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.
