Lecture 3.4 Information Transfer in Biology — Translation

HAZEL SIVE: We're well along our sequence of molecular information transfer. The thing that I want to talk with you about now is kind of the culmination of information transfer. And that is the process of translation. Translation refers to a process that produces protein from an RNA template. If we think about DNA replication and transcription, they are, in a way, quite similar. DNA replication is just copying one language into the same language. Transcription is kind of like changing fonts in your Word document. You go from Calibri to Georgia. The letters are the same. You can recognize them. There's a U, the UT, thing. But otherwise, the letters are the same. You're in the same language with RNA transcription from DNA. Translation is different. We're producing a protein here that's made of amino acids, its own language, from a different language, the language of nucleic acids. And that sense of how does nucleic acid encode protein, a different language, was a huge question for early molecular biologists. This process is done in the cytoplasm, unlike DNA replication and transcription, which are done in the nucleus. And it uses the genetic code. We'll write down what the genetic code is. And then we'll do some explanation. The genetic code refers to triplets, three sets of three, of RNA bases, each of which, each triplet, encodes one amino acid. These triplets are called codons. And each encodes an amino acid. The process requires lots of components. The key ones are a kind of RNA called tRNA, or transfer RNA, and also the organelles called ribosomes that we mentioned when we flew through the cell and listed the organelles. I will also mention that the kind of RNA that is used for translation is a special RNA called messenger RNA, or mRNA. And that is the protein coding type of RNA. The RNA is read 5-prime to 3-prime. And the protein is made from the amino to the carboxyl end. The RNA is read 5-prime to 3-prime. And the protein is made from amino to carboxyl end, OK? Let's look at a slide here. This is what the genetic code looks like. You always see it as a kind of table. And there are three parts to every codon. These are the triplets-- GUU, GUC, GUA, GUG. Those all encode the amino acid valine. AUG, very important one, encodes methionine. All proteins start with that, you'll see why in a moment. And then there are some codons that don't code for anything. They're UAA, UAG, and UGA. Those stop protein synthesis. We'll talk about those more in a moment. So let's carry on with some board work and lay out a bit more about how translation works. In any kind of language change, you need an interpreter. You need someone who understands both languages. You need something in the case of translation that understands both the language of the nucleic acid and the language of the protein. And as we started alluding to, this has got something to do with the codons that encode each amino acid. But the interpreter is this type of RNA called tRNA, small RNAs that are present throughout living cells. And those are really the interpreters for translation. So tRNA is the interpreter. And it's the interpreter for two reasons. Firstly, it recognizes codons on the mRNA. A tRNA has got something called an anticodon that base pairs with a codon on the mRNA. For example, if this is an mRNA codon, it goes 5-prime AUG 3-prime. You know it's RNA because of that U. The tRNA would have the complement of that 3-prime TAC-- not T, ah, UAC and 5-prime because it's also RNA. This would be the codon on the mRNA and the anticodon on the tRNA. They can base pair with one another. So tRNAs will base pair three nucleotides at a time to the messenger RNA. That's part of being interpreters. The other part is that tRNAs also carry the corresponding amino acid that is covalently bonded to the tRNA, very cool. For example, for this particular codon, for the mRNA 5-prime AUG, this encodes the amino acid methionine. And a particular tRNA that has the anticodon 5-prime CAU also carries methionine. Methionine is kind of interesting because it's the amino acid that all proteins start with. I'll make a note of it down here. Methionine is the starting amino acid. So let's think how this works. Let's write a small piece of nucleic acid, and then turn it into protein. Let's write 5-prime AUG AAA and ACU 3-prime. This is our messenger RNA. When we look at our messenger RNA, the first thing to do is to break it into codons. I like to put a slash. You can underline them, if you want. But we can put a slash. We'll break our messenger RNA sequence into codons. Then we can start thinking about what amino acids correspond to those codons. So codon 1 will correspond to methionine. Codon 2 corresponds to lysine. And codon 3 corresponds to threonine. These are the three letter codes for the amino acids. The way translation works is that you start at methionine. That is your start signal. You read the mRNA code without spaces. And you keep translating until you come to a stop codon. The name there is a bit misleading because they're not actually codons. They don't code for any amino acid. That's why they stop translation. The whole thing falls apart. The ribosome falls apart. The protein comes off the ribosome. And you're done when you get to a stop codon. So stop codons do not code. And they end translation. That is their job. Let's look again at the genetic code bearing what I've told you in mind. The way it is written is with the first letter-- it's actually the first nucleotide, the first base-- the second letter, and the third letter, as indicated. So the first letter-- here, all of these start with an A. The second letter, all of them start with a U. And the third letter is different. You can see that, for some amino acids, there are a number of possibilities. This one here called isoleucine has a number of possibilities. It has three possibilities. This one here, threonine has, again, a number of different possibilities. Methionine has just one possible codon, OK? For each of these codons, again, there's a matching tRNA that carries the right amino acid and recognizes the correct codon. Now, let's look how we go from DNA to protein. Here is a DNA template. I'm showing you just one strand of the DNA template. I've taken away the other one-- the 3-prime to 5-prime strand. It is translated-- it is transcribed, excuse me, into RNA in the anti-parallel direction, 5-prime to 3-prime. And I'm telling you that it is messenger RNA. And it's coding RNA. And I've broken it up now into codons. And each of those codons then corresponds to a particular amino acid. So transcription is going in the 5-prime to 3-prime direction, as all nucleic acid synthesis does. Protein synthesis goes in the amino to carboxyl direction, as we discussed. And you can match this all up to go from the DNA template to the corresponding complementary RNA and the corresponding protein that would be translated from that particular RNA. We're not going to go into detail of how ribosomes work and the exquisitely complicated, beautiful process of translation in the same way as we haven't for replication or transcription. The idea is that you get the basics, you get the general idea of translation. And when you take a higher level course, you'll be able to understand a lot more of the details. But the idea here is, just as we have shown, the messenger RNA 5-prime to 3-prime. The tRNA is the little anticodons indicated that have their 5-prime to 3-prime, their anti-parallel bonding to particular codons on the messenger RNA, and are also carrying the corresponding amino acids. Coming in sequentially into the ribosome as the amino acids are brought next to each other by virtue of the tRNAs, they form peptide bonds between them. And voila, you get the protein chain that is busy growing from amino to carboxyl end. I want you now to go to the assignment that has to do with translation and get some good practice on how you go from RNA to protein and from gene to protein.