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.
Definition: DNA (Deoxyribonucleic Acid)
DNA is the molecule that carries the genetic instructions for life. It is composed of two nucleotide strands that coil around each other to form a double helix.
Definition: RNA (Ribonucleic Acid)
RNA is a single-stranded polynucleotide that helps in the transfer of genetic information and in the interpretation of this information to synthesize proteins. In some viruses, RNA carries the genetic material instead of DNA.
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.
Key Term: Nucleotide
A nucleotide is a monomer of a nucleic acid polymer. Each nucleotide consists of a pentose sugar, a phosphate group, and a nitrogen-containing base.
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.
Example 1: Describing the Structure of RNA
Which of the following correctly describes the structure of an RNA molecule?
- An RNA molecule is formed of a single strand of nucleotides joined by a sugar–phosphate backbone.
- An RNA molecule is formed of two strands of amino acids joined by hydrogen bonds that form between the amino groups.
- An RNA molecule is formed of a single strand of amino acids joined by peptide bonds.
- An RNA molecule is formed of two strands of nucleotides joined by hydrogen bonds that form between complementary base pairs.
- An RNA molecule is formed when two DNA molecules interact and join together to form a more complex structure.
In all living cells, there are two types of nucleic acids involved in the storage, transfer, and interpretation of genetic information. DNA, or deoxyribonucleic acid, is the molecule that stores genetic information in the form of a genetic code. In its various forms, RNA helps convey this genetic information to the protein-synthesizing apparatus of the cell and also helps convey the genetic code into proteins, which proceed to carry out different functions in the cell.
Since RNA is a nucleic acid, it is made of individual units called nucleotides, which are linked together to form a long polynucleotide chain. Of the options given in the question, we can straightaway eliminate the ones that describe RNA as strands of amino acids, since we know that nucleotides, not amino acids, make up RNA.
Each nucleotide in RNA is made up of a pentose sugar, a phosphate group, and a nitrogenous base. 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. The spine of the RNA molecule is, therefore, called the sugar–phosphate backbone, and this is what holds the nucleotides together in the polynucleotide chain.
Although DNA occurs naturally in the double-stranded helix shape that we are familiar with, RNA generally exists as a single strand of nucleotides. With this information, we can look through the options in the question and select the description that best fits RNA.
The correct option is option A. An RNA molecule is formed of a single strand of nucleotides joined by a sugar–phosphate backbone.
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.
Key Term: Central Dogma
The central dogma of molecular biology describes the process of converting the instructions encoded by DNA into RNA and then into functional proteins.
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 replicates itself. This represents the first process involved in the central dogma.
Key Term: DNA Replication
DNA replication is the process by which two identical DNA molecules are produced from a single original DNA molecule.
Transcription, the second process, describes the mechanism by which DNA is converted into 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.
Key Term: Transcription
Transcription is the process of converting a DNA sequence into RNA.
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.
Key Term: Translation
Translation is the process of converting an mRNA sequence into a polypeptide that can fold into a protein.
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.
Definition: mRNA (Messenger RNA)
mRNA is a message that is transcribed from the DNA of a gene and can be translated to produce the corresponding protein.
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 amino acid is coded for by a specific codon; for example, the codon CAG represents the amino acid glutamine.
A codon is a sequence of three nucleotides of DNA or RNA that corresponds to a specific amino acid.
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.
Example 2: Identifying the Type of RNA Involved in Transcription
RNA can be categorized into different types based on its role in the body. Which type of RNA is formed after a section of coding DNA has undergone transcription?
In all living cells, DNA is the genetic material that resides in the nucleus or nuclear region of the cell and carries the instructions for the synthesis of the proteins that the cell and, in turn, the organism need to survive and function. During transcription, a section of DNA is converted into RNA so as to convey the genetic information from the DNA of the cell to the protein-synthesizing apparatus, or the ribosome.
Let’s look at the options that we have been given in the question. Each option outlines a type of RNA, identified by a lowercase letter. Let’s examine the full names of all of these types of RNA molecules. This should give us clues about the functions these molecules perform. We can then match the names to the role performed by RNA during transcription, as we have learned.
rRNA stands for ribosomal RNA. This type of RNA forms one of the components that make up ribosomes, which are the protein-synthesizing units of the cell. This, however, does not fit the type of RNA we are looking for, so we can rule it out.
tRNA, or transfer RNA, is a molecule that behaves as an adapter during the process of translation. It carries amino acids to the ribosomes for protein synthesis. Since this type of RNA plays no role in transcription, this is not the correct answer.
siRNA stands for small interfering RNA. This is a highly specialized form of RNA that can regulate the synthesis of proteins in the cell by binding to and silencing stretches of RNA. This, however, does not match the description of the RNA involved in transcription.
mRNA, or messenger RNA, behaves as a means of conveying information from the DNA in the nucleus to the ribosomes in the cytoplasm, where proteins are synthesized. Stretches of DNA are converted into mRNA via transcription. This perfectly matches the description that we have learned.
Let’s take a look at the final option, ncRNA. This stands for noncoding RNA, which describes types of RNA that are not converted into proteins. The purpose of transcription is to generate a stretch of RNA that can be converted into proteins, which means that this option is also incorrect.
The correct answer is, therefore, mRNA.
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 polypeptides, 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.
Key Term: rRNA (Ribosomal RNA)
rRNA is a type of noncoding RNA that forms one of the major components of ribosomes.
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 polypeptides 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.
Example 3: Understanding the Function of Ribosomes
Ribosomes are formed from ribosomal RNA and polypeptides. What role do ribosomes have in a cell?
- Controlling what enters and leaves the cell
- Providing the site for aerobic respiration
- Transporting enzymes around the cell
- Intracellular signaling
- Providing the site for protein synthesis
Every living cell carries its genetic information in the form of DNA. The information contained in the DNA encodes the instructions for the synthesis of proteins. This involves several levels of communication within the cell.
When a section of DNA needs to be converted into proteins, the sequence of DNA in question must first be converted into a molecule of mRNA via transcription. This enables the information to be conveyed from the DNA to the protein-synthesizing apparatus of the cell.
In this question, we are being asked to identify the role played by ribosomes in a cell. Ribosomes, as mentioned in the question, are formed from ribosomal RNA and polypeptides. A ribosome attaches to a molecule of mRNA after transcription in order for this molecule to be converted into a protein via the process of translation. Here, carried by tRNA adapter molecules, amino acids are delivered to the ribosome in the order encoded by the mRNA, forming a polypeptide chain, which becomes a functional protein.
The correct option is, therefore, that ribosomes provide the site for protein synthesis.
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.
Definition: tRNA (Transfer RNA)
tRNA is an adapter molecule that carries amino acids to the ribosome for translation.
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.
An anticodon is the complementary sequence to a codon in mRNA that is found in a tRNA molecule.
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.
- In every living cell, DNA carries the genetic material to be converted into proteins. RNA, in its various forms, supports and enables this process.
- In RNA, the pentose sugar is ribose, rather than deoxyribose as in DNA.
- In RNA, the nitrogenous base thymine is replaced with uracil.
- RNA generally exists as a single strand, rather than a double strand.
- There are three main types of RNA: mRNA, rRNA, and tRNA.
- During transcription, a stretch of DNA is converted into mRNA in order to convey the genetic information to the ribosomes.
- Ribosomes are protein-synthesizing organelles composed of rRNA and polypeptides.
- In the ribosomes, the molecules of tRNA behave as adapters, delivering amino acids in the order specified by the mRNA. This process is called translation.