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Lesson Video: Transcription Biology

In this video, we will learn how to describe the process of transcription and outline the roles of DNA, mRNA, and RNA polymerase.


Video Transcript

In this video, we’ll discuss the process of transcription where DNA is converted into mRNA. We’ll talk about the major enzyme involved in this process, RNA polymerase, and see how it can copy a sequence of DNA and turn it into a sequence of mRNA. Finally, we’ll discuss posttranscriptional modifications like RNA splicing and polyadenylation.

Nearly all the cells in our body contain DNA. DNA is a double-stranded helix, and organized in the DNA are genes that contain instructions for building different proteins needed for life. If we zoom in a little closer, we can see that the sequence of DNA is a series of Gs, Cs, As, and Ts that we call nucleotides. These nucleotides or bases as they’re sometimes called include guanine or G for short, cytosine, adenine, and thymine. Nucleotides on one strand can base-pair with nucleotides on the opposing strand of DNA. Guanine can base-pair or form hydrogen bonds with its complementary base cytosine, whereas adenine can base-pair with thymine.

The sequence of nucleotides that make up a gene can be converted into a protein. This process involves a few steps. The first step is called transcription and involves copying the sequence of the gene and turning it into a transcript or mRNA. The sequence in the transcript can then be converted into amino acids that can fold and form the protein. In this video, we’ll be describing the process of transcription. In eukaryotic cells, which contain a nucleus like the one shown here, transcription takes place in the nucleus, whereas in prokaryotic cells, which lack a nucleus, transcription actually occurs in the cytoplasm.

The major enzyme involved in transcription is called RNA polymerase. This enzyme, represented here as this pink outline attached to DNA, is able to copy the DNA sequence in a gene, for example, and turn it into an mRNA sequence during transcription. Here’s an example for us to work with of DNA. Notice that there’s an upper sequence and a lower sequence. This is to indicate the sequences on each of the two strands. You may recall that DNA has directionality, and its upper sequence is written in the five prime to three prime direction, while the lower sequence is written in the three prime to five prime direction. During transcription, RNA polymerase unwinds the double helix. And you can see this here on the right and here on the left as well.

Now that the two strands are separated, RNA polymerase can use one of the strands as a template to start forming the mRNA transcript. The three prime to five prime strand is used as a template, and bases are added following complementary base-pairing rules, so C pairs with G. In DNA, normally thymine would base-pair with adenine. However, in RNA, thymine isn’t used and uracil takes its place. G pairs with C and T pairs with A. Here’s the rest of the sequence, and a blue line has been added to show that this is a strand of mRNA. Now, let’s add this information to the diagram on the right. There, the mRNA has been added right here to correspond with what we see on the left.

Now, RNA polymerase can move forward, and it unwinds the helix in front of it to access the next segment of the DNA sequence. While this happens, a segment of the DNA sequence behind it winds up again, and the corresponding segment of mRNA begins to be released. Now that we’ve seen this on the right, let’s turn our attention to the left and see how this looks here. Again, transcription and RNA polymerase move in the five prime to three prime direction. So first, a bit of the helix opens up, while a bit behind it closes up and a portion of the mRNA molecule begins to be released. The process repeats, and now this section of DNA is transcribed. Why don’t you pause the video to see if you can figure out what the corresponding sequence of mRNA will be?

Thymine pairs with adenine, cytosine pairs with guanine, and since there is no thymine in RNA, adenine pairs with uracil. We can also complete this on the right. And this process repeats itself until a special transcription termination sequence is reached and RNA polymerase detaches from DNA and releases the mRNA transcript. In eukaryotes, transcription occurs in the nucleus, so the mRNA is released in the nucleus as shown here. If this is a prokaryote and there was no nucleus, transcription would take place in the cytoplasm. And that’s where we would find that mRNA. Now, let’s close up the sequence on the left so we can see our final transcript.

Note that this is an unrealistically small transcript made up of only nine nucleotides. In real life, transcripts can be much larger. Human dystrophin is a protein found in muscle tissue and its gene is 2.3 million base pairs long. RNA polymerase needs about 16 hours to transcribe this gene. This works out to about 40 nucleotides per second, which is pretty impressive even for this muscular individual here. Anyways, back to our tiny mRNA, we saw that this mRNA sequence was assembled using the three prime to five prime strand as a template. This is because mRNA is synthesized by RNA polymerase in the five prime to three prime direction.

You’ll notice that this mRNA sequence matches the corresponding sequence in DNA on the five prime to three prime strand, with the exception of uracil of course. For this reason, the five prime to three prime DNA strand is sometimes called the sense strand because it matches the mRNA sequence, while the three prime to five prime DNA strand is sometimes called the antisense strand because it’s complementary to the mRNA sequence.

A certain species of mushroom, called the death cap mushroom, is able to inhibit transcription. It can do this by producing a protein called 𝛼-amanitin, which combine very tightly to RNA polymerase. This protein can constrain the motion of RNA polymerase and slow down the process of transcription dramatically. Normally, RNA polymerase produces mRNA at a rate of thousands of nucleotides per minute. But 𝛼-amanitin can slow down this process to just a few nucleotides per minute. Because the cell can’t perform transcription effectively, protein production slows down and can’t meet the demand of the cell, so cells begin to die. This is why 𝛼-amanitin is such a deadly poison and it’s why eating unidentified wild mushrooms is not a great idea.

Now that we’ve seen our transcription can produce mRNA, this mRNA transcript is actually not quite finished yet. It still needs to be processed by posttranscriptional modifications. And before we get started, let’s put this DNA back inside the nucleus where it belongs. That looks better now. Now, let’s turn our attention to this mRNA molecule shown here in blue. The mRNA at this stage is called pre-mRNA, and this needs to be processed with posttranscriptional modification. There’s a couple of these that we’ll cover. The first one we’ll cover is called RNA splicing. Most of our DNA is actually made up of noncoding regions or regions that don’t code for proteins.

In pre-mRNA, these noncoding regions, shown here in orange, need to be removed. These noncoding regions are called introns, while the coding regions are called exons and these need to be joined together. During RNA splicing, these introns are removed, while the exons are joined together, and this process can be repeated on the other end. The spliced RNA with its joined exons can then go through another posttranscriptional modification. During polyadenylation, multiple adenine nucleotides are added to the three prime end of the mRNA molecule. This improves the mRNA stability and can assist with it being exported from the nucleus.

Another posttranscriptional modification can occur on the five prime end of the mRNA, where specialized nucleotide is added. This is called the five prime cap. Once all the posttranscriptional modifications have occurred, we now have a mature mRNA. In eukaryotes, mature mRNA can now exit the nucleus and enter the cytoplasm. Here, it can then be translated to form the protein for the corresponding gene.

Now that we understand transcription in more detail, let’s try out a practice question.

A single strand of DNA undergoing transcription reads three prime to five prime AATCCGATCG. Reading five prime to three prime, what will the sequence on the complementary strand of mRNA be? (A) TTCGGATCGA, (B) GGAUUCGAUC, (C) UUAGGCUAGC, (D) AATCCGATCG, or (E) TTAGGCTAGC.

This question is asking us to transcribe a sequence of DNA into mRNA. You’ll recall that when a gene needs to be expressed as a protein, it first needs to be transcribed or copied into mRNA. This process is called transcription. This mRNA transcript can then be converted into a sequence of amino acids in the polypeptide. This is called translation, and once the polypeptide is formed, it can go on to fold into a protein with a specific function. The enzyme that converts DNA into mRNA is called RNA polymerase, which attaches to the DNA double helix as shown here. Once attached, RNA polymerase can unwind the helix and begin copying one of the DNA strands to form an mRNA transcript of the gene.

RNA polymerase moves along the DNA until it reaches the end of the gene and the mRNA transcript is released. Let’s look at this process of transcription in a bit more detail to see how this looks in the DNA sequence. The sequence we’ll use is the sequence in the question. Here you can see the two strands of DNA. You’ll recall that DNA has directionality. So, one strand is in the five prime to three prime direction, while the opposing strand is in the three prime to five prime direction. The sequence in this question is on the three prime to five prime strand. The three prime to five prime strand is actually what’s used as a template during transcription. So, once RNA polymerase binds and unwinds the helix, which is now represented here, RNA polymerase can start adding nucleotides to build the mRNA molecule.

Since the three prime to five prime strand is used as a template, the corresponding mRNA, shown here as this green arrow, will be assembled in the five prime to three prime direction. mRNA is synthesized using the same complementary base-pairing rules as in DNA. In DNA, guanine or G pairs with cytosine by forming hydrogen bonds indicated here as these black dots, and adenine pairs with thymine. There is one exception. In RNA, there is no thymine, and thymine is actually replaced by another nucleotide called uracil or U for short.

Now, let’s start filling in the mRNA sequence by adding the complementary bases. Adenine normally base-pairs with thymine, but since we’re forming mRNA and there is no thymine, uracil is used instead. Thymine in DNA pairs with adenine in mRNA, cytosine in DNA pairs with guanine in mRNA, and guanine pairs with cytosine. Why don’t you pause the video and see if you can work out the rest of the sequence?

Alright, now let’s fill it in. Therefore, the sequence of mRNA read in the five prime to three prime direction is UUAGGCUAGC.

Now, let’s look at some of the key points that we covered in this video. Genetic information flows from DNA to mRNA to protein. Transcription is the process of converting DNA into mRNA. The major enzyme involved in transcription is RNA polymerase. In eukaryotes, transcription occurs in the nucleus, whereas in prokaryotes, transcription occurs in the cytoplasm. And finally, mRNA can undergo posttranscriptional modifications such as RNA splicing and polyadenylation.

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