Lesson Video: Translation Biology

In this video, we will learn how to describe the process of translation and outline the roles of mRNA, tRNA, and ribosomes.


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.

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