Transcription 2


– 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 this 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 our, the backbone of the RNA, and the DNA has, well, let’s say that the orange bases represent the nitrogen base 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’s 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 our, the
backbone of our RNA, and DNA Strand B has a bunch of guanines, and so the mRNA will have cytosines. Nine 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 when, 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’s our mRNA, the nucleotide sequence on it is actually identical to A. These two have an identical
nucleotide sequence. Therefore, Strand A, 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 anti-sense strand. Anti-sense meaning it’s complementary to the sense strand,
but not identical to it, and sometimes it’s 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 gonna 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 p-o-l, 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 labeled 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 ’cause 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. An 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 nucelotides. 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 prokarotic cells, transcription happens at the rate of 40 nucleotides per second. Let’s go on to Termination. So I’m gonna move our RNA polymerase and put him 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 it’s 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.

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