- In an E. coli cell that is deficient in Dam methyltransferase, the mismatch repair (MMR) system cannot reliably distinguish the parental and daughter strands. If a G-T mismatch occurs due to a replication error (where T was incorrectly incorporated opposite a G on the template strand), and the MMR system incorrectly identifies the G-containing parental strand as the one to be repaired, what will be the ultimate outcome for the DNA sequence at this position after another round of replication?
Answer: The G on the parental strand will be excised and replaced with an A, resulting in a permanent G:C to A:T transition mutation.
When you encounter questions about DNA mismatch repair (MMR), focus on understanding the strand identification mechanism and the consequences of repair errors. Normal MMR relies on Dam methylation patterns to distinguish the newly synthesized (unmethylated) daughter strand from the methylated parental strand, ensuring errors are corrected on the daughter strand.
Without functional Dam methyltransferase, the MMR system loses this crucial guidance system. In this scenario, a replication error created a G-T mismatch where T was incorrectly incorporated opposite the template G. The MMR system mistakenly targets the parental strand containing G for repair instead of the daughter strand containing the erroneous T.
When the G is excised from the parental strand, DNA polymerase fills the gap using the daughter strand as template. Since the daughter strand contains T, an A will be incorporated opposite it, creating an A-T base pair. After the next replication round, one daughter cell will have the original G-C pair, but the other will have A-T, establishing a permanent G:C to A:T transition mutation.
Answer choice A is wrong because the error isn't corrected—it becomes fixed as a mutation. Choice B incorrectly suggests deletion rather than base substitution. Choice C describes normal MMR function where the daughter strand error is corrected, but that's not what happens when strand identification fails.
Remember: MMR errors don't just fail to fix mistakes—they can actively convert temporary mismatches into permanent mutations by "correcting" the wrong strand. Dam methylation is essential for proper strand discrimination.
- Transcription-coupled nucleotide excision repair (TC-NER) provides a mechanism for rapidly repairing lesions in actively transcribed genes. The signal that specifically initiates TC-NER, distinguishing it from global genome NER (GG-NER), is the:
Answer: physical blockage and stalling of an elongating RNA polymerase II complex on the template strand.
The defining feature of TC-NER is its link to transcription. When a transcribing RNA polymerase encounters a bulky lesion on the template DNA strand, it stalls. This stalled complex acts as a damage signal, recruiting TC-NER specific factors (like CSA and CSB) to the site, which then assemble the rest of the NER machinery. In contrast, GG-NER is initiated by the XPC complex recognizing helix distortions anywhere in the genome, independent of transcription.
- Base excision repair (BER) is initiated by a DNA glycosylase that recognizes and removes a specific damaged or inappropriate base. The existence of a specific uracil-DNA glycosylase is critical for genomic integrity primarily because:
Answer: uracil can arise from the spontaneous deamination of cytosine, and if unrepaired, it would be read as thymine during replication.
The chemical instability of cytosine leads to its spontaneous deamination to form uracil. Because uracil has the same base-pairing properties as thymine (pairing with adenine), if it is not removed before replication, a C:G base pair will be converted to a T:A base pair in one of the daughter duplexes. This is a common C to T transition mutation. The uracil-DNA glycosylase specifically removes U from DNA to prevent this outcome. Uracil is not bulky and does not significantly distort the helix, making it a poor substrate for other repair systems.
- Some oxidative lesions, such as 8-oxoguanine (8-oxoG), are repaired by base excision repair (BER). If an 8-oxoG paired with cytosine is not repaired and the DNA replicates, the 8-oxoG can mispair with adenine. How would the cell's repair machinery most likely handle the resulting 8-oxoG:A mismatch?
Answer: A specialized DNA glycosylase (MUTYH in humans) from the BER pathway would recognize and remove the adenine that is mispaired with 8-oxoG.
When you encounter questions about DNA repair mechanisms, focus on which specific repair pathway handles each type of damage and how different systems recognize their target lesions.
8-oxoguanine (8-oxoG) is a common oxidative DNA lesion that's normally repaired by base excision repair (BER). However, if it escapes initial repair and pairs with adenine during replication, the cell has a backup mechanism. The specialized DNA glycosylase MUTYH specifically recognizes 8-oxoG:A mismatches and removes the incorrectly incorporated adenine, allowing the BER pathway to complete repair by inserting the correct cytosine opposite 8-oxoG.
Option A is incorrect because nucleotide excision repair (NER) targets bulky lesions that distort the DNA helix, like UV-induced thymine dimers. The 8-oxoG:A mismatch doesn't create sufficient helix distortion to trigger NER.
Option B misrepresents how mismatch repair (MMR) works. While MMR does recognize base mismatches, it cannot distinguish between 8-oxoG and normal guanine, so it wouldn't specifically target this oxidative lesion. MMR would treat this as a standard G:A mismatch.
Option C is wrong because 8-oxoG:A mismatches don't typically cause replication fork collapse. These lesions are relatively minor and don't block DNA polymerase progression enough to trigger homologous recombination.
The key study tip: Remember that cells often have specialized backup repair mechanisms. MUTYH represents the cell's "second chance" to fix 8-oxoG lesions that escaped initial BER, demonstrating how multiple repair pathways can target the same type of damage at different stages.
- Which of the following correctly pairs an enzyme from base excision repair (BER) with an enzyme from nucleotide excision repair (NER) that performs an analogous, but distinct, function in their respective pathways?
Answer: BER: Uracil-DNA Glycosylase; NER: XPC protein
Both base excision repair (BER) and nucleotide excision repair (NER) are DNA repair pathways that detect and fix different types of damage, but they follow analogous steps: damage recognition, removal, and repair synthesis. The key to this question is identifying enzymes that perform the same functional role in their respective pathways.
Answer A correctly pairs damage recognition enzymes. Uracil-DNA Glycosylase in BER recognizes and removes specific damaged bases (like uracil that shouldn't be in DNA), while XPC protein in NER recognizes bulky DNA lesions like UV-induced thymine dimers. Both serve as the initial damage detectors that initiate their repair cascades.
Answer B incorrectly pairs enzymes from different repair steps. DNA Polymerase β fills in gaps during BER, while DNA Ligase I seals nicks during NER - these aren't analogous functions.
Answer C contains a fundamental error. DNA glycosylases only exist in BER, not NER. This attempts to trick you by using a real BER enzyme (AP Endonuclease, which cuts DNA after glycosylase action) with a nonexistent NER enzyme.
Answer D pairs enzymes with completely different functions. DNA Ligase III seals DNA breaks in BER, while TFIIH Helicase unwinds DNA in NER. One seals, the other opens - opposite functions.
Study tip: When comparing repair pathways, focus on the functional steps rather than enzyme names. Both BER and NER follow: recognize damage → remove damage → fill gap → seal nick. Learn which enzymes perform each step in both pathways.
- A key distinction between nucleotide excision repair (NER) and mismatch repair (MMR) is that:
Answer: MMR functions almost exclusively to correct errors made during replication, whereas NER primarily repairs DNA lesions caused by exogenous mutagens.
This question highlights the fundamental difference in the biological roles of the two pathways. Mismatch repair (MMR) is intrinsically linked to DNA replication, where its job is to correct misincorporated bases and small insertions/deletions, acting as a 'proofreader' for the replication machinery. Nucleotide excision repair (NER) is a general damage repair pathway that deals with lesions, often caused by environmental mutagens like UV light or chemical carcinogens, that distort the DNA helix.
- In a hypothetical eukaryotic cell, the primary mechanism for mismatch repair strand discrimination is the recognition of nicks that are exclusively present on the lagging strand due to Okazaki fragment synthesis. Given this specific mechanism, which of the following replication errors would be repaired least efficiently?
Answer: A G-T mismatch occurring on the continuously synthesized leading strand, far from the origin of replication.
The premise is that nicks serve as the signal to identify the new strand. The lagging strand is synthesized discontinuously as Okazaki fragments, and thus is transiently rich in nicks before they are sealed by DNA ligase. The leading strand, however, is synthesized continuously. Therefore, a mismatch on the leading strand, especially one far from the origin, would lack a nearby nick to direct the MMR machinery. This would make it difficult for the system to distinguish the new strand from the template strand, leading to inefficient or incorrect repair.
- In a cell with a loss-of-function mutation in the gene for AP endonuclease, what would be the structure of a DNA molecule immediately after a uracil-DNA glycosylase has successfully acted on a U:G mispair?
Answer: The DNA would contain an abasic site with an intact phosphodiester backbone at that position.
Base excision repair is a sequential process. The first step is the removal of the damaged base (uracil) by a glycosylase. This creates an abasic (AP) site but leaves the sugar-phosphate backbone intact. The second step is the cleavage of the backbone at the AP site by AP endonuclease. If AP endonuclease is non-functional, the pathway is blocked after the first step, resulting in the accumulation of AP sites with an intact backbone.
- A cell culture is exposed to a chemical mutagen that causes two primary types of damage: deamination of cytosine to uracil and formation of bulky benzo[a]pyrene adducts on guanine. If the cell's nucleotide excision repair (NER) pathway is specifically inhibited by a small molecule, what is the most likely long-term consequence for the cell's genome after several rounds of replication?
Answer: Uracil will be efficiently removed, but the benzo[a]pyrene adducts will persist, leading primarily to replication fork stalling and chromosomal breaks.
This question requires differentiating the lesions handled by Base Excision Repair (BER) and Nucleotide Excision Repair (NER). Deamination of cytosine to uracil is repaired by BER, initiated by uracil-DNA glycosylase. This pathway is functional. Bulky adducts like those from benzo[a]pyrene are repaired by NER. With NER inhibited, these lesions will persist. Bulky lesions are potent blockers of DNA replication, leading to stalled replication forks, which can collapse into double-strand breaks.
- A patient diagnosed with Xeroderma Pigmentosum (XP) shows extreme photosensitivity. Genetic analysis reveals a mutation in the XPG gene. This protein's function is critical for which specific step of the nucleotide excision repair pathway?
Answer: Making an incision in the damaged DNA strand 3' to the lesion.
The XPG protein is a structure-specific endonuclease. In the context of NER, it makes the 3' incision relative to the DNA lesion. The 5' incision is made by the XPF-ERCC1 complex. Distractor A refers to XPC or CSA/B. Distractor B refers to the helicases XPB and XPD. Distractor D refers to DNA polymerase δ/ε.
- A yeast strain is engineered to be deficient in both uracil-DNA glycosylase (UNG1) and the mismatch repair protein MSH2. This strain is then grown for many generations in a medium containing a chemical agent that moderately increases the spontaneous rate of cytosine deamination. Which two types of mutations are expected to accumulate at the highest frequency in this strain's genome?
Answer: C:G to T:A transitions and small insertions/deletions in repetitive sequences.
This is a two-part problem. First, deficiency in uracil-DNA glycosylase (UNG1) cripples base excision repair of uracil in DNA. Cytosine deamination produces uracil. If not repaired, this uracil will pair with adenine during replication, leading to a C:G to T:A transition mutation. Second, deficiency in MSH2, a key recognition protein in mismatch repair (MMR), prevents the correction of replication slippage errors. These errors occur most frequently in repetitive sequences (microsatellites) and manifest as small insertions or deletions.
- In the base excision repair (BER) pathway, the process is initiated by a DNA glycosylase. Which statement most accurately describes the primary consequence of this enzyme's action?
Answer: It generates an abasic (AP) site by cleaving the N-glycosidic bond between the damaged base and its deoxyribose sugar.
The defining role of a DNA glycosylase is to recognize a specific type of damaged or incorrect base and catalyze the hydrolysis of the N-glycosidic bond that links the base to the deoxyribose sugar of the DNA backbone. This action removes the base, leaving behind an 'abasic' or 'apurinic/apyrimidinic' (AP) site, while the sugar-phosphate backbone remains intact. The nick is created in the next step by AP endonuclease.
- In eukaryotes, repair of an abasic (AP) site via base excision repair can proceed through a short-patch or long-patch pathway. The switch to long-patch BER often occurs if the 5' deoxyribose phosphate (dRP) residue cannot be easily removed. Which protein's activity is essential for completing the long-patch BER pathway but is not required for short-patch BER?
Answer: Flap endonuclease 1 (FEN1).
When you encounter questions about DNA repair pathways, focus on the key differences between short-patch and long-patch base excision repair (BER). Both pathways start similarly but diverge based on whether the 5' deoxyribose phosphate (dRP) residue can be easily removed.
In short-patch BER, AP endonuclease (APE1) creates a nick 5' to the abasic site, DNA polymerase β fills in a single nucleotide while removing the dRP group with its lyase activity, and DNA ligase III seals the nick. However, when the dRP residue is chemically modified or difficult to remove, the pathway switches to long-patch BER.
Long-patch BER involves DNA polymerase β (or δ/ε) adding 2-10 nucleotides, which displaces the problematic dRP-containing strand as a "flap" structure. This is where flap endonuclease 1 (FEN1) becomes essential—it specifically cleaves these displaced DNA flaps, allowing the repair to proceed. Without FEN1, the flap cannot be removed and repair stalls.
Looking at the wrong answers: (A) APE1 is required in both pathways to create the initial nick. (B) DNA polymerase β participates in both pathways for gap filling. (C) DNA ligase III functions in both pathways to seal the final nick. Only (D) FEN1 is uniquely required for long-patch BER because short-patch repair doesn't generate flap structures.
Remember this pattern: when DNA repair creates displaced flaps or overhangs, FEN1 is usually involved to clean them up. This principle applies beyond BER to other repair mechanisms as well.
- In eukaryotic nucleotide excision repair (NER), several proteins act in a coordinated sequence. Which of the following represents the correct temporal order of key events after the initial damage recognition?
Answer: DNA unwinding by helicase → Excision by endonucleases → DNA synthesis → Ligation.
The logical and experimentally determined order of events in NER after damage recognition is as follows: first, the DNA around the lesion must be opened and unwound by helicases (XPB/XPD in TFIIH). This creates a bubble that allows access for the endonucleases (XPF/XPG), which then excise the damaged oligonucleotide. Next, a DNA polymerase fills in the resulting gap. Finally, DNA ligase seals the nick to complete the repair.
- Both nucleotide excision repair (NER) and base excision repair (BER) restore the original DNA sequence after damage. However, the NER pathway is significantly more complex. Which of the following is a key requirement for NER that is NOT typically required for the simplest form of BER (short-patch BER)?
Answer: ATP-dependent helicase activity to unwind the DNA around the lesion.
When comparing DNA repair mechanisms, focus on the scope and complexity of the damage being repaired. Base excision repair (BER) handles single damaged bases, while nucleotide excision repair (NER) tackles bulky lesions that distort the DNA helix structure.
The correct answer is D because NER requires ATP-dependent helicase activity to unwind DNA around bulky lesions like thymine dimers or chemical adducts. These large distortions create significant structural changes that must be unwound for proper access by repair enzymes. In contrast, short-patch BER deals with single base modifications (like uracil or 8-oxoguanine) that don't severely distort the helix, so helicases aren't needed.
Option A is wrong because both pathways require DNA polymerase - BER needs it to fill the single-nucleotide gap after base removal, while NER needs it to fill the longer gap after excision. Option B is incorrect since both pathways require DNA ligase to seal the final nick and restore the continuous phosphodiester backbone. Option C is wrong because both pathways begin with damage recognition proteins - BER uses glycosylases that recognize specific damaged bases, while NER uses proteins that detect helix distortions.
Remember this key distinction: BER is like precision surgery on individual bases, while NER is like major reconstruction around bulky damage. The structural complexity of NER lesions demands additional machinery (helicases) that simple base modifications in BER don't require. Focus on the relationship between lesion size, structural distortion, and repair complexity.
- The choice of DNA repair pathway can be influenced by the cell cycle phase. Mismatch repair (MMR) is most tightly coupled with which phase of the cell cycle, and for what reason?
Answer: S phase, because MMR's primary function is to correct errors made by DNA polymerase during DNA synthesis.
Mismatch repair (MMR) corrects errors of replication, such as misincorporated bases and small insertions/deletions. These errors are generated during DNA synthesis, which occurs in the S phase of the cell cycle. Furthermore, the strand discrimination mechanisms that allow MMR to identify the newly synthesized (and erroneous) strand are also linked to the replication process itself (e.g., transient nicks in the lagging strand). Therefore, MMR function is temporally and mechanistically coupled to S phase.
- A cell line derived from a patient with Lynch syndrome (HNPCC) is found to have a high degree of microsatellite instability. This phenomenon is a direct consequence of the failure of the mismatch repair system to correct which type of error?
Answer: Slippage of DNA polymerase during replication of short tandem repeats.
Lynch syndrome is caused by inherited mutations in mismatch repair (MMR) genes. Microsatellites are regions of repetitive DNA (e.g., ...CACACACA...). These regions are prone to errors during replication because DNA polymerase can 'slip,' creating a small loop of inserted or deleted bases on the new strand. The MMR system is responsible for recognizing and repairing these insertion/deletion loops. When MMR is defective, these errors accumulate, causing the length of microsatellites to change, a phenomenon called microsatellite instability.
- In E. coli, the mismatch repair (MMR) system relies on the hemimethylated state of GATC sequences. The Dam methyltransferase is responsible for methylating the adenine in these sequences. What is the most likely consequence of a hyperactive Dam methyltransferase that methylates the new strand almost instantly after the replication fork passes?
Answer: The MMR system would be unable to distinguish the template from the new strand, leading to random repair and an increased mutation rate.
When you encounter questions about DNA mismatch repair (MMR), focus on the critical timing of methylation that allows the system to identify which strand contains the error. In E. coli, the MMR system depends on GATC sequences being hemimethylated (only the template strand is methylated) immediately after replication, creating a temporary window where the system can distinguish old DNA from newly synthesized DNA.
The correct answer is A because hyperactive Dam methyltransferase would eliminate this crucial distinction. Normally, there's a brief period after replication where only the template strand is methylated while the new strand remains unmethylated. This hemimethylated state tells the MMR machinery which strand is "correct" (the methylated template) and which potentially contains errors (the unmethylated new strand). If Dam methyltransferase instantly methylates the new strand, both strands become fully methylated immediately, leaving the MMR system with no way to determine which base in a mismatch is correct.
Answer B incorrectly assumes faster methylation improves repair efficiency, but rapid methylation actually disrupts the targeting mechanism. Answer C misunderstands the consequence—the system wouldn't become hyperactive against correct pairs, but rather unable to function properly at all. Answer D incorrectly suggests that nucleotide excision repair would compensate, but these systems target different types of DNA damage and don't functionally substitute for each other.
Remember: MMR effectiveness depends on asymmetric methylation patterns that create directional information. Without this asymmetry, the repair machinery loses its ability to make informed corrections.
- The figure shows two DNA molecules that are intermediates in distinct repair processes. Which statement accurately identifies the repair pathways and the next enzymatic step for each?
Refer to the figure.
Answer: Molecule 1 is in BER; next step is incision by AP endonuclease. Molecule 2 is in NER; next step is dual incision by endonucleases.
Molecule 1 shows an abasic (AP) site, which is the hallmark intermediate of base excision repair (BER) after a DNA glycosylase has removed the damaged base. The next step is for an AP endonuclease to cleave the phosphodiester backbone. Molecule 2 shows a bulky lesion within a 'bubble' of unwound DNA, which is the characteristic intermediate of nucleotide excision repair (NER) after damage recognition and DNA unwinding by helicase. The next step is the dual incision by the XPF and XPG endonucleases to excise the damaged segment.
