Lesson Video: Types of RNA Biology

In this video, we will learn how to describe the components that make up RNA molecules and differentiate between tRNA, mRNA, and rRNA.

14:49

Video Transcript

In this video, we will learn to describe the components that make up RNA molecules and differentiate between three types of RNA: tRNA, mRNA, and rRNA.

In learing about genetics and molecular biology, DNA is often in the spotlight. But DNA doesn’t act alone. Like a movie star can’t shine without the movie crew, DNA wouldn’t be able to play its role without RNA. So let’s turn the spotlight on RNA and give it the credit it deserves. As we’ll see, RNA and DNA share some common similarities and differences. Let’s look at this in a bit of detail as it relates to the composition and structure of these molecules. So here, we have a molecule of RNA on the left and a molecule of DNA on the right. RNA is normally single-stranded, while DNA is double-stranded. Let’s zoom in on both of these molecules to see what the chemical structures are.

RNA and DNA are both considered nucleic acids which are made up of repeating subunits called nucleotides. Each nucleotide is made up of three different parts: the phosphate group as shown here and here, the pentose sugar as shown here and here, and the nitrogenous base. You may have noticed that the only difference between these nucleotides is the nitrogenous base. There’s different types of these bases that we’ll talk about in a moment. In both molecules, the phosphate group plays a role in establishing a phosphodiester bond between two consecutive nucleotides. These phosphodiester bonds form a backbone for both RNA and DNA that we call the sugar phosphate backbone. You can see these boxed in pink here.

Notice that RNA has one sugar phosphate backbone because it’s single-stranded, while DNA has two because it’s double-stranded. You can also see the sugar phosphate backbone indicated in black in both the RNA and the DNA diagrams, while the different coloured lines indicate the nitrogenous bases. Now that we understand the general structure of RNA and DNA, let’s look at some of the differences.

One of the differences has to do with the pentose sugar. We’ll start with the pentose sugar that’s in RNA. The carbons are numbered around the pentose sugar with one prime, being a carbon that bonds to the nitrogenous base, then two prime, three prime, four prime, and five prime bonds to the phosphate group. Now let’s look at the pentose sugar in DNA. There, the carbons are numbered one prime to five prime, just like in RNA. Why don’t you pause the video and see if you can see what’s different between the two? The difference is in the group that’s attached to the second carbon of the pentose sugar.

The hydroxy group in RNA is what makes this sugar a ribose. This is why we call it ribonucleic acid, whereas in DNA we’re missing an oxygen and just have the hydrogen instead. And this is why we call it deoxyribonucleic acid. This hydroxy group in RNA makes it more chemically reactive. Because of this, RNA is less stable or more fragile than DNA. This is why DNA is preferred by organisms as a support for genetic information, and short-lived RNA is preferred for transient messages in the cell.

Now let’s compare the nitrogenous bases of RNA and DNA. You may recall that in DNA there are four different nitrogenous bases as shown here: adenine as shown in green, thymine as shown in pink, guanine as shown in blue, and cytosine as shown in orange. Of these, adenine and guanine are purines and have a double ring structure, whereas thymine and cytosine are pyrimidines and have a single ring. You may also remember that nucleotides can make hydrogen bonds with other nitrogenous bases. According to the rules of complementary base pairing, adenine binds the thymine with two hydrogen bonds, and guanine binds the cytosine by three hydrogen bonds. This is what forms these base pairs as indicated here.

The two strands of DNA are held together by these hydrogen bonds which are individually weak but collectively strong. This double-stranded DNA molecule can then twist and form the double helix shape that you are familiar with. Now let’s look at the nitrogenous bases that we find in RNA. As you can see, the nitrogenous bases of RNA are very similar to those of DNA with only one key difference. In RNA, we see uracil in place of thymine. Uracil also belongs to the pyrimidine family, and it pairs with adenine in the same way that thymine does, using two hydrogen bonds. We can see that here. Although the nitrogenous bases of RNA can base pair just like DNA, RNA generally doesn’t form a double-stranded molecule like DNA. However, the ability of RNA to form complementary base pairs with DNA is incredibly important, during the formation of any type of RNA during transcription. Before we get into this, let’s first review how RNA can transmit genetic information contained in DNA to the rest of the cell.

This is a universal process in all organisms. It has been summed up in what’s called the central dogma of molecular biology. So here we have a stretch of DNA, and within this DNA is a gene. A gene is a section of DNA that codes for a functional unit such as a protein or some type of functional RNA molecule like rRNA or tRNA that we’ll discuss later. This gene is transcribed in a process called transcription. Transcription is the process of converting DNA into RNA. Any type of RNA can be produced from transcription. But in this example, we’ll be discussing a special type of RNA called messenger or mRNA. Genes that code for proteins produce mRNA after transcription. And this mRNA sequence can be converted into an amino acid sequence during a process called translation. These amino acids can then fold to form a protein with a specific function.

Now we’re going to look at this process of transcription in more detail using mRNA as an example. Then, we’ll see how this mRNA can be translated. During translation, there’s two other types of RNA that we’ll be looking at. These are called rRNA and tRNA. We’ll first look at how transcription works to produce mRNA as an example. But remember, transcription produces any kind of RNA including the rRNA and tRNA that we’ll be discussing in a moment. First, an enzyme called RNA polymerase binds to the DNA. Then, the double helix is unwound by breaking the hydrogen bonds between the nitrogenous bases. This exposes the different nucleotides which then act as a template for mRNA synthesis. So here we can see the mRNA molecule that’s being synthesized by RNA polymerase.

Different RNA nucleotides are matched to the complementary DNA nucleotide, which are then added to the growing mRNA molecule by RNA polymerase. RNA polymerase moves forward as this mRNA molecule elongates. You can see the mRNA emerging from RNA polymerase here. Transcription continues until RNA polymerase reaches the end of the gene. Now that we have our mRNA molecule, the nucleotides within it need to be translated into a sequence of amino acids to form a protein. This process is called translation, and it involves two more types of RNA that we’ll be discussing called rRNA and tRNA. Translation is performed by an organelle called the ribosome. The ribosome is made of two separate structures called the large and small subunits. These two subunits combine together to form the ribosome. The ribosome is made up of a combination of proteins and ribosomal RNA or rRNA.

Ribosomal RNA does not code for proteins like mRNA. Instead, it’s used as a structural and functional component for the ribosome. The ribosome combine to mRNA and start translation. Now that the ribosome is bound to mRNA, specific amino acids need to be brought in to be matched to the corresponding nucleotides in mRNA. This is done by another type of RNA called transfer or tRNA. Like rRNA, tRNA is not converted into a protein like mRNA. Instead, the RNA nucleotides fold upon themselves to form hydrogen bonds as indicated by these black dots. This gives tRNA its characteristic cloverleaf shape.

At one end of the tRNA molecule is an amino acid. The amino acid that’s attached to the tRNA is not a random choice. In fact, there is a sequence of three nucleotides at the other end of the tRNA molecule that determines which amino acid is attached. This sequence is called the anticodon site. The three nucleotides in the anticodon site can base pair with three complementary nucleotides in mRNA that we call a codon. Each codon in mRNA codes for a specific amino acid. So when these tRNA molecules bind to mRNA, they bring the corresponding amino acid with them which can then be linked together as we can see here. The ribosome can move along the mRNA sequence to bring in the next tRNA molecule to join the next amino acid, while the earlier tRNA exits.

This process continues, codon after codon, until a special codon is reached called the stop codon. This causes the ribosome to separate and the amino acid chain or polypeptide can be released. It’s important to point out that, in reality, polypeptides can be much longer than what’s indicated here. If we look at a diagram of the different parts of an mRNA molecule, we can see the stop codon indicated here. The ribosome actually binds toward the beginning of the molecule, but it doesn’t start translation until it reaches a start codon. The region in between is a coding sequence, and this is what’s actually translated to the amino acid sequence.

The poly-A tail is another structure that’s made up of around 200 adenine nucleotides. This helps to stabilize mRNA to prevent its degradation so it can be translated. Now that we understand more about the different types of RNA, let’s try out a practice question.

Ribosomes are formed from ribosomal RNA and polypeptides. What role do ribosomes have in a cell? (A) Controlling what enters and leaves the cell, (B) providing the site for aerobic respiration, (C) transporting enzymes around the cell, (D) intracellular signalling, or (E) providing the site for protein synthesis.

In every living cell, DNA carries the genetic information that gives the instructions the cell needs to properly function. These instructions come in the form of genes which are sections of DNA that can be converted into functional units, such as proteins. When a gene needs to be converted into protein, the sequence of DNA must first be converted into a molecule of messenger RNA or mRNA by transcription. Then, the sequence of mRNA is converted into a sequence of amino acids or a polypeptide in a process called translation. This polypeptide can then go on to fold into a protein with a specific function.

Translation is performed by an organelle called the ribosome, which is made up of a special kind of RNA called ribosomal RNA and polypeptides, as indicated in the question. This ribosome can attach to mRNA and convert its sequence into a sequence of amino acids. This forms a polypeptide, which can then fold to form a protein. Therefore, the role of the ribosome in the cell is to provide the site for protein synthesis.

Now let’s take a moment to go over the key points that we covered in this video. RNA, like DNA, is made up of repeating subunits called nucleotides. RNA contains the nitrogenous bases adenine, uracil, guanine, and cytosine. Unlike DNA, RNA is normally single-stranded instead of double-stranded, contains a ribose sugar instead of a deoxyribose, and contains uracil instead of thymine. In order to form proteins, genetic information flows from DNA to mRNA by transcription which is then translated into a protein. Transcription can produce any type of RNA including rRNA and tRNA. Ribosomes, the organelle that performs translation, are made up of rRNA and polypeptides. During translation, tRNA delivers amino acids to the ribosome to be matched to the sequence in mRNA.

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