Using your knowledge of DNA repair pathways, choose the pathwa...

Question

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.

A cytosine that has been deaminated to uracil

Accepted Answer

Welcome back, everyone. Let's look at our next question. Which type of DNA damage can be repaired by the base excision repair pathway or B er pathway choice. A double strand of DNA breaks B DNA adducts C DNA mismatches or D oxidized bases. Well, we can eliminate two things fairly easily from this right away. Uh Choice C DNA mismatches, mismatches get their own special repair pathway and that's called mismatch repair. So that's not what we're talking about here. So we're gonna go ahead and cross that out and that mismatch repair, of course, senses that there's a wrong pairing and methylation status is used to distinguish the template strand from the new strand. The template strand will already be methylated while that brand new strand doesn't have methyl groups yet. And that's the way the repair system recognizes which one is the original and which one to correct the other one we can eliminate right away is choice. A double stranded DNA breaks, these have their own special pathways. They're very serious damage because it means a whole chromosome just breaks both strands. And there's two pathways that deal with those breaks. Uh non homologous end joining or N H N H E J pathway or homologous recombination. But again, we're talking about base excision repair, cutting out a base with an error in it and not dealing with an entire break across both strands of the DNA double helix. So we'll go ahead and cross out choice A and just focus on B and D. So we're talking about what type of DNA damage can be repaired by the B er pathway. So we've got DNA addic and oxidized bases. So to understand which will be repaired by this particular pathway, let's think about what these are DNA add ups are bulky chemical groups added to a base and this is most likely caused by uh chemical Mutis. So exposure to very harmful chemicals that can stick these big old bulky chemical groups onto DNA, obviously causing the potential for erroneous replication. And that's why these chemicals can be Mutis, oxidized bases are a smaller change, a smaller chemical change to a an individual base and oxidized bases are a fairly common mistake that happens. And so you need to have a pathway to get rid of these bases. So now we need to think about which of these errors will be repaired by B er base excision repair. Now, here's where it gets tricky because we also have nucleotide excision repair or N er and it can be really hard to keep track of which is which even with similar names, nucleotide versus base. How can I remember? Which is which? And the key here is if we think about the fact that base excision repair is for damage to a single base. And although nucleotide, oxygen repair has the singular word nucleotide, it refers to damage to a patch of nucleotides or excuse me, removing, I should say, removing a patch of nucleotides in the process of repair. So it's not a foolproof way to remember. But for me, I find it a little helpful to focus on those words. So since B er just exercises a single base and then replaces it, it's used for cases where you have damage to a single base with little to no distortion to the DNA structure. So you've just got this small molecule stuck on there. A base has been oxidized as our problem asks us, asks us about alkalized. Maybe you have a uracil stuck in your DNA caused by DEA where you shouldn't have it. So relatively small chemical changes so they can cause the potential for mismatches. And that's why they're a problem, but it's not a huge giant uh distortion to our DNA nucleotide excision repair has to excise an entire patch of nucleotides because it's a method of repair for bulky lesions. N DNA result of often U V radiation damage or DNA adducts the addition of large chemical groups from harmful chemical exposure. So things that damage DNA in a way that causes significant distortion to our double helix and that's actually the signal for this pathway. And the enzymes recognize this distortion in the double helical structure. So that's kind of the big distinction there if we remember B er single base excision, because we have a small problem with our base N er nucleotide excision excesses a whole patch of nucleotides because you have a significant distortion to the DNA molecule, whether from significant amount of damage U V radiation causing diers to form covalent bonds between neighboring bases or DNA addicts adducts large chemical groups being stuck onto our bases. So with that walk through, we can see that our correct answer here is choice. D oxidized bases are repaired by the B er pathway and we can eliminate choice B DNA addic as being usually repaired by the N er pathway. So once again, which type of DNA damage is replaced by base excision repair. Choice the oxidized bases. See you in the next video.

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.
A cytosine that has been deaminated to uracil

Key Concepts

DNA Repair Mechanisms

Base Excision Repair (BER)

Deamination of Cytosine

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