Learn what a mutagen is. Understand types of mutagens, explore how mutagens cause mutations, and see examples of how mutations affect the real world.
Updated: 11/21/2023
Mutagen Definition, Types & Examples
Table of Contents
- What is a Mutagen?
- Types and Examples of Mutagens
- How a Mutagen Works
- Mutations in the Real World
- Lesson Summary
Frequently Asked Questions
What is a mutagen in biology?
A mutagen refers to any agent capable of producing damage to DNA. This results in the formation of mutations, the primary source of genetic diversity in populations.
What are the three types of mutagens?
There are three main types of mutagens: physical, chemical, and biological. Each type results in damage to DNA during the processes of replication and repair.
Table of Contents
- What is a Mutagen?
- Types and Examples of Mutagens
- How a Mutagen Works
- Mutations in the Real World
- Lesson Summary
A mutagen refers to any agent found in an organism's environment capable of producing genetic mutations in DNA. Under normal circumstances, DNA replication occurs without the introduction of mutations or changes in the genetic code of an organism. This is due to the presence of DNA repair mechanisms such as base excision repair (BER), nucleotide excision repair (NER), and mismatch repair (MMR). In base excision repair, a damaged base is removed from the DNA during the G1 stage of the normal cell cycle. Nucleotide excision repair occurs during the G1 and G2 stages of the cell cycle and deletes lesions that form on nucleotide sequences in DNA. Even after the cell cycle is complete, mismatch repair can occur after replication to ensure that base pair matching is maintained within the DNA.
However, in some cases, mutations persist in the DNA due to the presence of mutagenic agents. What does mutagenic mean? The term mutagenic is defined as the ability to induce genetic changes in the DNA of an organism. There are many sources of mutagenic agents, including physical, chemical, and biological mechanisms. Some examples of physical mutagens include UV or gamma radiation while alkylating agents and azides represent potential chemical mutagens. Biological mutagens often consist of viruses and different bacterial species capable of initiating changes in an individual's DNA.
Examples of these different types of mutagens and their potential effects on the human body are explored as part of this lesson.
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Mutagens are often classified based on their origin and their effects upon DNA. There are three basic types of mutagens:
- Physical mutagens
- Chemical mutagens
- Biological mutagens
Physical mutagens include ultraviolet and gamma radiation. Both sources induce the creation of free radicals or molecules having unpaired electrons. These free radicals can then cause unintentional bonds to form between bases, such as covalent bonds between thymine molecules. The creation of the thymine dimer prevents the double helix from forming wherever this dimer exists in the nucleotide sequence. The condition xeroderma pigmentosum, in which individuals have an increased risk of skin cancer due to extreme light sensitivity, is caused by the lack of an enzyme needed to remove thymine dimers from DNA sequences.
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Chemical mutagens are commonly found in compounds that produce toxicity and concomitant damage to DNA. Alkylators, one type of chemical mutagen, directly impact bases by creating G (guanine) residues resulting in depurination or loss of a purine. This creates a mismatch between base pairs and induces DNA repair mechanisms to allow these mutations to persist in the DNA sequence. Other chemical mutagens like nitrous acid cause base switching in which cytosine is converted to uracil, adenine is changed to hypoxanthine, and guanine is transformed to xanthine. These altered bases still undergo base pairing, even though this often results in miscoded regions of DNA.
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Biological mutagens consist of bacteria and viruses capable of producing mutations in DNA. Human papillomavirus (HPV), for example, results in long-term infection in individuals unable to clear the virus from their bodies. Infected tissues are located in the cervix, throat, and other regions of the reproductive tract in men, while women undergo mutagenesis, in which precancerous cells develop in response to the virus.
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Some specific mutagen examples include:
| Agent | Mutagen Type | Effect |
| UV radiation | Physical | Thymine dimers |
| Gamma radiation | Physical | Thymine dimers |
| Ethyl methane sulfonate | Chemical | G residues, depurination and base mispairing |
| Methyl methane sulfonate | Chemical | G residues, depurination and base mispairing |
| Diethylsulfate | Chemical | G residues, depurination and base mispairing |
| Nitrosoguanidine | Chemical | G residues, depurination and base mispairing |
| Human papilloma virus | Biological | Cancers of the reproductive and digestive tract |
| Epstein Barr virus | Biological | Lymphoma |
| Hepatitis B, C viruses | Biological | Liver cancer |
| Helicobacter pylori | Biological | Stomach cancer |
| Chlamydia trachomatis | Biological | Cervical cancer |
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Mutagens create damage to DNA by disrupting the normal replication and genetic repair mechanisms. In healthy individuals under normal conditions, DNA is built by adding nucleotides to the 3' ends of DNA strands. There are at least fifteen different forms of DNA polymerase that facilitate this process by performing replication, repair, and proofreading of DNA. When exposed to a mutagen, errors in DNA replication occur. This results from the replacement of normal human DNA polymerase with Y-family polymerases that bypass DNA repair mechanisms. The main mutagenic effects produced by this switch include an increase in error-prone replication, creation of frameshift mutations, and prevention of genetic replication.
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Some specific actions of mutagens include:
- Alkylation: the addition of alkyl groups to bases, leading to increases in DNA mutations
- Oxidation: the production of free radicals causing lesion formation in DNA
- Amination: the addition of an amine group leading to the development of precancerous and cancerous cells
- Hydrolysis: breaking the glycosyl bonds between adjacent bases in DNA, leading to the loss of genetic information
- Photo addition: the addition of C5-C6 double bonds in pyrimidine bases, resulting in mutations that evade the normal DNA repair mechanisms
- Coordination: exposure to heavy metal ions resulting in the crosslinking of DNA, in which nucleotides on the same or opposite strands of DNA form covalent links that block replication, repair, and transcription
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While mutations are often viewed as having a deleterious impact on organisms, in reality, mutations are capable of putting a combination of advantageous and disadvantageous traits in individuals. Beneficial mutations, for example, occur when the introduction of a new trait in an individual leads to increased fitness or the ability to pass these traits onto one's offspring. In humans, for instance, there are genetic mutations that reduce the risk of type 2 diabetes despite the presence of high-risk factors such as obesity and high blood pressure. Other beneficial mutations found in humans are associated with a reduced risk for neurodegenerative disorders such as Parkinson's disease and resistance to infection.
However, there are also instances in which mutations have negative consequences on human health. As mentioned above, exposure to the human papillomavirus (HPV) results in an increased risk for cancers of the reproductive tract in both men and women. When exposed to HPV, some individuals are unable to get rid of the virus from their bodies and instead experience long-term infection. In individuals with harmful mutations to the DNA repair pathways, cells infected with HPV enable the virus to replicate by using these mutations to facilitate viral replication. In turn, this results in the transformation of healthy cells to precancerous cells (carcinogenesis) or the production of cancer cells.
Overall, mutations represent the primary source of introducing new traits into a population. When these mutations enhance the fitness of a population, or the ability to pass these beneficial traits to offspring, natural selection can act upon these traits and facilitate the process of evolution, in which new species are formed. Indeed, mutations represent the primary source of genetic variability and diversity in populations of organisms.
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Mutagens represent any agent capable of producing damage to DNA. Three main types of mutagens include:
- Physical mutagens: different forms of radiation that result in the production of free radical damage to DNA
- Chemical mutagens: compounds that create direct damage to bases, resulting in base pair mismatching and miscoded regions of DNA
- Biological mutagens: viruses and bacteria that produce DNA damage resulting in cancer formation
Mutagens affect DNA by disrupting the normal repair mechanisms that typically detect the presence of errors in DNA production and replication. Moreover, mutagens cause regular human DNA polymerases to be replaced with Y-family polymerases that bypass the normal mechanisms of DNA proofreading and repair. While many mutations are considered harmful to organisms, mutations also represent the primary source of genetic diversity in populations and thus contribute to the process of evolution.
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Video Transcript
Introduction to Mutagens
Have you ever wondered how mutations arise in living things? You've probably heard stories about mutated beings emerging from toxic waste sites or secret laboratories. While these urban legends usually turn mutations into fantasy, it is true that mutations often come from external factors.
The environment we live in has a real impact on whether we experience genetic mutations. The quality of water we drink and the air we breathe can actually affect the integrity of our DNA. Our bodies are designed to correct any mistakes, but dangers from the environment can increase our chances of ending up with a mutation. An environmental agent that causes a mutation is called a mutagen.
Mutation Formation
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Remember that all mutations are changes in the nucleotide sequence of DNA. While chromosome mutations involve large portions of DNA, point mutations are smaller and typically affect one or two bases. Point mutations include base substitutions and insertions and deletions. While base substitutions only switch one base for another, insertions and deletions change the length of a DNA code. But, it's rarely the original DNA strand that undergoes a mutation. Most of the time, mutations occur while a new DNA molecule is being formed through DNA replication.
Recall the work of the enzyme DNA polymerase. During replication, this enzyme's job is to build the daughter DNA strand along the parent strand. Every once in a while, DNA polymerase makes a mistake in matching the right nucleotide to the parent strand. It puts the wrong nucleotide down and produces a daughter strand that is slightly incorrect. Normally, this only happens in about one in a billion nucleotides. But, there are certain things that can make it more likely that DNA polymerase will make a mistake. One of these is radiation from ultraviolet, or UV, rays. When UV radiation hits the cells in your body, it can change the way DNA polymerase works and increase the probability of a replication error. Ultraviolet radiation is an example of a mutagen.
Mutagens
Things that cause mutations are not always found outdoors. Mutagens are any chemical or physical agents that cause a mutation in an organism's DNA. We said before that mutagens come from the environment. But remember, the 'environment' for an organism is nothing more than the chemical and physical factors surrounding it. A bacterium's environment might only be the inside of your intestine. A lab mouse's environment might only be its cage, its food, and its water. Mutagens can be found in any part of the environment. The word 'mutagen' comes from the word part 'gen,' which is found in lots of scientific terms. 'Gen' means the 'origin' or 'creation' of something. Just like an allergen is anything that causes an allergy, a mutagen is anything that causes a mutation.
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UV radiation is just one of many mutagens that exist in our environment. Other mutagens include X-rays, extreme heat, or chemicals that react with DNA molecules. These agents can change the way molecules bond and react with one another, which increases the likelihood that a mistake will be made in the nucleotide sequence. An error in the DNA can cause problems for that cell. Skin cells are especially vulnerable to UV radiation because, obviously, they have the most direct exposure to the UV light that comes from the sun. This is why we try to protect our skin from the sun's potentially harmful rays. Though a point mutation may seem insignificant at the molecular level, we know that a single insertion, deletion, or base substitution can drastically harm an organism.
Mutations and DNA Polymerase
Fortunately for us, DNA polymerase always double-checks its work to make sure all the nucleotides have been replicated correctly. When it finds an error in the DNA sequence, it quickly stops and fixes it before replication. That way, the mistake won't get bigger with every new generation of DNA. But nobody's perfect, and some mistakes do get through. It's all about probability. The best thing we can do to avoid mutations is to decrease our exposure to mutagens that are found in our environment.
Mutations in the Real World
As we've seen in previous lessons, mutations can cause a variety of disorders. Human diseases caused by point mutations include cystic fibrosis, color blindness, Tay-Sachs disease, sickle-cell anemia, and cancer. We usually think of point mutations as being harmful to living things. They often make organisms more vulnerable, less healthy, or otherwise less likely to survive and reproduce in their respective environments.
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On the other hand, there are plenty of cases of advantageous mutations. Don't get the false impression that all mutations are 'bad.' Sometimes accidents are a good thing. In fact, mutations can take credit for many of the adaptations we see in organisms today. In any population, there are individuals that turn out significantly different from the rest. Mutations arise randomly in all species of organisms. If a mutation doesn't cause significant harm, then it may be passed down through successive generations. Over time, the mutation may become more and more common, until it becomes the 'new normal.' In fact, this is one way that species branch out and evolve into new varieties of organisms. In the long run, all of us are the result of some kind of mutation. So, we can thank mutations for the diversity we see in plants and animals today.
Keep in mind, however, that simple mutations can't give an organism some amazing new structure or power. A person couldn't instantly sprout wings or fins because he got a mutation. All those legends about mutant monsters and lab-created superheroes are just fantasy. If you really want to gain super-human powers, you're better off just hitting the gym.
Lesson Summary
Point mutations are small changes in the DNA sequence that occur through mistakes during DNA replication. DNA polymerase normally matches the daughter nucleotides to the parent strand and corrects any mistakes through a proofreading step. Mutations are more likely to form when an organism is exposed to a mutagen. Mutagens are chemical or physical agents that cause mutations. While mutations often cause harmful disorders, they also increase the genetic diversity of populations. Advantageous mutations are passed on through generations and are ultimately responsible for the variety of living things.
Learning Outcome
You will be able to define mutagens and explain how they occur by the conclusion of this lesson.
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