Understanding Transcription and Translation

All right, so this is our second lecture on, well, second lecture on chapter four, although I guess it's our fourth if you go through. So we've had two labs on chapter four. Now we're going to be talking about transcription and translation. So we briefly mentioned, and I put a really good video from the Amoeba Sisters up about DNA replication in lab. So kind of go over that. Transcription is going to be a little similar in a lot of ways to DNA replication, but obviously with RNA instead. So Transcription and translation are going to be very important. And one of the important things that we need to talk a little bit about is DNA. And so what DNA is and where it is located. So DNA is housed in the nucleus. And we learned in Chapter 2 that it's composed of repeated monomers called nucleotides, right? DNA has deoxyribonucleotides, right? So it has that deoxyribose sugar in it. Whereas RNA has just ribonucleotides. It has the ribose sugar. So each deoxyribonucleotide is composed of the five carbon sugar deoxyribose, a phosphate, and one of four different nitrogenous bases. So in DNA, we have adenine, cytosine, guanine, and thiamine. And so we learned about this already in chapter two, so this should hopefully be a review. These nucleotides are linked together by phosphodiester bonds, and each molecule has two complementary strands of nucleotides. So DNA is double-stranded. It looks a little bit like a twisted ladder or a spiral ladder. The sugar and phosphates form the struts of the ladder, and the pairs of nucleotide bases form the rungs. And these base pairs are connected by hydrogen bonds, right from one strand to the other complementary strand. This houses most of the genetic material in the cell. The human has 46 separate double-stranded DNA molecules, or 23 pairs. Each double helix is wound around nuclear proteins called histones. And together we call this a nucleosome because otherwise we'd have so much DNA, like a lot of DNA. When not dividing, DNA is in a finely filamented mass called chromatin. So the non-compacted form of DNA is called chromatin. When it is dividing, the chromatin becomes tightly coiled or supercoiled into chromosomes. And in our last lab, we talked about how the chromosomes form. during the beginning of prophase and we start seeing them during all those phases of mitosis. But when they're coiled in these chromosomes, they can't really be, it can't, all the pro, the enzymes can't really get to them and so if they're coiled into chromosomes they can't undergo transcription or, you know, transcription or DNA replication. So here this is our double stranded DNA. These are our base pairs. They're held together by hydrogen bonds. We've got three between the C and the G and two between the A and the T. And if it is loosely coiled as chromatin, then it's going to be more accessible and we're going to be able to transcribe it. So how is DNA related to genes and how are those related to proteins? And how in the world does this molecule here dictate like your whole body? And so it's a little bit complicated, but the functional unit of DNA is basically a gene. And a gene is basically a segment of nucleotides that's going to code for a specific. protein. And those specific proteins might be enzymes or transport channels or something, but those proteins are going to change how your cells work. And if they change how your cells work, then it is going to be changing like your phenotype, change what you look like, change how you are. So the beginning of the gene has what's called a promoter region. That's like the start signal that tells your body to start transcribing that gene. And the end of the gene has a terminal region or a stop signal. And cellular activities depend upon the protein synthesis, right? Different proteins and different enzymes are going to cause each cell to function differently and this is directed by the DNA, right? Because we said genes code for proteins. So the way that our cells are gonna figure out what proteins to make and make proteins is called transcription and translation. And so the very short, very brief thing to remember is that transcription happens in the nucleus And this is going to start with DNA. So the DNA is opened up and it's copied. And we copy it into an mRNA strand. And so we have this mRNA that's copied. So we transcribe the DNA into mRNA. We copy that gene. It's a copy of a gene. Then that mRNA is going to leave the nucleus through a nuclear pore and go into the cytosol and bind to a ribosome. And it's going to then be translated. So we're going to translate the language of RNA into the language of proteins. And we're going to actually make the protein that's coded for in this mRNA that was originally coded in this gene here. And this is going to use a ribosome as well as this special transfer RNA to bring amino acids to the ribosome to start making the linear structure of the amino acids, which is the primary structure of the protein. So DNA is the major structure required in transcription, right? This is like the blueprint. So we're going to be transcribing DNA. This is going to serve as a template for the complementary RNA molecule. So RNA is a little bit different. It's single-stranded. It just binds to the DNA, right? So it makes that one single strand. And instead of a T, it has a U, right? And so instead of deoxyribosugar, it has a ribosugar. So it has an A, a U, a C, and a G. And it's only single-stranded. So transcription requires DNA because we have to copy it. It requires all these ribonucleotides to make that mRNA and the enzyme RNA polymerase. And this is located in the nucleoplasm of the nucleus. So RNA polymerase in this picture is like this big purple enzyme here. It's going to unzip the DNA and then start copying it into this mRNA strand and then re-zip the DNA up. So before we start talking about exactly how... transcription happens, I just want to mention that there are three types of RNA that are produced during transcription. The one we'll talk about in class here that goes on to make proteins is called messenger RNA, but we also have transfer RNA. Transfer RNA or tRNA is going to be really important in translation, right? Translation, t. And it's going to help transfer the amino acids to the ribosome, the correct amino acid to the ribosome, and then ribosomal RNA for our RNA is going to be part of the ribosome. So ribosomes are made up of our RNA and protein, right? And remember that they were made, the nucleolus is going to help make those ribosomal subunits in the middle of the nucleus, and then it's going to ship them out to the cytoplasm. All right, so there are three events in transcription, so we'll go through each, initiation, elongation, and termination. So initiation is the start of transcription. So we have RNA polymerase, and it's going to start. need to bind to DNA. So DNA is unwound by some specific enzymes that we don't need to name because this is only an overview, that make it accessible to RNA polymerase. RNA polymerase, look, ACE, A-S-E at the end, it's an enzyme that catalyzes the synthesis of that messenger RNA. Remember, we're talking about messenger RNA here. So that RNA polymerase is going to attach those ribonucleotides to their complementary strands of DNA. So if we had an A DNA, it would bind to a U RNA. If we had a T, it would bind to an A. If we had a C, it would bind to a G. If we had a G, it would bind to a C and so on. And so this DNA is the template strand and it's going to make an RNA strand that matches. But remember that instead of T's, RNA is going to have a U. So RNA is going to attach to the DNA. RNA polymerase is going to attach to the DNA strand and locate the promoter region. That promoter region is the first part, the start of the gene, and that's going to start that gene transcription. So it's going to break the bonds between DNA and it's going to open up that little area. And this open area, that's where we can copy. We're copying the DNA to make mRNA. The copied DNA strand is called this template strand and the other DNA strand, the coding strand, is not copied. So let's look here. So this is what's happening. The RNA polymerase is going to open it up, right? And so then we can copy our template strand, which is this one down here. So next we have elongation, right? So elongation is going to be when we have those free ribonucleotides are base paired. They're going to pair with the exposed bases on the DNA template strand and then hydrogen bonds form between the ribonucleotide and the complementary DNA base. New phosphodiester bonds form between the ribonucleoclides and that makes an RNA polymer. And then RNA polymerase moves down the DNA and continues until the entire gene is transcribed. So right here, we're going to be building this mRNA. Now remember, if the DNA has a T, it binds to an A in the RNA. If the DNA has an A, it binds to a U in the RNA. Remember that in RNA, we have uracil instead of thiamine. If the DNA has a C, it binds to a G. If the DNA has a G, it binds to a C, and so on. And then finally we have termination. So whenever the RNA polymerase reaches the terminal region of the gene, it releases RNA polymerase. The hydrogen bonds are then broken between the DNA and the new mRNA strand, so the mRNA is released. And then the DNA rewinds. The coding strand and the template strand are going to stick back together and it's going to rewind into its double helix. So then we have DNA is back to normal as it started with, and now we have this copied area. So we've copied the instructions from the DNA. And this strand is going to be called a pre-mRNA strand because it still needs to undergo a couple of modifications before it goes out and into the cytosol to go to the ribosome. And so it's going to undergo certain changes before leaving the nucleus, before it can form the mature mRNA that's used as the... the recipe to make the protein. And so it's going to put on a couple of things like a poly-A tail, the 5-prem cap. But one of the important things that happens is called splicing. So splicing is going to mean that there's a bunch of non-coding, like nonsense regions of the pre-RNA that are called introns. And so these introns, they don't code for anything specific, they're just nonsense, they're kind of just extra stuff in there to confuse you. And so we're taking them out. So we're removing the introns. And so these introns are removed from the pre-mRNA. The exons are going to be the rest of it that stays in. So the exons then are going to, like all the exons attach to each other and they become the final mRNA strand. And those exons will all be, they will undergo translation. And so they're spliced together by something called a spliceosome. And so this, they can actually be spliced together in different orders and different ways. And so this provides a means of producing a large number of proteins from DNA. So one pre-mRNA could be spliced in different ways to form different proteins. So other changes, as I said, there's a ribonucleotide. that contains guanine that binds to the leading end of the mRNA. This prevents digestion by the enzymes in the cytoplasm, and it also has a poly-A tail that's going to be placed on the end of the DNA. So lots and lots and lots and lots of adenines. And so we've spliced out the introns, we put on the guanine cap and the poly-A tail, and then we have now our mature mRNA that's going to leave the nucleus and go to the cytoplasm. and undergo translation. So translation is the next part. It is the synthesis of a new protein. So we go from mRNA to protein in translation. This occurs at the ribosome within the cytoplasm. mRNA binds to a ribosome and it's threaded through. The ribosome has two subunits, a big one and a small one. And the mRNA kind of sits right in the middle. The codes in the nucleotide sequence of the mRNA is translated and it's going to be converted into the amino acids that cause it. form of protein. So some things we need for translation to happen. We need a ribosome, right? And we're going to find those in the cytoplasm. Some of them are free and some of them are bound to the rependoplasmic reticulum. We need the mRNA, right? That we just made in transcription. We also need that special tRNA or the transfer RNA. And we'll talk about that as we go through. And we need amino acids, right? Because the amino acids are the building blocks of protein. Ribosomes, as we said, are composed of rRNA and protein, and the nucleolus makes those ribosomal subunits and ships them out. The large subunit has three different sites, the A site, the P site, and the E site, and then it has a small subunit that kind of sits under here, and those are going to clamp together and kind of form, like clamp around the mRNA. The rRNAs serve as catalysts during assembly of amino acids into a protein molecule. Remember the mRNA we just made in transcription and it carries the instructions for synthesizing the protein, which is the linear sequence of the amino acids. When translation happens mRNA is read three base pairs at a time. So three, three base pairs at a time. So like A, T, and G. So it reads those three things as a word. Imagine if all the words in in the world were just three letters. So every word could be made by three different letters. And it reads in three letter segments. There's a couple of different types of codons. A start codon is AUG. It signals for methionine. That signals to begin the protein synthesis. Then all the codons, after that, all the three different codons are going to code for a different amino acid. Well, some codons code for the same amino acid, but there is a, like they all code for a specific amino acid. And then the stop codon, there are a few stop codons, tell the mRNA to stop. And then we stop making that protein. So tRNA is another, it's the transfer RNA. The transfer RNA has an anticodon on one side, and the anticodon is going to match the codons of the mRNA. And on the other side, it's bound to a specific amino acid, right? That matches. that codon. It's kind of a clover leaf shape and there's a picture of it over here. So this is going to be, we have the anticodon right here and the amino acid acceptor ends. We'll have the amino acid on one side and then the anticodon on the other side. And the anticodon here is going to match with the codons on the mRNA. And it's going to, when it matches, then it's going to bring the correct amino acid. So here we have our ribosome with our E and our P and our A site, the large subunit and then the small subunit. our messenger RNA and our transfer RNA. Let's see, as we said, the tRNA has the anticodon region. This one, each anticodon is going to correspond to a specific amino acid and it's going to bond to a specific codon on the mRNA. And this is catalyzed by the amino acetyl tRNA synthetase. After binding a The amino, the tRNA equals charged tRNA. So we have, once it binds the amino acid, it becomes charged. And it serves as the adapter site for binding the tRNA to the complementary codon of mRNA. So right, we have the amino acid on one side, and we've got the anticoagulant on the other side. And it's going to make sure that we're bringing the right amino acid to the right area, so we have the right order of proteins. There are 20 different amino acids found in the proteins of living things. And so each of these antipodons is going to code for a specific one of these 20 amino acids. So once again, here's an amino acid. This is an amino acid. We have a strand of amino acids. That's going to be the primary sequence of the protein. And then eventually it's going to, you know, we have these alpha helixes and the beta sheets are going to be those secondaries. And then this is a globular protein. So once we have that. protein made it's gonna form this big protein. As we can see here this is like the the dictionary right so if we said the codon is like a word let's say each of these three letter words they code for a different amino acid and so UUU codes for phenylalanine UUC also codes for phenylalanine. Some of them you'll have a lot of them if you see that all of these four of them all code for leucine and this actually allows there to be some mistakes. Because like if we had any of these, CUU, CUC, CUA, or CUG, any of those, all of them code for leucine. So if we have a couple of little mistakes, hopefully it won't matter. Methionine is really important, it's the start codon. We've got a couple of start, stop codons over here. So you guys can actually like, if you knew the sequences, we can now figure out what each of these codons code for. So there are also three events in translation. We have the same three events there were in transcription so it's easy to remember. Initiation, elongation, and termination. So first we have initiation. This is where mRNA is going to come out and form a complex between the ribosomal subunits right. So the ribosome is going to bind across the mRNA and then that first tRNA is going to come. So those that tRNA is going to have an anticoagulant on and it's going to sit on the specific anticoagulant. It's going to find the codon. And that first codon is going to be methionine, right? It's going to be AUG. That's the start codon. And methionine is always bound to that first tRNA. And that's going to be the first amino acid and protein synthesis. And later it might be removed, but that's just how we started out. And the start codon starts on the ribosomal P site. And so here we have our initiation over here. We're going to start with the methionine on the P site, which is that middle site, and it's going to bring that methionine part. So we've got our mRNA is sitting in the ribosome, the ribosome is attached to it, and now we have our first, we found the AUG, which is the start codon, and we have our first amino acid. Next we have elongation, which is going to be... the matching of antichodons and codons. So if we look right here, the next three is another antichodon, we have to find the right tRNA that matches the antichodon that matches. When we find that antichodon that matches, then that amino acid is going to bring, or that tRNA brings the next amino acid. That amino acid is then going to be bound. Those two are bound together. A peptide bond is formed. And then eventually that initial amino acid is going to move from the P site to the E site. The amino acid in the A site then moves to the P site. And then a new amino acid is going to come on. And it's going to bring, it's going to be a specific tRNA that matches the next anticodon. And it's going to bring the next amino acid. And then it's going to bind to that one over here. And then Eventually the E site, I think of that as like the exit site, that's once they move to the E site, they're going to exit. So it will, the tRNA is going to release its amino acid and then it's uncharged and it's going to move back to the cytosol and it can get a new amino acid and bring it back later. And so that's going to be part of elongation. And this repeats until the entire mRNA sequence is translated. And that yields the linear strand of amino acids, which is the primary structure of the protein. Termination happens when we have a stop codon that goes into the A-site, and that ends translation. There's something called a release factor that enters the A-site instead of the charged tRNA. That's going to cause the two ribosomal subunits to separate, and the mRNA is released, and a new protein is released. Now we can actually have this happening a bunch of times on the same mRNA, because one mRNA can be attached to multiple ribosomes at different areas along it, and it can be making multiple proteins. very quickly. So here we could have another ribosome somewhere else attached somewhere else on here making another protein. So how exactly does DNA change the cell? Even if we have different proteins made it doesn't seem like it would change the cell that much but it really can. And so DNA directs the synthesis of proteins that carry out body functions, but it is also indirectly responsible for other metabolic changes. So it directs the synthesis of steroids, right? Those are a lipid hormone. It's important in the enzymatic pathway of glucose oxidation, right? It makes those enzymes and things. It controls the enzymes responsible for decomposition and synthesis of chemical structure. So all these enzymes are proteins that are originally going to be coded for by DNA. All right. So this one was a little bit shorter. I would... I recommend watching the Crash Course Ed Puzzle video to get some more information about transcription and translation.

So we briefly mentioned, and I put a really good video from the Amoeba Sisters up about DNA replication in lab. So kind of go over that. Transcription is going to be a little similar in a lot of ways to DNA replication, but obviously with RNA instead. So Transcription and translation are going to be very important. And one of the important things that we need to talk a little bit about is DNA.

And so what DNA is and where it is located. So DNA is housed in the nucleus. And we learned in Chapter 2 that it's composed of repeated monomers called nucleotides, right? DNA has deoxyribonucleotides, right?

So it has that deoxyribose sugar in it. Whereas RNA has just ribonucleotides. It has the ribose sugar.

So each deoxyribonucleotide is composed of the five carbon sugar deoxyribose, a phosphate, and one of four different nitrogenous bases. So in DNA, we have adenine, cytosine, guanine, and thiamine. And so we learned about this already in chapter two, so this should hopefully be a review.

These nucleotides are linked together by phosphodiester bonds, and each molecule has two complementary strands of nucleotides. So DNA is double-stranded. It looks a little bit like a twisted ladder or a spiral ladder. The sugar and phosphates form the struts of the ladder, and the pairs of nucleotide bases form the rungs.

And these base pairs are connected by hydrogen bonds, right from one strand to the other complementary strand. This houses most of the genetic material in the cell. The human has 46 separate double-stranded DNA molecules, or 23 pairs. Each double helix is wound around nuclear proteins called histones. And together we call this a nucleosome because otherwise we'd have so much DNA, like a lot of DNA.

When not dividing, DNA is in a finely filamented mass called chromatin. So the non-compacted form of DNA is called chromatin. When it is dividing, the chromatin becomes tightly coiled or supercoiled into chromosomes.

And in our last lab, we talked about how the chromosomes form. during the beginning of prophase and we start seeing them during all those phases of mitosis. But when they're coiled in these chromosomes, they can't really be, it can't, all the pro, the enzymes can't really get to them and so if they're coiled into chromosomes they can't undergo transcription or, you know, transcription or DNA replication.

So here this is our double stranded DNA. These are our base pairs. They're held together by hydrogen bonds. We've got three between the C and the G and two between the A and the T.

And if it is loosely coiled as chromatin, then it's going to be more accessible and we're going to be able to transcribe it. So how is DNA related to genes and how are those related to proteins? And how in the world does this molecule here dictate like your whole body? And so it's a little bit complicated, but the functional unit of DNA is basically a gene.

And a gene is basically a segment of nucleotides that's going to code for a specific. protein. And those specific proteins might be enzymes or transport channels or something, but those proteins are going to change how your cells work. And if they change how your cells work, then it is going to be changing like your phenotype, change what you look like, change how you are. So the beginning of the gene has what's called a promoter region.

That's like the start signal that tells your body to start transcribing that gene. And the end of the gene has a terminal region or a stop signal. And cellular activities depend upon the protein synthesis, right? Different proteins and different enzymes are going to cause each cell to function differently and this is directed by the DNA, right? Because we said genes code for proteins.

So the way that our cells are gonna figure out what proteins to make and make proteins is called transcription and translation. And so the very short, very brief thing to remember is that transcription happens in the nucleus And this is going to start with DNA. So the DNA is opened up and it's copied. And we copy it into an mRNA strand.

And so we have this mRNA that's copied. So we transcribe the DNA into mRNA. We copy that gene. It's a copy of a gene.

Then that mRNA is going to leave the nucleus through a nuclear pore and go into the cytosol and bind to a ribosome. And it's going to then be translated. So we're going to translate the language of RNA into the language of proteins. And we're going to actually make the protein that's coded for in this mRNA that was originally coded in this gene here.

And this is going to use a ribosome as well as this special transfer RNA to bring amino acids to the ribosome to start making the linear structure of the amino acids, which is the primary structure of the protein. So DNA is the major structure required in transcription, right? This is like the blueprint. So we're going to be transcribing DNA. This is going to serve as a template for the complementary RNA molecule.

So RNA is a little bit different. It's single-stranded. It just binds to the DNA, right? So it makes that one single strand. And instead of a T, it has a U, right?

And so instead of deoxyribosugar, it has a ribosugar. So it has an A, a U, a C, and a G. And it's only single-stranded.

So transcription requires DNA because we have to copy it. It requires all these ribonucleotides to make that mRNA and the enzyme RNA polymerase. And this is located in the nucleoplasm of the nucleus.

So RNA polymerase in this picture is like this big purple enzyme here. It's going to unzip the DNA and then start copying it into this mRNA strand and then re-zip the DNA up. So before we start talking about exactly how...

transcription happens, I just want to mention that there are three types of RNA that are produced during transcription. The one we'll talk about in class here that goes on to make proteins is called messenger RNA, but we also have transfer RNA. Transfer RNA or tRNA is going to be really important in translation, right? Translation, t. And it's going to help transfer the amino acids to the ribosome, the correct amino acid to the ribosome, and then ribosomal RNA for our RNA is going to be part of the ribosome.

So ribosomes are made up of our RNA and protein, right? And remember that they were made, the nucleolus is going to help make those ribosomal subunits in the middle of the nucleus, and then it's going to ship them out to the cytoplasm. All right, so there are three events in transcription, so we'll go through each, initiation, elongation, and termination. So initiation is the start of transcription. So we have RNA polymerase, and it's going to start.

need to bind to DNA. So DNA is unwound by some specific enzymes that we don't need to name because this is only an overview, that make it accessible to RNA polymerase. RNA polymerase, look, ACE, A-S-E at the end, it's an enzyme that catalyzes the synthesis of that messenger RNA. Remember, we're talking about messenger RNA here. So that RNA polymerase is going to attach those ribonucleotides to their complementary strands of DNA.

So if we had an A DNA, it would bind to a U RNA. If we had a T, it would bind to an A. If we had a C, it would bind to a G. If we had a G, it would bind to a C and so on.

And so this DNA is the template strand and it's going to make an RNA strand that matches. But remember that instead of T's, RNA is going to have a U. So RNA is going to attach to the DNA.

RNA polymerase is going to attach to the DNA strand and locate the promoter region. That promoter region is the first part, the start of the gene, and that's going to start that gene transcription. So it's going to break the bonds between DNA and it's going to open up that little area.

And this open area, that's where we can copy. We're copying the DNA to make mRNA. The copied DNA strand is called this template strand and the other DNA strand, the coding strand, is not copied.

So let's look here. So this is what's happening. The RNA polymerase is going to open it up, right?

And so then we can copy our template strand, which is this one down here. So next we have elongation, right? So elongation is going to be when we have those free ribonucleotides are base paired.

They're going to pair with the exposed bases on the DNA template strand and then hydrogen bonds form between the ribonucleotide and the complementary DNA base. New phosphodiester bonds form between the ribonucleoclides and that makes an RNA polymer. And then RNA polymerase moves down the DNA and continues until the entire gene is transcribed. So right here, we're going to be building this mRNA.

Now remember, if the DNA has a T, it binds to an A in the RNA. If the DNA has an A, it binds to a U in the RNA. Remember that in RNA, we have uracil instead of thiamine. If the DNA has a C, it binds to a G. If the DNA has a G, it binds to a C, and so on.

And then finally we have termination. So whenever the RNA polymerase reaches the terminal region of the gene, it releases RNA polymerase. The hydrogen bonds are then broken between the DNA and the new mRNA strand, so the mRNA is released.

And then the DNA rewinds. The coding strand and the template strand are going to stick back together and it's going to rewind into its double helix. So then we have DNA is back to normal as it started with, and now we have this copied area. So we've copied the instructions from the DNA. And this strand is going to be called a pre-mRNA strand because it still needs to undergo a couple of modifications before it goes out and into the cytosol to go to the ribosome.

And so it's going to undergo certain changes before leaving the nucleus, before it can form the mature mRNA that's used as the... the recipe to make the protein. And so it's going to put on a couple of things like a poly-A tail, the 5-prem cap. But one of the important things that happens is called splicing. So splicing is going to mean that there's a bunch of non-coding, like nonsense regions of the pre-RNA that are called introns.

And so these introns, they don't code for anything specific, they're just nonsense, they're kind of just extra stuff in there to confuse you. And so we're taking them out. So we're removing the introns. And so these introns are removed from the pre-mRNA.

The exons are going to be the rest of it that stays in. So the exons then are going to, like all the exons attach to each other and they become the final mRNA strand. And those exons will all be, they will undergo translation.

And so they're spliced together by something called a spliceosome. And so this, they can actually be spliced together in different orders and different ways. And so this provides a means of producing a large number of proteins from DNA. So one pre-mRNA could be spliced in different ways to form different proteins.

So other changes, as I said, there's a ribonucleotide. that contains guanine that binds to the leading end of the mRNA. This prevents digestion by the enzymes in the cytoplasm, and it also has a poly-A tail that's going to be placed on the end of the DNA.

So lots and lots and lots and lots of adenines. And so we've spliced out the introns, we put on the guanine cap and the poly-A tail, and then we have now our mature mRNA that's going to leave the nucleus and go to the cytoplasm. and undergo translation. So translation is the next part. It is the synthesis of a new protein.

So we go from mRNA to protein in translation. This occurs at the ribosome within the cytoplasm. mRNA binds to a ribosome and it's threaded through.

The ribosome has two subunits, a big one and a small one. And the mRNA kind of sits right in the middle. The codes in the nucleotide sequence of the mRNA is translated and it's going to be converted into the amino acids that cause it.

form of protein. So some things we need for translation to happen. We need a ribosome, right?

And we're going to find those in the cytoplasm. Some of them are free and some of them are bound to the rependoplasmic reticulum. We need the mRNA, right? That we just made in transcription. We also need that special tRNA or the transfer RNA.

And we'll talk about that as we go through. And we need amino acids, right? Because the amino acids are the building blocks of protein.

Ribosomes, as we said, are composed of rRNA and protein, and the nucleolus makes those ribosomal subunits and ships them out. The large subunit has three different sites, the A site, the P site, and the E site, and then it has a small subunit that kind of sits under here, and those are going to clamp together and kind of form, like clamp around the mRNA. The rRNAs serve as catalysts during assembly of amino acids into a protein molecule. Remember the mRNA we just made in transcription and it carries the instructions for synthesizing the protein, which is the linear sequence of the amino acids. When translation happens mRNA is read three base pairs at a time.

So three, three base pairs at a time. So like A, T, and G. So it reads those three things as a word.

Imagine if all the words in in the world were just three letters. So every word could be made by three different letters. And it reads in three letter segments. There's a couple of different types of codons.

A start codon is AUG. It signals for methionine. That signals to begin the protein synthesis.

Then all the codons, after that, all the three different codons are going to code for a different amino acid. Well, some codons code for the same amino acid, but there is a, like they all code for a specific amino acid. And then the stop codon, there are a few stop codons, tell the mRNA to stop.

And then we stop making that protein. So tRNA is another, it's the transfer RNA. The transfer RNA has an anticodon on one side, and the anticodon is going to match the codons of the mRNA. And on the other side, it's bound to a specific amino acid, right?

That matches. that codon. It's kind of a clover leaf shape and there's a picture of it over here. So this is going to be, we have the anticodon right here and the amino acid acceptor ends.

We'll have the amino acid on one side and then the anticodon on the other side. And the anticodon here is going to match with the codons on the mRNA. And it's going to, when it matches, then it's going to bring the correct amino acid. So here we have our ribosome with our E and our P and our A site, the large subunit and then the small subunit. our messenger RNA and our transfer RNA.

Let's see, as we said, the tRNA has the anticodon region. This one, each anticodon is going to correspond to a specific amino acid and it's going to bond to a specific codon on the mRNA. And this is catalyzed by the amino acetyl tRNA synthetase.

After binding a The amino, the tRNA equals charged tRNA. So we have, once it binds the amino acid, it becomes charged. And it serves as the adapter site for binding the tRNA to the complementary codon of mRNA.

So right, we have the amino acid on one side, and we've got the anticoagulant on the other side. And it's going to make sure that we're bringing the right amino acid to the right area, so we have the right order of proteins. There are 20 different amino acids found in the proteins of living things. And so each of these antipodons is going to code for a specific one of these 20 amino acids. So once again, here's an amino acid.

This is an amino acid. We have a strand of amino acids. That's going to be the primary sequence of the protein.

And then eventually it's going to, you know, we have these alpha helixes and the beta sheets are going to be those secondaries. And then this is a globular protein. So once we have that. protein made it's gonna form this big protein.

As we can see here this is like the the dictionary right so if we said the codon is like a word let's say each of these three letter words they code for a different amino acid and so UUU codes for phenylalanine UUC also codes for phenylalanine. Some of them you'll have a lot of them if you see that all of these four of them all code for leucine and this actually allows there to be some mistakes. Because like if we had any of these, CUU, CUC, CUA, or CUG, any of those, all of them code for leucine.

So if we have a couple of little mistakes, hopefully it won't matter. Methionine is really important, it's the start codon. We've got a couple of start, stop codons over here.

So you guys can actually like, if you knew the sequences, we can now figure out what each of these codons code for. So there are also three events in translation. We have the same three events there were in transcription so it's easy to remember.

Initiation, elongation, and termination. So first we have initiation. This is where mRNA is going to come out and form a complex between the ribosomal subunits right.

So the ribosome is going to bind across the mRNA and then that first tRNA is going to come. So those that tRNA is going to have an anticoagulant on and it's going to sit on the specific anticoagulant. It's going to find the codon.

And that first codon is going to be methionine, right? It's going to be AUG. That's the start codon. And methionine is always bound to that first tRNA.

And that's going to be the first amino acid and protein synthesis. And later it might be removed, but that's just how we started out. And the start codon starts on the ribosomal P site. And so here we have our initiation over here.

We're going to start with the methionine on the P site, which is that middle site, and it's going to bring that methionine part. So we've got our mRNA is sitting in the ribosome, the ribosome is attached to it, and now we have our first, we found the AUG, which is the start codon, and we have our first amino acid. Next we have elongation, which is going to be... the matching of antichodons and codons.

So if we look right here, the next three is another antichodon, we have to find the right tRNA that matches the antichodon that matches. When we find that antichodon that matches, then that amino acid is going to bring, or that tRNA brings the next amino acid. That amino acid is then going to be bound.

Those two are bound together. A peptide bond is formed. And then eventually that initial amino acid is going to move from the P site to the E site.

The amino acid in the A site then moves to the P site. And then a new amino acid is going to come on. And it's going to bring, it's going to be a specific tRNA that matches the next anticodon.

And it's going to bring the next amino acid. And then it's going to bind to that one over here. And then Eventually the E site, I think of that as like the exit site, that's once they move to the E site, they're going to exit.

So it will, the tRNA is going to release its amino acid and then it's uncharged and it's going to move back to the cytosol and it can get a new amino acid and bring it back later. And so that's going to be part of elongation. And this repeats until the entire mRNA sequence is translated. And that yields the linear strand of amino acids, which is the primary structure of the protein.

Termination happens when we have a stop codon that goes into the A-site, and that ends translation. There's something called a release factor that enters the A-site instead of the charged tRNA. That's going to cause the two ribosomal subunits to separate, and the mRNA is released, and a new protein is released.

Now we can actually have this happening a bunch of times on the same mRNA, because one mRNA can be attached to multiple ribosomes at different areas along it, and it can be making multiple proteins. very quickly. So here we could have another ribosome somewhere else attached somewhere else on here making another protein. So how exactly does DNA change the cell? Even if we have different proteins made it doesn't seem like it would change the cell that much but it really can.

And so DNA directs the synthesis of proteins that carry out body functions, but it is also indirectly responsible for other metabolic changes. So it directs the synthesis of steroids, right? Those are a lipid hormone. It's important in the enzymatic pathway of glucose oxidation, right?

It makes those enzymes and things. It controls the enzymes responsible for decomposition and synthesis of chemical structure. So all these enzymes are proteins that are originally going to be coded for by DNA.

All right. So this one was a little bit shorter. I would...

I recommend watching the Crash Course Ed Puzzle video to get some more information about transcription and translation.

Transcript for:Understanding Transcription and Translation

Transcript for:
Understanding Transcription and Translation