In this video, we're going to begin our introduction to transcription. Recall from our previous lesson video that DNA within a gene is the coding template, and this is the process of building RNA. Also, recall that genes are small units of DNA that encode a product, for example, a protein. In order to create a protein, genes will need to be both transcribed and translated. Here we're talking about the first step of the process, which is transcription, being transcribed.
In order to better understand transcription, it's helpful to describe some specific sequences of DNA that mark where transcription of a gene begins and ends. The first sequence of DNA that you should note is called the promoter. The promoter is a small stretch of DNA sequences where transcription begins. The promoter is just the site where the RNA polymerase is going to attach, and the RNA polymerase is going to be the primary enzyme involved in transcription. RNA polymerase, as mentioned down below here, is going to an enzyme, the primary enzyme that is going to polymerize or build RNA from scratch. It turns out that RNA polymerases do not need a primer like DNA polymerases do. RNA polymerase does not require a primer. We're going to talk a lot more about RNA polymerase and transcription as we move forward in our course here. But it's important to note that the promoter is the sequence where transcription begins because this is the site where the RNA polymerase first attaches.
The terminator, on the other hand, is also going to be a stretch of DNA sequences, but this is where transcription is going to end. If we take a look at our image down below, what you'll notice is on the far left, we have a chromosome, a replicated chromosome, which we know consists of DNA and protein. But if you unravel this chromosome, what you'll find is there is DNA in here, some of this DNA are going to be genes that encode a product like a protein. If we take a look at a typical gene and zoom into it, what you'll find is that it's going to have a green region here that we're going to be referring to as the promoter. And the promoter is, again, the small stretch of DNA sequences that allows the RNA polymerase to bind. This pink structure that you see here is the RNA polymerase, the primary enzyme involved in transcription. The RNA polymerase, what it does, is it will bind to the gene specifically at this promoter region, and then the RNA polymerase will transcribe the coding sequence of this gene. The terminator over here at the end is, of course, where transcription is going to end. Collectively, the promoter, the coding sequence, and the terminator, this entire region right here, is referred to as the gene.
Sometimes there are other important sequences that surround a gene, so it's important to refer to the directionality of those sequences that might be outside of the gene. The terms used are upstream of the gene and downstream. It turns out that downstream of the gene refers to DNA sequences in the same direction as transcription. You can see the arrow down here. You can think of little fishes downstream, and downstream represents DNA sequences outside of the gene that are in the same direction of transcription. In this case, because the RNA polymerase binds here, it's going to be transcribing in this direction, so downstream is in the same direction of transcription. The opposite direction is going to be referred to as upstream. Upstream refers to DNA sequences in the opposite direction of transcription. The RNA polymerase binds at the promoter and transcribes in this direction, but upstream of the gene is in the opposite direction of the transcription direction.
This concludes our brief introduction to transcription. As we move forward in our course, we're going to continue to learn more about transcription. I'll see you all in our next video.