Lesson Explainer: Mutations Biology

In this explainer, we will learn how to describe mutations and explain the impacts mutations can have on organisms.

You may have heard of mutations—lots of Hollywood superheroes and villains are often created by “mutations”! In real life, however, a mutation is rarely going to give a person superpowers.

A mutation is a change in the DNA base sequence. Some mutations, as we will learn in this explainer, can be harmful. However, mutations are still incredibly important as they create genetic diversity. They drive evolution and allow species to adapt to changing environments and conditions.

Key Term: Genetic Mutation

A genetic mutation is a change in the base sequence of a DNA molecule.

DNA, or deoxyribonucleic acid, is a long molecule with two deoxyribose sugar–phosphate strands that twist around each other to form a double helix. Inside the helix, attached to the sugar–phosphate backbone, are four nucleotide bases, adenine (A), guanine (G), cytosine (C), and thymine (T). You can see the structure of a DNA molecule in Figure 1. Within DNA molecules, the order of the bases encodes genetic information to create characteristics and traits.

Figure 1: This figure shows the basic structure of DNA. The sugar–phosphate backbone is depicted by the yellow circles and gray pentagons and the nucleotide bases are depicted by the colorful blocks within the center.

DNA sequences that encode information for creating characteristics are called genes. A gene is a sequence of DNA that may code for a particular protein, which in turn will produce a particular characteristic or version of a characteristic. For example, our genes are responsible for determining our eye color, natural hair color, and blood type and even play a role in how tall we can grow!

Definition: Gene

A gene is a section of DNA that contains the information needed to produce a functional unit (e.g., a protein).

A specific gene contains instructions to make a specific protein. The sequence of DNA will be “transcribed” into a sequence of messenger RNA (or mRNA), which is another form of genetic material that cells can use to make proteins. This mRNA is then "translated" into an amino acid sequence. Lots of amino acids will join together to form a polypeptide, which will then form a protein (or part of one). A brief summary of this process is given in Figure 2 below.

Figure 2: A simple outline of the process of producing a protein. A sequence of DNA is transcribed into mRNA, and this mRNA is translated into a sequence of amino acids that will eventually form a protein.

Definition: Transcription

Transcription is the process of converting a DNA sequence into mRNA.

Definition: Translation

Translation is the process of converting an mRNA sequence into a polypeptide that can fold into a protein.

If the sequence of DNA bases is changed by a mutation, this could result in a different protein—or no protein at all—being produced. We may even be able to see this change in an organism’s physical characteristics, also called a phenotype.

Definition: Phenotype

The phenotype is the observable traits of an organism and is determined by its genotype.

If mutation occurs in a noncoding region of DNA, this means the DNA in this region does not contain the bases that will be converted into mRNA, then an amino acid, so mutation may not have an effect in this case. However, if this noncoding region of DNA regulates the transcription of other genes, mutation in these regions may disrupt the transcription of these genes.

Example 1: Describing the Potential Effects of a Mutation in the Regulatory Region of OCA2

A mutation occurs in a noncoding section of DNA. This section of DNA regulates transcription of the gene OCA2. Which of the following best describes what could happen?

  1. The OCA2 is not translated into DNA, so the protein it codes for cannot be made.
  2. The OCA2 gene is not transcribed into DNA, so the protein it codes for cannot be made.
  3. The OCA2 gene is not translated into mRNA, so the protein it codes for cannot be made.
  4. The OCA2 gene is not replicated, so the protein it codes for cannot be made.
  5. The OCA2 gene is not transcribed into mRNA, so the protein it codes for cannot be made.

Answer

To help us answer this question, let’s first make sure we understand the key terms.

A mutation is a change in the DNA base sequence of an organism. For example, it may be that at one position in the DNA sequence, there is an adenine (A) base present. Then, this molecule of DNA mutates and that adenine is replaced with a guanine (G).

Within sections of DNA called genes, there are instructions for making proteins. A specific sequence of DNA will make a specific protein with a specific function. To do this, the DNA sequence undergoes two main processes: transcription, which converts it into a molecule called mRNA, then translation, where the mRNA is translated into an amino acid sequence. Lots of these amino acid sequences will then come together to form a protein.

In our question, we are told that a mutation occurs in noncoding DNA. This means the DNA in this region does not contain the bases that will be converted into mRNA, then an amino acid. So, because it does not directly code for a protein, a mutation here will have no effect, right?

Not necessarily. We are also told that this section of noncoding DNA regulates the transcription of the OCA2 gene. Just because DNA is noncoding does not mean it is not important. Mutations in noncoding DNA can have just as significant impacts as mutations in DNA that codes for proteins.

So, if this section of DNA regulates the transcription of the OCA2 gene, we can assume that a mutation here will disrupt the ability of the cell to transcribe the DNA sequence of this gene into mRNA.

Looking at our answer choices, we can exclude all the choices that do not mention the impact on transcription (A, C, and D). Between our remaining answers, we are looking for the one that describes a potential impact on transcribing DNA into mRNA.

Therefore, we can see that our correct answer is E: the OCA2 gene is not transcribed into mRNA, so the protein it codes for cannot be made.

Mutations can be caused by the removal of bases, the addition of extra bases, or the substitution of one base for another. The different types of mutations that can occur are outlined in Table 1 below.

Table 1: A summary of insertion, deletion, and substitution mutations.
Mutation TypeDescriptionExample
InsertionAn additional base is inserted into a sequence.ATTAGGATTCAGG
DeletionA base is removed from a sequence.ATTAGGATAGG
SubstitutionOne base is swapped for another in a sequence.ATTAGGATTCGG

Mutations can occur anywhere in the genome. Sometimes, a base can change but have no impact on the protein that is eventually produced. These types of mutations are considered “neutral,” as there is no change in the organism’s phenotype. This means that they have no impact on an organism’s characteristics. This can happen because mutations may occur in regions of DNA that do not code for or regulate the expression of proteins or because they occur in a protein-coding region but do not actually change the final amino acid sequence that is produced.

A mutation can also change a gene sequence to beneficially change its corresponding protein. For example, some people have a mutation in the CCR-5 gene that changes the receptor that HIV uses to get into immune cells, giving them resistance to HIV infections. The organism will then benefit from the mutation. These types of mutations are considered “beneficial.”

Unfortunately, mutations can also be considered “harmful.” These mutations will likely occur in important sections of DNA that codes for a particular protein or noncoding DNA that is involved in regulating the synthesis of a protein. Harmful mutations usually have a significant effect on the organism.

Example 2: Determining If All Mutations Have Harmful Effects

A student said, “All mutations have harmful effects.”

Is this true or false?

Answer

Genetic mutations are changes in a DNA base sequence. Let’s consider an example.

A sequence of DNA reads AATTGCGCG.

A mutation occurs, and the new sequence reads AATAGCGCG.

Can you spot the difference? In the mutated sequence, the 4th base (a T in the original sequence) has been replaced by an A.

Mutations can be harmful. If they occur in a gene that codes for a very important protein in the body, they can have significant impacts on a biological function or a person’s physical characteristics. However, mutations are also incredibly important. Mutations are a driver of evolution, as they provide populations with variation (e.g., a single base mutation in a gene that produces the lactase enzyme allowed humans to digest milk). Humans were largely lactose intolerant until this mutation occurred and persisted within populations. By being able to digest milk, humans had access to the wide range of nutrients and minerals available in milk from other species, like cows.

Many mutations are considered “neutral.” This means that they have no impact on an organism’s characteristics. This can happen because mutations may occur in regions of DNA that do not code for or regulate the expression of proteins or because they occur in a protein-coding region but do not actually change the final amino acid sequence that is produced.

So, in response to the question, we now know that the statement is false: not all mutations have harmful effects.

Let’s have a look at a specific example of a disorder caused by a genetic mutation: sickle cell disease.

Hemoglobin is the protein in red blood cells that binds to and carries oxygen throughout the body. Sickle cell disease is a blood disorder caused by a single nucleotide change in the gene that codes for hemoglobin. This change in DNA is outlined in Figure 3.

Figure 3: A diagram to show the change in the DNA base sequence that causes hemoglobin to change shape.

This mutation causes a change in the shape of a red blood cell, which is known as a “sickle” shape. You can see a diagram of what these sickled cells may look like in Figure 4 below.

Figure 4: A diagram to demonstrate the shape of a normal red blood cell and a sickled red blood cell. The sickled red blood cell cannot carry as much oxygen.

If a person inherits this mutated gene from both parents, it is likely the majority of their red blood cells will be “sickled.” Because the sickled shape significantly reduces the oxygen-carrying capacity of the red blood cells, this causes many problems in a person with sickle cell disease. They may suffer from periods of intense pain in their body, they are more vulnerable to infections like colds and flus, and they are likely to suffer from headaches, dizziness, and fainting.

The mutation that causes sickle cell disease is random and genetic and is not influenced by external factors. These types of mutations are generally referred to as spontaneous, as they occur due to an error in a natural biological process.

However, some mutations can be caused by a person’s environment or exposure to certain substances. These mutations are referred to as induced, as they are initiated by something in the environment of an organism.

If these mutations occur in a cell that will later become a reproductive cell (known as a germline cell), or is a reproductive cell (known as a gamete), the mutation is likely to be passed on to any offspring that organism has. If they occur in any other body cells (somatic cells), the effects of the mutation will be limited to that organism and will not be passed on to their offspring.

Example 3: Differentiating between Induced and Spontaneous Mutations

Which of the following correctly differentiates between spontaneous and induced mutations?

  1. Induced mutations are caused by errors in natural biological processes, whereas spontaneous mutations are caused by mutagenic agents in the environment.
  2. Spontaneous mutations are caused by errors in natural biological processes, whereas induced mutations are caused by mutagenic agents in the environment.

Answer

Mutations are changes in DNA base sequences and can be classified into various categories. Two of these categories are induced mutations and spontaneous mutations. Let’s take a look at each type so we can distinguish between the two.

Spontaneous mutations are, as the name suggests, spontaneous. This means they occur naturally and not due to an external influence. A likely cause of a spontaneous mutation is an error occurring when a section of DNA is being replicated prior to a cell dividing. If this mistake is not identified and rectified, this can lead to the mutation being present in the cell’s DNA.

Induced mutations are mutations that are caused—or “induced”—by an external factor. Most commonly, they arise because an organism has been exposed to a mutagenic agent in their environment. For instance, a high amount of exposure to radiation can cause mutations to occur in cells. If you have had an X-ray, you may have noticed that the medical professionals will leave the room—this is because it may be safe for you to have a few X-rays in a year, but it’s not safe for them to be exposed to many X-rays every day for years and years!

We should now have enough information to correctly summarize spontaneous and induced mutations.

Looking back at our answer choices, we can see the correct answer is B: spontaneous mutations are caused by errors in natural biological processes, whereas induced mutations are caused by mutagenic agents in the environment.

Mutagenic substances, or environmental conditions, are those that can cause mutations. Examples include prolonged or intense exposure to UV radiation or the intake of harmful chemicals. Exposure to these can be incredibly dangerous; some mutations can damage the cell’s DNA in a way that disrupts the replication of the cell. Uncontrolled cell division can lead to tumors forming in the body, and these can form cancer. Mutagenic substances that cause cancer are known as carcinogens.

This is why it is important for people to limit their exposure to these mutagenic agents. For instance, when you go out in the sun, remember to wear sunscreen to protect yourself from the UV rays, and avoid substances that contain many carcinogenic compounds, like cigarettes.

Let’s summarize what we have learned about genetic mutations.

Key Points

  • A mutation can be defined as a change in the DNA sequence, and it can affect the proteins produced by particular genes.
  • Some mutations can be harmful, a lot of mutations have no effect (are neutral), and some mutations can be beneficial; these drive variation in organisms.
  • Mutations can be classified into various groups including insertion, deletion, substitution, and spontaneous or induced.
  • If a mutation occurs in the reproductive cells of an organism, it may be passed on to any offspring that organism has, whereas if a mutation occurs in the somatic cells, it will be limited to that organism.
  • The occurrence of sickle cell disease in humans is an example of a genetic disease caused by a mutation.
  • Causes of induced mutations include prolonged or intense exposure to UV radiation and carcinogenic substances.

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