I hope that general overview of transcription and translation helped put the big picture in perspective for you in terms of how the information in DNA ends up being turned into these proteins that are either parts of cells or become enzymes that do the work of cells. One thing that I will point out about that video is it focused on eukaryotic cells. And there are some features of eukaryotic transcription that are a little bit different.
One of them is that... eukaryotic RNA, messenger RNA, is processed to remove introns. These are pieces of information inside genes that actually aren't part of the coding region, and so they get spliced out of the transcript in eukaryotic cells. And bacteria don't have introns. Prokaryotic cells don't have introns.
So it's a little bit simpler in bacteria, which I'm sure is okay with you. Let's go over some terms that we're going to be using just to review so that we don't get confused in the middle of things. I'll just remind you that RNA, as opposed to DNA, has uracil instead of thiamine.
They both have the same information. They mean the same thing in terms of which amino acids are being coded for, but uracil is the base that's used in RNA. And RNA contains ribosugars in the backbone, not deoxyribosugars like we see in DNA. When you have a piece of DNA that has a gene, there has to be a part of that gene where RNA polymerase says, aha, this is where I'm going to bind, this is where I start. And that piece of DNA is called a promoter.
So the promoter is where RNA polymerase binds to DNA to begin mRNA synthesis. And then there's also a spot where RNA polymerase says, ah, the gene's finished, I'm all done. And that's called the terminator. So RNA polymerase will bind to the promoter. It will then synthesize messenger RNA until it hits a terminator, and then it's going to fall off, and the messenger RNA is going to fall free.
The DNA will wind back up, and that messenger RNA can then go out to a ribosome, and it can be translated by the ribosome into protein. Now, the steps going from the initiation of messenger RNA synthesis, we call that initiation, so we're going to begin synthesis, the process where we're continuing to make the RNA and add RNA bases to our lengthening mRNA chain is called elongation. And then when RNA polymerase falls off, we call that termination. So it's got an initiation step, an elongation step, and a termination step. Here is a picture of this whole process.
And this here is the RNA polymerase. And you can see what's happened is it's opened up. the DNA just forms a little short bubble, so it's actually quite a bit simpler than DNA synthesis.
And you can see that what's happening here is we have a strand here that is the non-template strand or in the other picture I showed you they called it the coding strand. And then we have the template strand. So the template strand is the sequence of DNA that's used to actually make the messenger RNA. And of course, these won't be identical.
They will be complementary, right? They're going to be complementary. So if there's a C in the template strand, there will be a G added to the messenger RNA.
And where there are A's in the template strand, we're going to add U's to our mRNA, all right? G, we're going to add C. A T here in our DNA, we're going to add an A, right?
So the messenger RNA is going to be complementary to the template strand. And it's going to match. The coding strand, do you see that it's actually they're missing a base here, but I think if we go here, TT, remember that T and U are informationally the same, right?
So TTCA, UUCA, CG, CG, CACA. So the sequence of messenger RNA that we make is going to match the sequence on the non-template or the coding strand of the DNA molecule. So this RNA polymerase is moving this direction because we're synthesizing our messenger RNA 5'to 3'. So it's going to go this direction, and it's going to keep moving and synthesizing messenger RNA until it hits a terminator.
And as it moves along, the DNA will wind back up. It's already been copied. All right, so we said our steps were initiation.
elongation and termination. And this picture does a really nice job showing all three steps. So here's initiation. This is showing our DNA. It's still wound, but we're the promoter region, and the promoter is where RNA polymerase binds, is being highlighted.
So this protein recognizes this sequence of DNA. It says, aha, this is where I'm going to bind. This is where I need to start synthesizing messenger RNA.
It will bind. It will begin unwinding the DNA. And as it unwinds a little bit of the DNA, it's going to synthesize RNA.
So it's going to, again, using the template strand, it's going to make a complementary strand of messenger RNA. And as it moves along, the DNA will wind back up. Okay, so it will unwind the DNA to make the copy.
And then as it keeps going, the DNA behind it will wind back up. Eventually, there's going to be a termination sequence or a stop sequence. And when RNA polymerase hits the sequence, it says, aha.
I've come to the end of the gene. It's time to stop synthesizing messenger RNA. And RNA polymerase will fall off, the messenger RNA will leave, and the DNA will wind back up. And of course, DNA can be used over and over and over again to make many, many copies of messenger RNA from a single gene.
All right, so now what I'm going to do is play for you another video that animates the process of transcription. Just like the first overview I showed you, they use eukaryotic cells as their exemplar. And so just know that it's a little bit simpler.
There are some, they talk about a TATA box and a methyl G cap and a poly A tail. Don't worry about these. These are features of eukaryotic transcription.
But I just want you to get kind of see the animation and see how the messenger RNA is built. Because the idea of taking DNA. and making a messenger RNA from a transcript is the same in both cell types, even if some of the peripheral processing steps are slightly different.