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, forming the foundation of molecular biology and genetics.

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 refer to small units of DNA that encode a product, for example, a protein. Genes, in order to create a protein, 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. It is the site where RNA polymerase, the primary enzyme involved in transcription, will attach. 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.

It's important to note that the promoter is the sequence where transcription begins because this is the site where RNA polymerase first attaches. The terminator, on the other hand, is a stretch of DNA sequences where transcription ends. If we take a look at our image, on the far left, we have a chromosome, a replicated chromosome, which consists of DNA and protein. But if you unravel this chromosome, you will find DNA, and some of this DNA will be genes. They encode a product like a protein.

If we zoom into a typical gene, you will find a green region referred to as the promoter. Again, this small stretch of DNA sequences allows the RNA polymerase to bind. The pink structure you see here is the RNA polymerase, the primary enzyme involved in transcription. The RNA polymerase binds to the gene specifically at this promoter region and transcribes the coding sequence of this gene. The terminator over here at the end is, of course, where transcription ends. Collectively, the promoter, the coding sequence, and the terminator, this entire region, is referred to as the gene.

Sometimes there are other important sequences surrounding a gene, and it's important to refer to the directionality of those sequences outside of the gene. The terms used are "upstream" of the gene and "downstream." Downstream of the gene refers to DNA sequences in the same direction as transcription. You can think of little fish downstream; downstream represents DNA sequences outside of the gene that are in the same direction of transcription. In this case, since the RNA polymerase binds here, it transcribes 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. Again, the RNA polymerase is binding 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 and 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 details. 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 carry 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 this 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.

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 strand and 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, coding strand sequence, it 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 gonna 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 in 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 detail, looking at it 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 process of RNA synthesis. Unlike DNA polymerase, RNA polymerase does not require a primer to start building RNA. It reads the template strand of DNA and synthesizes a complementary RNA strand by pairing free RNA nucleotides with the DNA template. The RNA strand is built from its 5' end to its 3' end, following Watson and Crick base pairing rules, where adenine (A) pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G). The process continues until RNA polymerase reaches the terminator sequence, where transcription ends.

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

In transcription, the coding strand and the template strand are the two strands of DNA in a gene. 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, serves as the actual template for RNA synthesis. RNA polymerase reads the template strand to build the RNA molecule, which will be complementary to the template strand and identical to the coding strand (with the T to U substitution). This ensures that the genetic information is accurately transcribed into RNA.

What are the promoter and terminator sequences in transcription?

The promoter and terminator sequences are specific regions of DNA that mark the beginning and end of transcription, respectively. The promoter is a small stretch of DNA where RNA polymerase binds to initiate transcription. It is located upstream of the gene and signals the start of RNA synthesis. The terminator is another stretch of DNA that signals the end of transcription. When RNA polymerase reaches the terminator sequence, it stops transcribing and releases the newly synthesized RNA molecule. These sequences ensure that transcription occurs at the correct location and produces the correct RNA product.

How does base pairing work during transcription?

During transcription, base pairing follows Watson and Crick rules to ensure accurate RNA synthesis. RNA polymerase reads the template strand of DNA and pairs free RNA nucleotides with the complementary DNA bases. Adenine (A) in the DNA template pairs with uracil (U) in RNA, while cytosine (C) pairs with guanine (G). Thymine (T) in the DNA template pairs with adenine (A) in RNA. This base pairing ensures that the RNA sequence is complementary to the DNA template strand and identical to the DNA coding strand, except for the substitution of uracil for thymine.

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

The 5' to 3' direction is crucial in RNA synthesis because RNA polymerase can only add nucleotides to the 3' end of the growing RNA strand. This directionality ensures that the RNA molecule is synthesized in a consistent and accurate manner. The 5' end of the RNA molecule is the starting point, and nucleotides are added sequentially towards the 3' end. This 5' to 3' synthesis is essential for proper gene expression and the accurate transfer of genetic information from DNA to RNA, which will later be translated into proteins.