Methods of Inducing Mutations: Videos & Practice Problems

Induced mutations are deliberate changes in DNA caused by external agents known as mutagens, which can be chemical or physical. Chemical mutagens include alkylating agents that modify nitrogenous bases, base analogs that resemble normal bases, and intercalating agents that insert between base pairs, increasing mutation rates. Radiation mutagens, such as UV light and X-rays, cause DNA distortions like thymine dimers or breaks, leading to mutations. Understanding these mechanisms is crucial for studying genetic variations and their implications in fields like cancer research and biotechnology.

Methods of Inducing Mutations

This video, we're going to begin our lesson on methods of inducing mutations. Recall from our previous lesson videos that induced mutations are controlled, deliberate mutations that occur due to an external source. Now, in order to increase the mutation rate in bacteria that are being studied, scientists will typically use mutagens to increase the mutation rate. Mutagens are defined as any chemical or physical agent that can cause mutations. Carcinogens are specific types of mutagens that contribute to the development of cancer, which is characterized by uncontrollable cell growth.

If we take a look at our image below, notice that the top half of the image focuses on the term "mutagen", whereas the bottom half focuses on the term "carcinogen". The top half of the image with the mutagen shows that a mutagen is any chemical or physical agent used to increase mutation rates or to cause mutations. Notice that a mutagen chemical is being added here to this bacteria, leading to mutations. However, mutagens do not necessarily lead to cancer; this mutagen does not lead to cancer but does cause a mutation, a change in the DNA sequence without necessarily leading to cancer.

The carcinogen, a specific type of mutagen, leads to the development of cancer. The carcinogen here is added to the bacterial cell and it leads to a mutation, but this mutation leads to the development of cancer. Notice the uncontrolled division of these mutated cells over here, due to the carcinogen. Thus, mutagens lead to mutations, but not necessarily to cancer, and carcinogens lead to mutations that result in the development of cancer.

Over here on the right, what we're showing you is the OSHA symbol, where OSHA stands for Occupational Safety Health Administration. This is the OSHA hazard symbol for carcinogens and mutagens. If you see this symbol on any kind of chemical or physical agent, you should be careful because that substance is going to be either a carcinogen or a mutagen.

This concludes our brief introduction to methods of inducing mutations, and we'll be able to talk more about other methods of inducing mutations as we move forward in our course. I'll see you all in our next video.

Chemical Mutagens

This video, we're going to talk a little bit more about chemical mutagens or chemicals that can cause mutations in the DNA. Now, some chemical mutagens can cause chemical modification, and this is referring to chemicals that have the ability to modify or alter the structure of the nitrogenous bases in the DNA. An example of a chemical mutagen that can chemically modify the nitrogenous bases are alkylating agents. These chemicals that can chemically modify can also alter the base pairing abilities of the nucleotides that were modified. If we take a look at our image down below at this chemical modification box here in purple, notice on the left-hand side over here, we're showing you a guanine nitrogenous base. This guanine nitrogenous base normally forms 3 hydrogen bonds, represented by these three dotted lines that you see here. Usually, the guanines are going to base pair with cytosines or 'c's, and so you could put a 'c' here, for the guanine base pairs with cytosine. However, if a chemical mutagen that's capable of chemical modification is added, such as an alkylating agent, then it can chemically modify an existing nitrogenous base and change its properties. Notice in this case that a methyl group is being added here at this position where the carbonyl group used to be. Now there is one less hydrogen bond that can form here. And so, there are only 2 hydrogen bonds that form. Because this is a chemically modified base where it's modified with a methyl group, we actually call this methylguanine. Methylguanine, which is this base that you see here, has different base pairing abilities. Instead of base pairing with cytosines, it can base pair with thymine, and that is not supposed to happen. Guanines are supposed to base pair with 'c's, cytosines, not with 't's, thymines. This chemical modification can lead to mutations in the DNA and cause issues.

Next, what we're going to talk about are base analogs, and base analogs are actual chemicals that are just structurally similar to nitrogenous bases. However, these base analogs, although they're very similar to the nitrogenous bases, they are not exactly identical, and so they do have different base pairing properties. The base analogs are not necessarily going to be chemically modified like what we've seen up above. Instead, these base analogs can sometimes exist in nature. And so, these base analogs can sometimes mistakenly be incorporated instead of incorporating the normal base. If we take a look at our image down below, notice on the left-hand side over here, we're showing you a normal base. And on the right-hand side over here, we're showing you a base analog. This one could exist naturally in nature, like I mentioned. Notice that it is different, only different from the normal base at this position here. The normal base has a methyl group whereas the base analog here has a bromine group. And so occasionally, sometimes base analogs are mistakenly incorporated instead of incorporating the normal base. When base analogs are mistakenly incorporated, they can have different hydrogen bonding abilities and different base pairing abilities. This can once again lead to mutations in the DNA.

Finally, the last chemical mutagen that we're going to talk about are intercalating agents. These are really flat molecules that have the ability to insert between 2 base pairs and therefore increase the chance of a point mutation occurring. If we take a look at our image down below at the intercalating agents over here, notice on the left, we're showing you a DNA molecule. Here we're showing you the addition of an intercalating agent, proflavine. This intercalating agent is a flat molecule that has the ability to insert itself in between 2 base pairs. The insertion of this intercalating agent can change the structure of the DNA and increase the chance of a point mutation occurring. This is another example of a chemical mutagen. This here concludes our brief lesson on chemical and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

5-Bromouracil is structurally similar to a nucleobase and can be inserted into the DNA and perform base- pairing. 5-Bromouracil is a base ________ for the base thymine.

• A. Analog.
• B. Agent.
• C. Substitute.
• D. Modification.
• E. Replacement.

Answer: (A)

Which of the following statements about mutagens and carcinogens is true?

• A. A mutagen is a chemical or physical agent which causes mutations in DNA.
• B. A carcinogen is chemical or physical agent which causes mutations in DNA that lead to the development of cancer.
• C. All carcinogens are mutagens, but not all mutagens are carcinogens.
• D. All of the above statements are true.

Answer: (D)

Radiation Mutagens

This video, we're going to begin our lesson on radiation mutagens. The exposure to certain types of radiation can cause mutations to occur in the DNA. There are two types of radiation that are commonly used as mutagens. The first is UV light or ultraviolet light, and the second is x-rays. UV light or ultraviolet light causes the formation of nucleobase dimers that distort the DNA structure. More specifically, it causes the formation of what are known as Thymine dimers. Thymine dimers occur when covalent bonds form between adjacent thymine bases in the DNA. These thymine dimers cause issues because DNA replication and transcription cannot continue past the distortion caused by these thymine dimers. The cell typically ends up dying when these Thymine Dimers are present. If we take a look at our image below, notice that we're showing you UV light used as a method of being a mutagen. You can see that the ultraviolet light, which could be present in the sun, can cause these thymine dimers to form. The thymine dimers, you can see that two adjacent thymines are covalently linked and bound together, and it distorts the structure of the DNA. This can cause a mutation in the DNA, and these thymine dimers can cause cell death because they disrupt processes such as DNA replication and transcription.

X-rays cause single and double-stranded breaks in the DNA. Rather than causing thiamine dimers, with these x-rays you can see that the x-rays will cause the DNA strands to break. Sometimes it's a single-stranded break, other times it's a double-stranded break. The single-stranded DNA breaks shown here, when repaired, oftentimes introduce mutations. This is why x-rays can be used as mutagens in specific scenarios.

This concludes our brief lesson on radiation mutagens, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

Which of the following wavelengths of light cause Thymine dimer mutations?

• A. X-Rays.
• B. Microwaves.
• C. UV light.
• D. Visible light.

Answer: (C)

Here’s what students ask on this topic:

What are the different types of mutagens used to induce mutations?

Mutagens are agents that cause changes in DNA, leading to mutations. They can be classified into two main types: chemical and physical mutagens. Chemical mutagens include alkylating agents, which modify nitrogenous bases; base analogs, which resemble normal bases but have different pairing properties; and intercalating agents, which insert between base pairs, increasing mutation rates. Physical mutagens include radiation types like UV light and X-rays. UV light causes thymine dimers, distorting DNA and hindering replication and transcription. X-rays cause single and double-stranded breaks in DNA, leading to mutations during repair. Understanding these mutagens is crucial for genetic research and biotechnology.

How do alkylating agents induce mutations in DNA?

Alkylating agents are chemical mutagens that induce mutations by adding alkyl groups to nitrogenous bases in DNA. This chemical modification alters the base pairing properties of the affected bases. For example, guanine normally pairs with cytosine through three hydrogen bonds. When an alkylating agent adds a methyl group to guanine, forming methylguanine, it can only form two hydrogen bonds and may pair with thymine instead. This incorrect pairing leads to mutations during DNA replication. Alkylating agents are significant in studying genetic variations and understanding mutagenesis mechanisms.

What are thymine dimers and how do they affect DNA?

Thymine dimers are a type of DNA damage caused by UV light, a physical mutagen. They occur when two adjacent thymine bases in the DNA strand form covalent bonds, creating a dimer. This dimer distorts the DNA structure, preventing normal DNA replication and transcription. As a result, the cell cannot properly replicate its DNA or produce necessary proteins, often leading to cell death. Thymine dimers are significant in understanding the effects of UV radiation on genetic material and the mechanisms of DNA repair.

How do base analogs cause mutations in DNA?

Base analogs are chemicals structurally similar to normal nitrogenous bases but with different base pairing properties. They can be mistakenly incorporated into DNA during replication instead of the normal bases. For example, a base analog might resemble thymine but pair with guanine instead of adenine. This incorrect pairing leads to mutations during subsequent DNA replications. Base analogs are useful in studying mutagenesis and understanding how structural changes in bases can affect genetic stability.

What is the difference between mutagens and carcinogens?

Mutagens are agents that cause changes in DNA, leading to mutations. They can be chemical or physical and do not necessarily cause cancer. Carcinogens are a specific type of mutagen that leads to the development of cancer, characterized by uncontrolled cell growth. While all carcinogens are mutagens, not all mutagens are carcinogens. Understanding this distinction is crucial in fields like cancer research and occupational safety, where identifying and managing exposure to these agents is essential.