stars in here, and that's just to tell you that it's not always as simple as it seems. Some of those nucleotides are a wee bit modified.
But what's most important is this, that we have a 3-prime end and a 5-prime end. And we have some other things that are going to loom hugely in our discussion. First of all, let's talk about the 3-prime end. I'll show you in a few minutes what this really looks like. It's good to see something nice and organized looking, huh? You know and I know that molecules don't always seem this flat and planar. But let's talk about the 3-prime site.
At the 3 prime site of the mRNA is where an amino acid is going to attach. So an amino acid is going to attach right there. Which amino acid? Any amino acid? No. And for that, we have to look at the structure of this tRNA and look down here. Because at the bottom of the tRNA is something that's going to be called the anticodon loop. The anticodon loop contains something called, you guessed it, the anticodon. Now, does that sound like something you've heard of before? Let's do a quickie review here.
Let's go back to messenger RNA. Messenger RNA is going to have a sequence of hundreds of nucleotides in a strand. You already know, although I haven't completely told you why, that this is going to be read in triplets, or codons. Now you'll start to get an inkling of how these codons are going to be read, and why the specificity of the polypeptide is so important. You know, look back on proteins and the whole idea of the fact that proteins - three words - shape, shape, shape. The shape of the protein is crucial. And what determines the shape of the protein and the polypeptide? The sequence of amino acids. So how important is it which amino acids get put into the polypeptide and in what order? It's everything.
So therefore, DNA has to code for something that's going to carry its message rigorously. So this particular RNA strand, if you recall, came from DNA template that said TTT, blah, blah, blah. And it may be AAA, and it went on. So if I were going to figure this out, this being the DNA strand that made this. Why am I showing you this? I don't want to get back into DNA. I want you to understand the rigors of making the mRNA. And if you understand the rigors of making the mRNA, now you're going to see the preciseness that this molecule must have. This molecule must carry the proper amino acid.
So how is it going to do that? This anticodon loop will match up with a codon here. So if I were to bring in a tRNA here, there's only one tRNA that can land there. What would that be? Well, remember base pairing. A only bonds with what? Well, in RNA, it only bonds with U. So therefore, if I'm going to bring in a tRNA that's going to bond there, that tRNA and its anticodon loop is going to be UUU. Remember the crucial specificity question here. Therefore, if TTT in DNA says, "I want lysine," then mRNA is going to carry AAA, and you bet that this UUU will only have - at its amino acid - at that 3-prime site - lysine, which brings us back to this whole structure.
Here's the tRNA. The 3-prime site is going to bond the lysine. This is going to be the match, if you will - the thing that matches it to the mRNA. How are we doing so far? Are you getting this so far? So tRNA is going to bring the amino acids there. How? That's coming. An amino acid transfer system. How cool. But that's not all, because that's not all that's going to help.
There's another form of RNA that's going to help. This one was made in the nucleolus. And this was called ribosomal RNA. Ribosomal RNA, in many ways, is the house that DNA built. What ribosomal RNA, being made in the nucleus, what it's going to do is it's going to be a skeleton. It's going to act like a skeleton for an actual structure. And eventually, what's going to happen is you're going to make a structure out of RNA called a ribosome, thus ribosomal RNA. In fact, the ribosome, even though we look at it in the cell - and you guys might remember from our whole discussion of cells, that ribosomes are made and the nuclear membrane actually has these little bumps sticking off it that are the protein pores. And the ribosomes actually leave - the ribosomal subunits leave - through those pores.
What I want you to understand now is a couple of things about ribosomes. Number one, ribosomes are roughly 60 percent RNA and 40 percent protein - once again, a combination of RNA and protein. In prokaryotes, they're a little bit smaller. And they're a little bit different too. I've got a great story for you.
Did you realize that one of the ways that antibiotics work is because prokaryotic ribosomes are different, antibiotics like tetracycline and streptomycin actually destroy prokaryotic ribosomes, but not yours, because they're a little bit different. That's very cool. It has nothing to do with this, but it's a very cool story.
Anyway, so here's a ribosome. There's what they're made out of. What do they do? Well, a ribosome is actually going to come in two parts, and that's the way it's found in the cell. It's found with a smaller subunit and a larger subunit. And the larger subunit has three sites in it, which we're going to call the E site, the P site and the A site. Okay? Now you're saying, "Oh no, I thought ribosomes were one structure." Well I'll show you what one looks like when it's put together, but I want you to understand something. Ribosomes are never found like this in the cell unless they're attached to something. Want to guess what that thing is? You've got it - RNA.
So now you're starting to get the idea. What do ribosomes do? They must, somehow, bring the tRNA and the mRNA together as one big happy family. And it starts out this whole idea of translation. This is going to be fun.
Molecular Genetics
Translation
Translation: Ribosomal and Transfer RNA Page [1 of 2]