Lesson Explainer: Types of RNA Biology • Third Year of Secondary School

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

When we began learning about genetics and molecular biology, we were taught that DNA, or deoxyribonucleic acid, is the molecule of life. By encoding information in its sequences using the genetic code, DNA carries the instructions needed by every living organism to survive. However, although this molecule is extremely important, it does not function alone. In order for DNA to be able to dictate every physical and functional aspect of life, a highly efficient system functions behind the scenes, driven by another nucleic acid, RNA.

RNA, or ribonucleic acid, is essential for the transport and interpretation of the genetic information carried by DNA, and it plays a key role in carrying out the instructions that DNA encodes. These tiny RNA molecules have a powerful impact on a cell’s survival and characteristics. Through research, scientists are constantly uncovering new ways in which RNA can be used, for example, in vaccinations and molecular therapy.

Before we delve into the different types of RNA and the functions they perform, let’s begin by learning about the structure of a molecule of RNA. We will also examine the differences between the structures of DNA and RNA.

Both DNA and RNA are nucleic acids, composed of polynucleotide chains. This means that they are long polymer chains made up of repeating units called nucleotides. Each individual nucleotide is composed of a pentose sugar, a phosphate group, and a nitrogenous base, as depicted in Figure 1.

To form the long polynucleotide chain, each nucleotide is linked to the next by a phosphodiester bond, which links the phosphate group of one nucleotide to the pentose sugar of the next.

At the nucleotide level, we encounter some of the fundamental differences between DNA and RNA. To begin with, although both DNA and RNA nucleotides have a pentose sugar, their sugar molecules differ slightly. For example, in DNA, the pentose sugar is called deoxyribose, whereas in RNA, the pentose sugar is called ribose. Figure 2 shows the structures of the two sugar molecules.

Can you spot where these two sugar molecules differ? At the carbon position, ribose has an oxygen atom, while deoxyribose does not. This is why the sugar molecule in DNA is called deoxyribose. It has one oxygen atom less than a ribose sugar molecule.

Another key difference between DNA and RNA lies in their nitrogenous bases. You may recall that, in DNA, there are four different nitrogenous bases: adenine (A), guanine (G), thymine (T), and cytosine (C). Of these, adenine and guanine are purines and have a double ring, whereas thymine and cytosine are pyrimidines and have a single ring.

You may also remember that the two strands of DNA are held together by hydrogen bonds between the nitrogenous bases of opposite strands, according to the rules of complementary base pairing: Adenine binds to thymine by two hydrogen bonds, and guanine binds to cytosine by three hydrogen bonds.

The nitrogenous bases of RNA are similar to those of DNA, with only one key difference: In RNA, thymine (T) is replaced with uracil (U). Uracil also belongs to the pyrimidine family, and it pairs with adenine in the same way that thymine does, by two hydrogen bonds. In Figure 3, you can see the structures of the different nitrogenous bases.

When we first studied the structure of DNA, one of the very first things that we learned is its characteristic double-stranded helical shape. Although the composition of RNA is very similar to that of DNA, it generally takes the form of a single strand. However, since it possesses nitrogenous bases, RNA can sometimes form double-stranded structures via complementary base pairing, as we will learn further on in this explainer. You can see a diagram of a single-stranded RNA molecule in Figure 4.

Now that we are clear on the structure of RNA and its differences from DNA, we can turn our attention to the different types of RNA and the diverse functions they perform.

In 1958, Francis Crick, whose work contributed to the discovery of the double helix structure of DNA, proposed the central dogma of molecular biology, which summarizes the process of converting the instructions encoded by DNA into functional proteins. Figure 5 shows a molecular summary of the central dogma. RNA, in its various forms, ties the whole process together, performing diverse roles at every stage of the central dogma. Let’s quickly understand what the central dogma means.

When organisms grow and reproduce, their cells divide. In order to ensure that every new cell receives a copy of the genetic information, the DNA is replicated. This represents the first process involved in the central dogma.

Transcription, the second process, describes the mechanism by which genetic information carried on the DNA is copied to a molecule of RNA. By generating a strand of RNA complementary to the DNA, the cell provides a means for the genetic information to be transported from the nucleus to the protein-synthesizing apparatus of the cell.

Translation, the final process of the central dogma, involves the interpretation of the information carried in the RNA. Organelles called ribosomes function as protein-synthesizing units, where individual amino acids are linked together in the order specified by the RNA to form functional protein molecules.

Now that we are familiar with the basic outline of the central dogma, let’s take a closer look at each stage and understand the different types of RNA and their functions as we go along.

During transcription, a section of DNA is read in the to direction by the enzyme RNA polymerase. This enzyme uses the sequence of DNA nucleotides as a template and synthesizes a complementary strand of RNA. Figure 6 shows a simple diagram of the process of transcription.

The enzyme RNA polymerase synthesizes RNA in the to direction, and therefore it starts synthesis from the end of DNA, as shown in Figure 6.

There are several different forms of RNA, all of which are synthesized in the manner we have just discussed. mRNA, or messenger RNA, is responsible for carrying genetic information from the nucleus into the cytoplasm for protein synthesis. rRNA, or ribosomal RNA, is an integral part of the structure of ribosomes, whereas tRNA, or transfer RNA, behaves as an adapter to deliver amino acids during the process of protein synthesis. We will learn more about each of these in this explainer.

Interestingly, in prokaryotes, a single type of RNA polymerase is responsible for synthesizing all of these different RNA molecules, whereas in eukaryotes, there are multiple different types of RNA polymerase, each responsible for the synthesis of a different type of RNA.

Let’s now take a look at the different types of RNA molecules, beginning with mRNA, or messenger RNA. When a stretch of DNA is transcribed into mRNA, this mRNA then carries this genetic information from the nucleus to the cytoplasm, delivering the DNA’s instructions to the protein-synthesizing apparatus of the cell.

Depending on the cell’s requirements for specific proteins, specific stretches of DNA, called genes, are selected by the RNA polymerase, transcribed into mRNA, and eventually converted into these specific proteins.

Let’s take a closer look at the composition of an mRNA molecule, as depicted in Figure 7.

From what we have discussed earlier regarding the central dogma, we know that the next stage of the central dogma is translation, involving ribosomes, which perform protein synthesis. At the very beginning of the mRNA sequence, therefore, there is a ribosome binding site, which you can see as a blue circle in Figure 7. During translation, this site guides the ribosome to bind to the start of the mRNA molecule. This site belongs to a section of the mRNA transcript called the untranslated region, since this section serves to regulate the process of translation and does not carry any information for the synthesis of proteins. The untranslated regions of the mRNA transcript are depicted in blue in Figure 7.

We have also learned that nucleic acids carry information that codes for the synthesis of proteins through the sequences of their nitrogenous bases. Sequences of three nucleotides form what is known as a codon. The order of the codons specifies the order of amino acids in the final protein. The coding sequence of mRNA represented in yellow in Figure 7 contains the sequence of codons. Each codon codes for a specific amino acid except the stop codons; for example, the codon CAG represents the amino acid glutamine.

There are also codons that indicate to the ribosome where to begin protein synthesis and where to end it. These are called start codons and stop codons, respectively, and are represented in red in Figure 7. The start codon is always AUG, but there are three possible stop codons: UAA, UAG, or UGA. Any of these three codons is capable of stopping the process of protein synthesis.

In Figure 7, at the other end of the mRNA molecule, you will notice a repetitive sequence of A bases. This is called the poly-A tail and is generally composed of around 200 adenine residues. This poly-A tail helps stabilize the mRNA molecule, helps with its export from the nucleus, and protects it from degradation by enzymes in the cytoplasm.

Now, if we want to represent the mRNA in Figure 7 in a sequence of nucleotides, it may look something like the sequence in Figure 8.

It is interesting to note that, in prokaryotes, the process of translation can begin even before transcription is fully completed. Ribosomes in prokaryotic cells can begin translation at the beginning of a strand of mRNA, even if the strand has not yet been fully synthesized. In eukaryotes, however, a molecule of mRNA must first be fully synthesized and then subjected to certain forms of processing before it can exit the nucleus and convey its information to the ribosomes for protein synthesis in the cytoplasm.

We have mentioned the term ribosomes quite often in this explainer, and we are familiar with ribosomes as the organelles that drive protein synthesis. But what exactly are ribosomes made of?

Ribosomes are small, nonmembranous organelles found in the cytoplasm of every living cell. Each ribosome is composed of a large subunit and a small subunit, as shown in Figure 9. These two subunits exist separately in the cytoplasm of the cell and come together when they need to translate a sequence of mRNA. Each single ribosome is made of about 70 different kinds of proteins, along with a second type of RNA called rRNA, or ribosomal RNA. Because rRNA is very important for producing all the proteins that the organism needs, it is actually the most abundant type of RNA in the cell.

In eukaryotic organisms, ribosomes are produced by a small structure within the nucleus called the nucleolus. Just like mRNA, rRNA is synthesized via transcription from DNA. Unlike the information carried by mRNA, however, the rRNA itself does not get translated into proteins. The rate of production of rRNA molecules in the nucleolus can be several hundreds of thousands per hour. This is because eukaryotic DNA usually carries around 600 copies of the genes needed to produce rRNA. Ribosomal proteins are synthesized in the cytoplasm and then move across the nuclear envelope and into the nucleolus. Here, they are assembled along with rRNA into ribosomes.

So far, we have learned about two of the major components that a cell needs to convert its genetic information into proteins: mRNA, which provides the instructions, and ribosomes made up of rRNA, which provide the apparatus. The one tool missing now is some form of adapter, which can read the codons from the mRNA molecule and deliver the corresponding amino acids to the ribosome, attaching them together to form a chain that will eventually become a functional protein. This is where tRNA, or transfer RNA, comes into play.

Just like with rRNA, tRNA molecules are synthesized via the transcription of a stretch of DNA, but these molecules are not translated into proteins themselves. tRNA molecules have a characteristic cloverleaf shape, as you can see in Figure 10. Let’s take a closer look at the structure of this molecule.

As you can see, some sections of the tRNA molecule fold back upon themselves and form base pairs. This forms loops or arms, giving the molecule its cloverleaf shape. There are two aspects of this structure that are important to the process of translation, the anticodon site and the amino acid binding site, as depicted in Figure 10.

As we mentioned earlier, the molecule of tRNA is responsible for interpreting or translating the codons on the string of mRNA. As the ribosome moves along the mRNA molecule during translation, each mRNA codon is matched with a tRNA molecule carrying a complementary string of three nucleotides, or an anticodon, at its anticodon site, as depicted in Figure 10.

At the open end of the tRNA molecule, directly opposite the anticodon site, is a three-nucleotide sequence, CCA, followed by the site of amino acid attachment. Each given tRNA molecule has a fixed anticodon, corresponding to one specific amino acid attached at the end of the tRNA molecule.

Now, we can begin to understand how translation works. When a molecule of tRNA matches its anticodon to the mRNA codon, it carries its amino acid to the ribosome and binds to its complementary codon on the mRNA. The amino acid then detaches from the tRNA and attaches instead to the growing peptide chain, which eventually forms the protein.

Each codon in the mRNA is read, and a tRNA molecule with the complementary anticodon delivers the corresponding amino acid and then exits the ribosome. As the ribosome moves along the strand of mRNA, from its end to its end, each new amino acid is joined to the growing chain through a peptide bond, forming a polypeptide chain. This is represented in Figure 11. This continues until the ribosome reaches the stop codon of the mRNA strand, which puts an end to the protein synthesis process.

In Figure 12, we visually recap the different types of RNA that we have described in this explainer, as well as the main processes of transcription and translation that they participate in.

Let’s now quickly summarize what we have learned in this explainer.