Introduction to Transcription - Online Tutor, Practice Problems & Exam Prep

Transcription is the process where RNA is synthesized from a DNA template, beginning at the promoter and ending at the terminator. RNA polymerase binds to the promoter, transcribing the coding strand's sequence into mRNA, replacing thymine (T) with uracil (U). The RNA is built from its 5' to 3' end through base pairing, following Watson and Crick rules. Understanding transcription is crucial for grasping gene expression and protein synthesis, as it lays the foundation for translation, where mRNA is converted into proteins.

Introduction to Transcription

In this video, we're going to begin our introduction to transcription. Recall from our previous lesson video the coding template. 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 need to be both transcribed and translated. Here we're talking about the first step of the process, which is transcription, being transcribed.

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 the site where the RNA polymerase will attach, and the RNA polymerase will be the primary enzyme involved in transcription. As mentioned below, RNA polymerase will polymerize or build RNA from scratch. Unlike DNA polymerases, RNA polymerases do not need a primer. We will talk a lot more about RNA polymerase and transcription as we move forward in our course.

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 a stretch of DNA sequences, but this is where transcription will end. If we take a look at our image below, on the far left, we have a chromosome, a replicated chromosome, which consists of DNA and protein. If you unravel this chromosome, you'll find DNA, some of which are genes that encode a product like a protein.

A typical gene will have a green region referred to as the promoter. The promoter is the small stretch of DNA sequences that allow for the RNA polymerase to bind. The pink structure you see is the RNA polymerase, the primary enzyme involved with transcription. The RNA polymerase binds to the gene specifically at this promoter region and transcribes the coding sequence of this gene. The terminator, where transcription will end, is at the other end. Collectively, the promoter, the coding sequence, and the terminator—this entire region—is referred to as the gene.

Sometimes there are other important sequences that surround a gene. The terms upstream of the gene and downstream are used to refer to the directionality of those sequences outside of the gene. Downstream refers to DNA sequences in the same direction as transcription. You can think of little fishies downstream. Since the RNA polymerase binds here, it will be transcribing in this direction, so downstream is in the same direction of transcription. The opposite direction is 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 here 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.

Which of the following is the best definition of a gene?

• A. An RNA molecule transcribed from a sequence of DNA.
• B. A stretch of DNA that can be transcribed.
• C. A sequence of DNA where the process of transcription ends.
• D. A sequence of DNA that encodes a product like an RNA or a protein.
• E. A sequence of DNA where the process of transcription begins.

Which of the following statements is false?

• A. Transcription is the process that creates an RNA product from a sequence of DNA.
• B. RNA polymerase builds RNA molecules from a DNA template.
• C. A promoter is a sequence of DNA within a gene where RNA polymerase can begin transcription.
• D. RNA polymerase, like DNA polymerase, requires a primer to begin RNA synthesis.

Overview of Transcription

In this video, we're going to do an overview of the entire process of transcription. However, it's important to keep in mind that as we continue to move forward in our course, we're going to continue to break down transcription in more detail. And later in our course in a separate video, we'll talk about the individual steps of transcription in more detail. And here in this video, we're just doing an overview of the entire process. And so it's important to note that the two strands of DNA in a gene are referred to as either the coding strand or the template strand. Now, it turns out that during transcription, the RNA molecules that are being built are going to have the same exact sequence as the coding DNA strand. And the template DNA strand is just going to serve as a template for building the RNA. But ultimately, the RNA molecule is going to be carrying the message that's in the coding DNA strand. And so the only difference between the RNA sequence and the coding DNA sequence is going to be the fact that it's going to be replacing all of the T's or thymines with U's or uracils. And recall that is because RNA has uracils, whereas DNA is going to have thymines. And we'll be able to see an example or an application of this down below in our example. Now it's also important to note that during transcription, the RNA molecule itself is going to be built from its 5' end to its 3' end, and it's going to be built by pairing free RNA nucleotides that are floating around in the cell on a DNA template. And so we'll be able to see that down below in our image over here on this side. Now this nucleotide pairing is going to occur via Watson and Crick base pairing, which, recall, we already covered in our previous lesson videos, which basically says that A's or adenines will pair with T's or thymines. Or in the case of DNA base pairing with RNA, A's are going to be base pairing with U's, because remember that in RNA, U's are going to replace the T's. And then, of course, we know that G's are going to base pair with C's. And so if we take a look at our image down below, we can better understand the template DNA template DNA strand and for the mRNA transcript given the following coding strand. And so when we take a look at this side of the image over here, what you'll notice is we have a DNA molecule. And recall that DNA has 2 strands, and each of these two strands, one of them is going to be the coding strand, and the other one's going to be the template strand. In this case, the coding strand is at the top here, and the template strand is the one on the bottom. And so it wants us to determine the template DNA strand sequence along with the mRNA transcript sequence that's down below. And so, what's important to note here is that, when we look at the coding sequence, the coding strand sequence is GGATC, and we know that through Watson and Crick base pairing that the G's are going to base pair with C's, and so these G's here will base pair with C's on the DNA. So we know that C's are going to go in these positions, and that A's are going to base pair with T's in the DNA. And so, we will put T's here, and T's will base pair with A's, and, of course, C's base pair with G's. And so this sequence here is the sequence of the template strand sequence. Now, of course, the process of transcription represented by this arrow is going to use the DNA to build RNA. And, the RNA, as we mentioned up above, are going to have the same sequence as the coding DNA strand with this exception right here, replacing the T's with U's. And so what we can see here is that the coding sequence strand is right here at the top, and RNA is going to have the same sequence as the coding strand. So it is going to be GGA, and then, of course, replacing the T's with U's, as we mentioned here, replacing the T's with U's. And so here, instead of having a T at this position, we're going to have a U. And then, of course, the C here is going to go right here. And so this will be the sequence of the RNA. Now it turns out that in eukaryotic cells, the RNA that is first made is called a pre-mRNA transcript because it's not fully mature, but we're going to talk more details about that later in our course. Now over here on the right, we're kind of showing you more of a zoom out of the overall process of transcription. And recall that the RNA polymerase enzyme, which is this big pink circle in the background, is the primary enzyme involved with transcription. And so it will bind to the promoter region on a gene and start to transcribe the gene. And so when it transcribes a gene, it's going to be building the RNA, and the RNA is going to be built from its 5' end to its 3' end, again, by pairing free RNA nucleotides that are floating around in the cell on a DNA template. And so you can see that this RNA is being built from its 5' end over here towards its 3' end over here. And the direction of the RNA polymerase is to the right, so transcription is proceeding to the right. And so what you'll notice is that the DNA is going to unwind, and you can see that there's a coding strand and, again, a template strand. And the RNA, of course, is going to end up taking the same exact sequence as the coding strand. And again, this bottom one that's down here is going to be the template strand of DNA. Now you'll notice that there is going to be a region here highlighted by this box, which is representing an RNA DNA hybrid region. And so, you can see that these free RNA nucleotides that are floating around in the cell are going to be incorporated into this RNA molecule by the RNA polymerase enzyme. And so this here is representing the overall overview of transcription. And, again, as we move forward in our course, we're going to break down this process of transcription in more details, looking at it step by step by step. But for now, this here concludes our overview of transcription, and we'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you all in our next video.

The strand of DNA that has the same sequence as the RNA molecule being created during transcription is the:

• A. Lagging strand.
• B. Leading strand.
• C. Coding strand.
• D. Template strand.
• E. Parent strand.

Transcription is sometimes described as a process in which RNA is "copied" from the template strand of DNA. This statement is potentially misleading since _____.

• A. The nucleotides in RNA contain ribose and cannot be an exact copy of DNA.
• B. RNA molecules contain uracil instead of thymine and cannot be an exact copy of DNA.
• C. The RNA transcript has a sequence complementary to the template.
• D. The RNA transcript and the DNA template strand are antiparallel.
• E. All of the above.

Here’s what students ask on this topic:

What is the role of RNA polymerase in transcription?

RNA polymerase is the primary enzyme involved in transcription. It binds to the promoter region of a gene, initiating the transcription process. RNA polymerase synthesizes RNA by adding RNA nucleotides complementary to the DNA template strand, building the RNA molecule from its 5' end to its 3' end. Unlike DNA polymerase, RNA polymerase does not require a primer to start synthesis. It continues to elongate the RNA strand until it reaches the terminator sequence, where transcription ends. Understanding RNA polymerase's function is crucial for grasping how genetic information is transcribed from DNA to RNA.

What is the difference between the coding strand and the template strand in DNA?

In DNA, the coding strand and the template strand serve different roles during transcription. The coding strand has the same sequence as the RNA transcript, except that thymine (T) in DNA is replaced by uracil (U) in RNA. The template strand, on the other hand, is the strand that RNA polymerase uses as a template to synthesize the RNA molecule. The RNA nucleotides pair with the complementary bases on the template strand, following Watson and Crick base pairing rules. Thus, the coding strand's sequence is essentially copied into RNA, with the necessary base modifications.

What are promoters and terminators in the context of transcription?

Promoters and terminators are specific DNA sequences that play crucial roles in transcription. The promoter is a region where RNA polymerase binds to initiate transcription. It marks the starting point for RNA synthesis. The terminator, on the other hand, is a sequence that signals the end of transcription. When RNA polymerase reaches the terminator, it releases the newly synthesized RNA molecule and detaches from the DNA. These sequences ensure that transcription starts and stops at the correct locations, allowing for precise gene expression.

How does base pairing work during transcription?

During transcription, base pairing follows Watson and Crick rules. RNA nucleotides pair with their complementary DNA bases on the template strand. Adenine (A) in DNA pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G). For example, if the DNA template strand has a sequence of 3'-TACG-5', the corresponding RNA sequence will be 5'-AUGC-3'. This base pairing ensures that the RNA transcript accurately reflects the genetic information encoded in the DNA, with thymine (T) in DNA being replaced by uracil (U) in RNA.

What is the significance of the 5' to 3' direction in RNA synthesis?

RNA synthesis occurs in the 5' to 3' direction, meaning that RNA polymerase adds nucleotides to the 3' end of the growing RNA strand. This directionality is crucial because it ensures that the RNA molecule is synthesized correctly and efficiently. The 5' to 3' synthesis allows for the proper alignment of RNA nucleotides with their complementary DNA bases on the template strand. This directional synthesis is a fundamental aspect of transcription, ensuring that the genetic information is accurately transcribed from DNA to RNA.