Video Transcript
In this video, we’ll learn more
about the process of translation to produce a polypeptide from an mRNA
transcript. We’ll learn about the major
components of translation including the ribosome and transfer RNA and see how these
work together with an mRNA transcript to form a polypeptide.
Before we talk about translation,
let’s first review how a gene can be converted into a protein. Here’s a eukaryotic cell with its
nucleus and DNA inside. Suppose this gene here, shown in
blue, needs to be expressed as a protein. This actually happens in a couple
of steps. The first step is called
transcription. Here, a gene in double-stranded DNA
is transcribed or copied to make a single-stranded molecule of mRNA, or messenger
RNA.
In eukaryotes, this process occurs
in the nucleus, whereas in prokaryotes, which lack a nucleus, this process occurs in
the cytoplasm. This mRNA can then be converted
into a protein in a process called translation. During translation, the sequence of
nucleotides in mRNA is converted into a sequence of amino acids. This is called a polypeptide, and
it can go on to fold into a protein.
Now, let’s review how a sequence of
nucleotides in mRNA can be translated into a sequence of amino acids using the
genetic code. Here’s a sequence of mRNA for us to
work with. You’ll recall that mRNA contains
uracil instead of thymine as found in DNA and that mRNA is written in the five prime
to three prime direction. During translation, groups of three
nucleotides, called codons, are translated into specific amino acids using the
genetic code. We can use the codon wheel, as
shown here, to see how each codon can be translated into an amino acid. Let’s review how to do this using
the underlying codon ACA on the left.
We’ll start from the center, which
indicates the five prime end of the codon, and work our way outwards towards the
three prime end. So, with the codon ACA, we first
start with A, then C, and then A. This corresponds to the amino acid
threonine. Let’s try out the next codon,
UCA. So we start with U, then C, and
then A. This corresponds to the amino acid
serine. For this last codon, GGA, why don’t
you pause the video and see if you can work it out yourself? GGA corresponds to glycine. Now, why don’t we zoom out a bit so
we can see what this transcript looks like on a larger scale? Here’s the mRNA molecule shown in
blue. After mRNA is produced during
transcription, the entire transcript is not translated into protein; only a portion
of it is.
There’s a translation start signal
and a translation stop signal. During translation, these two
signals mark the beginning and end of the polypeptide. As we can see in the codon wheel on
the right, the start signal is indicated by the codon for the amino acid
methionine. So essentially, every protein
begins with the amino acid methionine. On the other hand, there are
several stop codons that can be used. These don’t code for amino
acids. Rather, they attract special
proteins to tell the ribosome to stop translation.
Now that we’ve covered how the
genetic code can be used to translate nucleotides in mRNA to amino acids in a
polypeptide, it’s time to talk about how this process takes place on the molecular
level. There’s a few key components to
this process. One is what we’ve covered already,
the mRNA molecule itself to provide the message that needs to be translated. The ribosome is a tiny organelle
made of special proteins that scan the mRNA and match the corresponding amino acids
to the codons and a special type of RNA, called transfer or tRNA, that physically
carries the amino acids needed to the ribosome. These components work together to
translate a series of nucleotides into amino acids.
Let’s take a closer look at these
components, starting with tRNA. tRNA is a specialized adapter molecule, which means
that it carries an amino acid that corresponds to a specific codon sequence in
mRNA. For each codon, there’s a
corresponding tRNA that goes with it. All tRNA molecules are made of a
single strand of RNA that folds upon itself to form this cloverleaf-type
structure. You can see the five prime and
three prime ends here. What’s not shown here are the
individual RNA nucleotides that are holding it together, so let’s have a closer
look. Here, you can see how the
individual nucleotides in tRNA can hydrogen-bond with each other as indicated by the
black lines. This is what allows the tRNA
molecule to take on this unique shape.
There are two important regions in
a tRNA molecule. At one end is the amino acid
attachment site. If we look closer, we can see an
asparagine amino acid attached to the three prime end of the tRNA molecule. At the other end of tRNA, there’s a
region called the anticodon. The anticodon is the sequence of
RNA nucleotides in tRNA, or UUA in this example, that is complementary and can form
hydrogen bonds with the codon in mRNA, or AAU as shown here. If we pull up our codon wheel
again, we can see that the codon AAU corresponds to the amino acid asparagine. And this is the amino acid that’s
attached to the three prime end of the tRNA molecule.
Now that we understand a bit more
about tRNA, let’s turn our attention to where translation physically takes place,
the ribosome. The ribosome is made up of two
subunits. On top is a large subunit and on
the bottom is a small subunit. These subunits are made up of
protein and a special kind of RNA called ribosomal RNA. These subunits come together along
with mRNA during translation. tRNA, carrying an amino acid, is also able to come
into the complex and match with its corresponding codon on mRNA, while the ribosome
builds a polypeptide. Now, let’s start from the beginning
and see how the entire process of translation takes place.
Translation begins with a small
subunit of the ribosome binding to a special sequence in mRNA called the ribosome
binding site. It then moves along mRNA scanning
it to locate a start codon. This codon is the nucleotide
sequence AUG, and this corresponds to the amino acid methionine. Next, the tRNA with methionine,
which has the complementary anticodon for AUG, UAC, binds to the start codon. Then, the large subunit binds to
mRNA and the small subunit to form the ribosome. The large subunit has three
compartments called E, P, and A. The A site is called the aminoacyl
site and is where the ribosome accepts a new tRNA for the next codon. The P site holds the growing
polypeptide chain, and the E site is where the previous tRNA exits.
When a large subunit first joins
the mRNA and small subunit complex, the tRNA from methionine is in the P site. The next tRNA with the matching
anticodon then comes into the ribosome at the A site. This codon CUG corresponds to the
amino acid leucine, which we can see here. Now, the ribosome can join these
two amino acids to form a peptide bond. It can do this because the large
subunit of the ribosome can act as an enzyme using its peptidyl transferase
activity. And here you could see the two
amino acids joined.
Next, the ribosome moves down the
mRNA in the five prime to three prime direction to the next codon. The tRNA molecules inside don’t
move, and they’re shifted over to the adjacent site. So the tRNA containing the
polypeptide is now in the P site, and the tRNA, which just lost its amino acid, is
now in the E site. This tRNA then exits the ribosome,
while the next tRNA, with its matching anticodon, can enter the ribosome at the A
site. This process repeats itself until a
stop codon is reached and special proteins called release factors cause the ribosome
to release the polypeptide and the mRNA. The resulting polypeptide can then
fold to give rise to a protein with the specific shape suited for its function.
Overall, translation as a process
can happen fairly quickly with a rate of about 10 amino acids per second. So for the protein insulin, which
is about 50 amino acids long, this would take the ribosome about five seconds to
synthesize from mRNA. There are some compounds that can
inhibit translation, such as ricin, which can be found in the seeds of the castor
oil plant.
Ricin is able to cleave a specific
component of the large ribosomal subunit. This can prevent it from
associating with special factors called elongation factors. One of these factors is involved in
bringing new tRNAs to the A site of the ribosome. This can lead to toxicity by
inhibiting protein synthesis, which can be fatal. Because only small doses are
needed, ricin has been used throughout history in various assassination
attempts. It also has potential in being used
to treat cancer as it can kill tumor cells.
Now that we know more about the
process of translation, let’s try out a practice question.
What is the purpose of tRNA in the
process of translation? (A) To provide the site within a
eukaryotic cell for translation to take place. (B) To carry amino acids to the
mRNA molecule being translated to form a polypeptide chain. (C) To catalyze the formation of
peptide bonds between amino acids in a polypeptide chain. Or (D) to provide the sequence of
nucleotides that determine the sequence of amino acids.
This question is asking us about
the purpose of tRNA during translation. To answer this question, let’s
first clear the answer choices so we have more room to work with. In order for a gene in our DNA to
be expressed and its protein made, it must first be transcribed or copied to form
mRNA. This process is called
transcription. This mRNA can then be translated to
form a protein. This process is called translation,
and it involves a few key components, namely, mRNA, a special organelle called the
ribosome, and tRNA. Let’s look at these in a little bit
more detail.
mRNA is a copy of a gene that’s
being expressed. It contains a nucleotide sequence
that contains the information needed for building a protein. Groups of three nucleotides, called
codons, can be decoded or translated to their corresponding amino acid. So the codon CGG corresponds to the
amino acid arginine. These nucleotides, when organized
into codons, can code for any amino acid in a protein. These amino acids are held together
by peptide bonds. The resulting polypeptide can then
fold into a protein.
mRNA can be translated in a special
complex called the ribosome. This molecular machine is able to
match the nucleotide sequence of mRNA with the corresponding amino acid. The ribosome can then form peptide
bonds between these amino acids and form the polypeptide. Finally, tRNA, or transfer RNA, is
a special adapter molecule that brings amino acids to the ribosome. Here, they’re matched to their
corresponding nucleotide sequence.
Therefore, the purpose of tRNA in
the process of translation is to carry amino acids to the mRNA molecule being
translated to form a polypeptide chain.
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. Translation is the process of
converting the nucleotide sequence in mRNA into amino acids in protein. There are several key components
during translation, including the mRNA itself, tRNA, and a tiny organelle called the
ribosome. tRNA is the molecule responsible for carrying amino acids to the
ribosome. The ribosome is where translation
occurs, and it’s made up of two subunits, the large and small subunit. Each has its own role during
translation.
