Let's say we wanted this piece of DNA to undergo transcription. So the question I want to ask you is, well, which strand is going to be transcribed? So let's just unzip our DNA and look at the two strands. And let's say that the strand over here is strand A, and let's say that this strand is strand B. So which one are we going to pick?
Well, first let's see what happens if we transcribe either one. Let's first see what happens if we transcribe A. So here's our RNA.
We'll draw it in another color. We'll draw it in purple. Here's the backbone of the RNA. And the DNA has, well, let's say that the orange bases represent the nitrogen-based cytosine.
And let's say that the blue bases represent guanine. So DNA strand A has a bunch of cytosine nucleotides. And so the mRNA will have the complementary guanine.
So let's draw the guanine nucleotides. So that's what the mRNA would look like. And each three nucleotides, or each codon, will code for one amino acid, and it happens to be, right here we have three codons that are each GGG, and GGG codes for the amino acid glycine. And so when this mRNA is translated in the ribosome, it will result in a polypeptide that looks like this. Glycine, glycine, glycine.
Let's see what happens if we decide to transcribe strand B. So let's draw the backbone of our RNA, and DNA strand B has a bunch of guanines, and so the mRNA will have cytosines, non-nucleotides of cytosine. And again, each three is going to code for one amino acid.
So we have three codons and each one reads CCC. CCC codes for the amino acid proline. And so if this mRNA is going to be translated, the polypeptide will look like this.
So the important thing to keep in mind is if we transcribe strand A, we're gonna get one polypeptide chain. If we transcribe strand B, we're going to get a completely different one. And so there's one correct strand that we're supposed to transcribe. And let's just say in our case that B is the one that we're going to transcribe.
So here's the strand we're transcribing. So let's get rid of the RNA that's made off of A. And if you look at the mRNA that was made, here let's label it, here is our mRNA, the nucleotide sequence on it is actually identical to A.
These two have an identical... nucleotide sequence. Therefore, strand E, the one that's not transcribed, is called the coding strand, because it has the exact same code as the RNA that's made.
And it's also called the sense strand. That's strand A. Strand B, the one off which we make the RNA, that's called the non-coding strand. Because the code on the strand B and the RNA are not identical. It's also called the antisense strand.
Antisense meaning it's complementary to the sense strand, but not identical to it. And some other, sometimes called the transcribed strand. And it's also called the template strand, because it's the template that's used to make the RNA.
Okay, so now that we've discussed which strand gets transcribed, let's see what transcription looks like in more detail. So here's a piece of DNA that I drew, and there are going to be three steps in transcription. The first one is initiation, basically how transcription starts. The second step is going to be elongation.
That involves actually making the strand of RNA, and you can see the word long in there because the strand is kind of long. And then the last step is going to be termination. And that's basically how transcription ends.
So we're going to start with... initiation. So what happens during initiation? So the enzyme RNA polymerase, that's the enzyme that puts together RNA, so let's label it. That's RNA and I'm just gonna write POL which stands for polymerase.
So the enzyme RNA polymerase is going to scan the DNA in this direction. I want to just pause for a second. Take note, I label the top strand as the non-coding strand, and so that's the strand that's going to be transcribed. And the bottom strand is the coding strand, which means that's the strand that's not going to be transcribed.
So that is not transcribed. Okay, back to the initiation step in transcription. So, RNA polymerase scans the DNA, kind of like a train that's going along train tracks, until it hits this sequence over here.
That's a promoter. And a promoter is a sequence on the DNA that tells RNA polymerase, here's where you should begin transcription. And so RNA polymerase is going to make this. kind of open up the DNA and form this bubble that I pre-drew. So that's a transcription bubble, because it kind of needs room to go there and transcribe.
Okay, so let's move our RNA polymerase. Let's put him right over here. So the RNA polymerase is right there, and it starts to put together an RNA in this direction.
And RNA is synthesized in the five prime to three prime direction. So, let's actually fill in some of the nucleotides in this top portion of the bubble to kind of get an idea of what's happening. And in the bottom right-hand corner, I drew this little key to help us keep track of which colors represent which.
nitrogen bases. So let's say that right here in the DNA there were some guanine nucleotides, I'll just draw a couple, and then let's say that after that there were some adenine nucleotides. Just like that.
So on the mRNA that's being made, well the nucleotides are going to be complementary. So wherever there are guanines, we'll have cytosines. And then wherever we have adenines, well, in DNA we would have thymine complementary to that. But in RNA, we have uracil instead of thymine. So what I just described to you, this step is elongation.
We're actually making this rather long RNA. And just an interesting fact to take note of, in prokaryotic cells, transcription happens at the rate of 40 nucleotides. per second.
Let's go on to termination. So I'm going to move our RNA polymerase and put them over here. So the RNA polymerase is here.
Well, it's going to hit this sequence, this sequence you see in red. That is the terminator. And the terminator is a sequence on the DNA that tells RNA polymerase, okay, here's where you should stop transcribing. And so when that happens, RNA polymerase is going to unlatch from the DNA. It's going to go on and do something else or be recycled.
The DNA closes back up, and the mRNA is going to be released, and its next step is it's going to go to the ribosome. And in the ribosome, it's going to be translated. And the ribosome's going to put together the corresponding polypeptide chain.