Mutagens: How the Environment Affects Mutation Rates

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

Types of point mutations

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.

X-rays, extreme heat and chemicals are examples of mutagens

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.

Example of a mutation that contributes to genetic diversity

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.