Microbiology, part 37: Genetics - Mutagens & DNA Repair

Hi, I'm Cathy with Level Up RN. In this video, I will be discussing mutagens as well as DNA repair. At the end of the video, I'm going to give you guys a little quiz to test your understanding of some of the key facts I'll be covering. So definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards so you can follow along with me. And of note, our flashcards are also available in digital format in Flashables.

Mutagens are chemical or physical agents capable of causing a genetic mutation or a change in an organism's DNA. Let's first talk about chemical mutagens, which include nucleoside analogs, nucleotide modifying agents, and intercalating agents. A nucleoside analog is a chemical that is structurally similar to a normal nucleotide base that is incorporated into the DNA during replication. An example of a nucleoside analog is the antiviral agent acyclovir, which is used to treat herpes simplex virus as well as varicella zoster virus. So acyclovir is structurally similar to a nucleoside found in the virus's DNA. So it gets incorporated into the viral DNA where it throws a wrench in the viral replication process, causing issues with base pairing and point mutations.

Nucleotide modifying agents are chemicals that modify normal DNA bases. An example of a nucleotide modifying agent is nitrous acid, which causes deamination or removal of the amino group of the base cytosine. And as you can see in the image, this causes cytosine to be converted into uracil, which leads to changes in base pairing during DNA replication and causes point mutations.

And finally, an intercalating agent is a chemical that inserts itself between base pairs, causing abnormal spacing between those base pairs. This, in turn, leads to frameshift mutations. An example of an intercalating agent is ethidium bromide, which is a fluorescent dye used in laboratories.

Now let's talk about how radiation can cause mutations in the DNA. Ionizing radiation, such as X-rays and gamma rays, leads to the formation of hydroxyl radicals, which causes breaks in the DNA strand. Ionizing radiation also damages nucleotides, leading to changes in base pairing and point mutations. Non-ionizing radiation, such as UV light, leads to the creation of thymine dimers. This is where two adjacent thymine bases become covalently bonded such that they cannot bond with their complementary bases on the opposite DNA strand. This leads to DNA replication errors and frameshift or point mutations.

Let's now talk about DNA repair. So when an error is introduced into an organism's DNA during DNA replication or as the result of a mutation, there are some means of correcting those errors. The first is proofreading that occurs during DNA replication. So during the replication process, DNA polymerase will proofread each new nucleotide as it is being added onto the new strand that is being synthesized. If an error occurs, then DNA polymerase will excise or cut out the incorrect base and replace it with the correct one before resuming the replication process. If proofreading fails to find an error during replication, then mismatch repair is a system that can be used after replication to fix the error. During mismatch repair, enzymes recognize the mismatched base, excise it, and replace it with the correct one.

Lastly, we're going to talk about the repair of thymine dimers, which we talked about earlier in this video. So thymine dimers are relatively common because organisms are often exposed to UV light. The two key mechanisms used to repair thymine dimers include nucleotide excision repair and light repair. With nucleotide excision repair, which is also referred to as dark repair, enzymes cut the DNA upstream and downstream from the thymine dimer and then remove the damaged section. DNA polymerase then replaces the missing nucleotides and ligase seals the strand where the cuts were made. This process is referred to as dark repair because it does not require light energy to perform the repair.

On the other hand, photoreactivation does require light energy to repair the DNA. During this process, the enzyme photoliase breaks the bond between the two thymine bases, allowing thymines to complementary base pair with adenines on the opposite strand. And again, this process occurs in the presence of visible light.

All right. It's quiz time, and I have three questions for you. Question number one, what do you call chemical mutagens that cause atypical spacing between base pairs leading to frameshift mutations? The answer is intercalating agents. Number two, which type of radiation creates thymine dimers? Ionizing or non-ionizing radiation? The answer is non-ionizing radiation. And number three, blank is the process in which light energy is used to repair thymine dimers. The answer is photoreactivation. All right, that's it for this video. I hope it was helpful. Take care and good luck with study.

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An example of a-- where it throws a wrench in the viral replicate-- in the viral replicate-- during the replicate.