1. In a segment of DNA exposed to chemicals, Cytosine has been deaminated to Uracil, Which of the following mechanisms will remove Uracil and incorporate the correct base?
A) Direct repair
B) Base excision repair
C) Nucleotide excision repair
D) Mismatch repair
E) Double strand break repair
Correct Answer: (B) Base Excision Repair (BER)
Reason:
BER specifically corrects small, non-bulky lesions, such as cytosine deamination to uracil.
- Uracil DNA Glycosylase (UDG) removes uracil.
- AP Endonuclease cuts the DNA backbone at the abasic site.
- DNA Polymerase inserts the correct cytosine.
- DNA Ligase seals the strand.
Why Others Are Incorrect:
(A) Direct repair – Fixes UV damage, not base changes.
(C) Nucleotide excision repair – Repairs bulky lesions, not single bases.
(D) Mismatch repair – Fixes replication errors, not deaminated cytosine.
(E) Double-strand break repair – Fixes severe DNA breaks, not base modifications.
2. A 45-year-old male factory worker has been exposed to high levels of benzopyrene over several years due to his job in an industrial plant. He is now diagnosed with squamous cell carcinoma of the lung. Which of the following mechanisms best explains the carcinogenic effects of benzopyrene?
A. Alkylation of DNA bases
B. Base pair deletions during replication
C. Covalent DNA adduct formation
D. Hydrolysis of phosphodiester bonds
E. UV-induced thymine dimer formation
Correct Answer: (C) Covalent DNA Adduct Formation
Reason:
Benzopyrene, a polycyclic aromatic hydrocarbon found in tobacco smoke and industrial pollutants, is metabolized into reactive benzopyrene diol epoxide (BPDE). BPDE forms covalent DNA adducts, particularly at guanine residues, leading to mutations in tumor suppressor genes (e.g., TP53), driving carcinogenesis.
Why Others Are Incorrect:
(A) Alkylation of DNA bases – Incorrect. Alkylating agents (e.g., methyl methanesulfonate) add alkyl groups, but benzopyrene primarily forms bulky adducts.
(B) Base pair deletions during replication – Incorrect. Benzopyrene adducts cause mispairing but do not directly induce base pair deletions.
(D) Hydrolysis of phosphodiester bonds – Incorrect. DNA backbone hydrolysis is caused by nucleases or extreme damage, not benzopyrene.
(E) UV-induced thymine dimer formation – Incorrect. Thymine dimers result from UV radiation, not chemical carcinogens.
Reason:
Ionizing radiation generates reactive oxygen species (ROS), which cause oxidative DNA damage. This leads to single-strand and double-strand breaks, base modifications (e.g., 8-oxo-dG), and increased genomic instability, contributing to acute tissue toxicity and carcinogenesis.
Why Others Are Incorrect:
(A) Base deamination leading to C→U transitions – Incorrect. Deamination is a spontaneous or enzymatic process, not primarily caused by ionizing radiation.
(B) Direct generation of apurinic (AP) sites – Incorrect. AP sites arise from base excision repair or spontaneous base loss, not directly from radiation.
(C) Induction of interstrand crosslinks – Incorrect. Crosslinks are typically caused by chemotherapy agents like cisplatin, not radiation.
(E) Single-base mismatch incorporation during replication – Incorrect. Replication errors occur due to polymerase mistakes, not ionizing radiation.
4. A 28-year-old woman presents with severe sunburn after prolonged sun exposure. She has a history of developing multiple skin lesions, some of which have been biopsied and diagnosed as basal cell carcinoma. Laboratory studies indicate defects in nucleotide excision repair. Which type of DNA damage is most likely associated with her condition?
A. DNA intercalation
B. Pyrimidine dimer formation
C. Single-strand breaks
D. Topoisomerase-induced damage
E. Bulky DNA adducts
Correct Answer: (B) Pyrimidine Dimer Formation
Reason:
Prolonged UV radiation exposure causes the formation of pyrimidine dimers, specifically thymine-thymine dimers, which distort the DNA helix. These lesions are normally repaired by nucleotide excision repair (NER). Defects in NER, such as in xeroderma pigmentosum (XP), lead to an accumulation of DNA damage and an increased risk of skin cancers, including basal cell carcinoma.
Why Others Are Incorrect:
(A) DNA intercalation – Incorrect. Intercalating agents (e.g., ethidium bromide) insert between DNA bases but are not caused by UV exposure.
(C) Single-strand breaks – Incorrect. UV light primarily causes bulky lesions (dimers), not strand breaks.
(D) Topoisomerase-induced damage – Incorrect. Topoisomerases cause strand breaks during DNA unwinding, not due to UV radiation.
(E) Bulky DNA adducts – Incorrect. While some chemical carcinogens (e.g., benzopyrene) form bulky adducts, UV-induced damage primarily leads to pyrimidine dimers, not adducts.
5. A 45-year-old woman undergoing chemotherapy with doxorubicin develops signs of cardiotoxicity, including fatigue and shortness of breath. Laboratory studies show increased lipid peroxidation and oxidative DNA damage in myocardial cells. Which of the following is the most likely mechanism of DNA damage caused by oxidative stress in this patient?
A. Apoptotic fragmentation of chromatin
B. Formation of single-strand breaks
C. Insertion of bulky adducts by metabolic carcinogens
D. Nucleotide excision repair inhibition
E. UV-induced formation of cyclobutane pyrimidine dimers
Correct Answer: (B) Formation of Single-Strand Breaks
Reason:
Doxorubicin generates reactive oxygen species (ROS) through redox cycling, leading to oxidative stress in myocardial cells. ROS cause oxidative DNA damage, including single-strand breaks (SSBs), 8-oxo-2′-deoxyguanosine (8-oxo-dG) formation, and lipid peroxidation, contributing to cardiotoxicity and genomic instability.
Why Others Are Incorrect:
(A) Apoptotic fragmentation of chromatin – Incorrect. While oxidative stress can induce apoptosis, the primary DNA damage is from ROS-induced strand breaks, not programmed fragmentation.
(C) Insertion of bulky adducts by metabolic carcinogens – Incorrect. Bulky adducts are formed by chemicals like benzopyrene, not oxidative stress.
(D) Nucleotide excision repair inhibition – Incorrect. NER repairs bulky lesions (e.g., thymine dimers), not oxidative strand breaks.
(E) UV-induced formation of cyclobutane pyrimidine dimers – Incorrect. Pyrimidine dimers result from UV exposure, not doxorubicin-induced oxidative stress.
6. A 24-year-old woman presents with recurrent skin lesions on sun-exposed areas. She reports developing freckles and blistering upon minimal sun exposure. Genetic testing reveals a defect in the DNA repair system. Which DNA repair mechanism is most likely defective in this patient?
A. Base excision repair
B. Mismatch repair
C. Nucleotide excision repair
D. Non-homologous end joining
E. Homologous recombination
Correct Answer: (C) Nucleotide Excision Repair (NER)
Reason:
This patient’s symptoms—photosensitivity, freckling, and blistering upon minimal sun exposure—are characteristic of xeroderma pigmentosum (XP), a condition caused by a defect in nucleotide excision repair (NER). NER is responsible for repairing UV-induced DNA damage, particularly pyrimidine dimers and other bulky lesions that distort the DNA helix.
Why Others Are Incorrect:
(A) Base Excision Repair (BER) – Incorrect. BER repairs small, non-bulky lesions (e.g., oxidative damage, alkylation, and deaminated bases) rather than UV-induced pyrimidine dimers.
(B) Mismatch Repair (MMR) – Incorrect. MMR corrects replication errors (e.g., base mismatches and insertion-deletion loops), not UV-induced lesions.
(D) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ repairs double-strand breaks, typically caused by ionizing radiation, not UV damage.
(E) Homologous Recombination (HR) – Incorrect. HR repairs double-strand breaks using a template, and its defect is linked to BRCA1/2 mutations in cancers, not UV sensitivity.
7. A 45-year-old woman is diagnosed with breast cancer. Genetic testing reveals a BRCA1 mutation. Her oncologist explains that this mutation impairs a critical DNA repair pathway.
Which DNA repair mechanism is defective in patients with BRCA1 mutations?
A. Nucleotide excision repair
B. Base excision repair
C. Mismatch repair
D. Non-homologous end joining
E. Homologous recombination
Correct Answer: (E) Homologous Recombination
Explanation:
BRCA1 and BRCA2 play essential roles in homologous recombination (HR), a high-fidelity DNA double-strand break (DSB) repair mechanism. When BRCA1 is mutated, cells rely on error-prone repair pathways like non-homologous end joining (NHEJ), increasing genomic instability and cancer risk.
Why Others Are Incorrect:
(A) Nucleotide Excision Repair (NER) – Incorrect. NER repairs bulky lesions (e.g., UV-induced pyrimidine dimers), not double-strand breaks.
(B) Base Excision Repair (BER) – Incorrect. BER fixes small base modifications (e.g., oxidative damage, alkylation), not DSBs.
(C) Mismatch Repair (MMR) – Incorrect. MMR corrects replication errors (mismatched bases), not DSBs.
(D) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ repairs DSBs but is error-prone and does not require BRCA1.
8. A researcher studying the effects of alkylating agents on DNA observes an increase in apurinic (AP) sites in treated cells. He hypothesizes that a specific repair pathway is activated to correct these lesions.
Which DNA repair mechanism is primarily responsible for repairing alkylation-induced apurinic sites?
A. Nucleotide excision repair
B. Mismatch repair
C. Base excision repair
D. Homologous recombination
E. Non-homologous end joining
Correct Answer: (C) Base Excision Repair (BER)
Explanation:
Alkylating agents modify DNA bases, leading to the formation of apurinic/apyrimidinic (AP) sites after spontaneous base loss. Base Excision Repair (BER) is the primary pathway for repairing these lesions. The process includes:
- DNA glycosylase removes the damaged base.
- AP endonuclease cleaves the DNA backbone at the AP site.
- DNA polymerase fills in the correct base.
- DNA ligase seals the strand.
Why Others Are Incorrect:
(A) Nucleotide Excision Repair (NER) – Incorrect. NER removes bulky lesions, such as UV-induced pyrimidine dimers, not small AP sites.
(B) Mismatch Repair (MMR) – Incorrect. MMR corrects replication errors (e.g., base mismatches and indels), not alkylation damage.
(D) Homologous Recombination (HR) – Incorrect. HR repairs double-strand breaks, not AP sites.
(E) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ is an error-prone mechanism for double-strand breaks, not base damage.
9. A 38-year-old man has a family history of colorectal cancer. Genetic testing identifies mutations in MLH1 and MSH2 genes. He is diagnosed with Lynch syndrome (hereditary nonpolyposis colorectal cancer, HNPCC).
Deficiency of which DNA repair pathway is associated with Lynch syndrome?
A. Nucleotide excision repair
B. Base excision repair
C. Mismatch repair
D. Homologous recombination
E. Non-homologous end joining
Correct Answer: (C) Mismatch Repair (MMR)
Explanation:
Lynch syndrome (HNPCC) is caused by germline mutations in DNA mismatch repair (MMR) genes, including:
- MLH1, MSH2, MSH6, PMS2
MMR corrects replication errors, such as mismatched bases and small insertion-deletion loops that occur due to DNA polymerase mistakes. Deficiency in this repair pathway leads to microsatellite instability (MSI) and increased risk of colorectal, endometrial, and other cancers.
Why Others Are Incorrect:
(A) Nucleotide Excision Repair (NER) – Incorrect. NER repairs bulky lesions (e.g., pyrimidine dimers from UV damage) but is not involved in mismatch correction.
(B) Base Excision Repair (BER) – Incorrect. BER fixes small base modifications (e.g., oxidation, alkylation), not replication errors.
(D) Homologous Recombination (HR) – Incorrect. HR repairs double-strand breaks, not mismatched bases.
(E) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ repairs double-strand breaks in an error-prone manner, but it does not correct mismatches.
10. A patient undergoing radiation therapy for lymphoma experiences genomic instability due to radiation-induced double-strand breaks (DSBs) in DNA. The treating physician explains that two major pathways exist to repair these breaks.
Which of the following pathways is an error-prone method of double-strand break repair?
A. Homologous recombination
B. Mismatch repair
C. Nucleotide excision repair
D. Base excision repair
E. Non-homologous end joining
Correct Answer: (E) Non-Homologous End Joining (NHEJ)
Explanation:
Double-strand breaks (DSBs) are the most severe form of DNA damage and can be repaired by two major pathways:
-
Homologous Recombination (HR) – High-fidelity
- Uses a sister chromatid as a template to accurately repair DSBs.
- Occurs in S and G2 phases of the cell cycle.
-
Non-Homologous End Joining (NHEJ) – Error-prone
- Directly ligates broken DNA ends without using a template.
- Leads to insertions, deletions, or chromosomal translocations, contributing to genomic instability.
- Occurs in G1 phase and is the primary repair mechanism in non-dividing cells.
Why Others Are Incorrect:
(A) Homologous Recombination – Incorrect. HR is an accurate repair process, not error-prone.
(B) Mismatch Repair (MMR) – Incorrect. MMR fixes replication errors (mismatched bases, indels), not DSBs.
(C) Nucleotide Excision Repair (NER) – Incorrect. NER corrects bulky lesions (e.g., UV-induced pyrimidine dimers), not DSBs.
(D) Base Excision Repair (BER) – Incorrect. BER repairs small base modifications (e.g., oxidative damage), not DSBs.
11. A 55-year-old man with glioblastoma is undergoing radiation therapy. He develops severe side effects, including neurological deficits. Genetic testing reveals a mutation in the ATM gene, which plays a role in the DNA damage response.
Which DNA repair pathway is primarily affected by an ATM mutation?
A. Nucleotide excision repair
B. Base excision repair
C. Homologous recombination
D. Mismatch repair
E. Non-homologous end joining
Correct Answer: (C) Homologous Recombination (HR)
Explanation:
ATM (Ataxia-Telangiectasia Mutated) is a key regulator of homologous recombination (HR) and is activated in response to DNA double-strand breaks (DSBs). It phosphorylates multiple repair proteins, including BRCA1 and RAD51, which are critical for error-free DSB repair via HR.
ATM mutations lead to:
- Defective homologous recombination → Increased genomic instability
- Sensitivity to ionizing radiation (which causes double-strand breaks)
- Neurological deficits (ataxia-telangiectasia syndrome)
Why Others Are Incorrect:
(A) Nucleotide Excision Repair (NER) – Incorrect. NER repairs bulky lesions (e.g., UV-induced thymine dimers), not double-strand breaks.
(B) Base Excision Repair (BER) – Incorrect. BER fixes small base modifications (e.g., oxidation, alkylation), not DSBs.
(D) Mismatch Repair (MMR) – Incorrect. MMR corrects replication errors (mismatched bases, small indels) but does not repair DSBs.
(E) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ repairs DSBs but does not depend on ATM; instead, DNA-PKcs is the key regulator.
12. A 62-year-old woman with ovarian cancer is being treated with cisplatin, a chemotherapy drug that induces DNA crosslinks. After initial tumor shrinkage, her cancer becomes resistant to treatment.
Which DNA repair mechanism is most likely responsible for the development of chemotherapy resistance?
A. Non-homologous end joining
B. Nucleotide excision repair
C. Base excision repair
D. Homologous recombination
E. Mismatch repair
Correct Answer: (D) Homologous Recombination (HR)
Explanation:
Cisplatin works by forming interstrand DNA crosslinks, preventing DNA replication and leading to cancer cell death. However, tumor cells can develop resistance by upregulating homologous recombination (HR), an error-free repair mechanism that efficiently removes DNA crosslinks.
- BRCA1/BRCA2-deficient tumors are hypersensitive to cisplatin because they lack effective HR.
- If HR is reactivated (e.g., secondary mutations restoring BRCA1/2 function), tumor cells can repair cisplatin-induced damage, leading to chemotherapy resistance.
Why Others Are Incorrect:
(A) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ is error-prone and does not efficiently repair DNA crosslinks; it is more involved in double-strand break repair.
(B) Nucleotide Excision Repair (NER) – Incorrect. While NER removes bulky DNA adducts, it is not the primary pathway for cisplatin-induced crosslinks.
(C) Base Excision Repair (BER) – Incorrect. BER corrects small base lesions (e.g., oxidation, alkylation), not crosslinks.
(E) Mismatch Repair (MMR) – Incorrect. MMR detects replication errors but does not repair interstrand crosslinks.
13. A 48-year-old man with uncontrolled diabetes has increased oxidative stress, leading to an accumulation of 8-oxo-guanine (8-oxoG), a common oxidative DNA lesion.
Which DNA repair mechanism primarily corrects oxidative DNA damage?
A. Nucleotide excision repair
B. Base excision repair
C. Homologous recombination
D. Mismatch repair
E. Non-homologous end joining
Correct Answer: (B) Base Excision Repair (BER)
Explanation:
Base Excision Repair (BER) is the primary repair mechanism for oxidative DNA damage, including 8-oxo-guanine (8-oxoG), which results from reactive oxygen species (ROS).
- The key enzyme OGG1 (8-oxoG DNA glycosylase) recognizes and removes 8-oxoG.
- AP endonuclease cleaves the DNA backbone at the abasic (AP) site.
- DNA polymerase inserts the correct base, and DNA ligase seals the strand.
Why Others Are Incorrect:
(A) Nucleotide Excision Repair (NER) – Incorrect. NER fixes bulky lesions (e.g., UV-induced thymine dimers), not oxidative base damage.
(C) Homologous Recombination (HR) – Incorrect. HR repairs double-strand breaks, not oxidative base modifications.
(D) Mismatch Repair (MMR) – Incorrect. MMR fixes replication errors (e.g., mismatched bases, small indels), not oxidative DNA lesions.
(E) Non-Homologous End Joining (NHEJ) – Incorrect. NHEJ repairs double-strand breaks but is error-prone and not responsible for oxidative damage correction.
14. A child is diagnosed with Xeroderma Pigmentosum (XP). The parents report extreme sensitivity to sunlight, and the child has developed multiple freckles and early skin lesions. A genetic defect in which of the following DNA repair pathways is most likely the cause of this condition?
A. Base excision repair
B. Direct damage reversal
C. Homologous recombination
D. Mismatch repair
E. Nucleotide excision repair
Correct Answer: (E) Nucleotide Excision Repair (NER)
Explanation:
Xeroderma Pigmentosum (XP) results from a defect in nucleotide excision repair (NER), which is responsible for fixing bulky DNA lesions, particularly UV-induced pyrimidine dimers (thymine dimers). Defective NER leads to:
- Extreme UV sensitivity
- Freckling and premature skin aging
- High risk of skin cancers (basal cell carcinoma, squamous cell carcinoma, melanoma)
Why Others Are Incorrect:
(A) Base Excision Repair (BER) – Incorrect. BER fixes small base modifications (e.g., oxidative damage) but does not remove bulky thymine dimers.
(B) Direct Damage Reversal – Incorrect. While some organisms use photolyase to reverse UV damage, humans lack this enzyme.
(C) Homologous Recombination (HR) – Incorrect. HR repairs double-strand breaks, not UV-induced DNA lesions.
(D) Mismatch Repair (MMR) – Incorrect. MMR corrects replication errors (mismatches, small insertions/deletions) but does not fix UV-induced damage.
15. A 35-year-old man presents with progressive ataxia, telangiectasia (dilated blood vessels), and an increased frequency of infections. He also exhibits hypersensitivity to ionizing radiation. Genetic testing reveals a mutation in the ATM gene, which plays a crucial role in the DNA damage response.
Which DNA repair pathway is primarily affected in this patient’s condition?
A. Base excision repair
B. Homologous recombination
C. Mismatch repair
D. Nucleotide excision repair
E. Non-homologous end joining
Correct Answer: (B) Homologous Recombination (HR)
Explanation:
This patient’s symptoms suggest Ataxia-Telangiectasia (A-T), a disorder caused by mutations in the ATM gene. ATM is a key regulator of homologous recombination (HR) and is activated in response to DNA double-strand breaks (DSBs), particularly those caused by ionizing radiation.
- Defective ATM function impairs HR, leading to genomic instability, radiosensitivity, and immune deficiencies.
- Patients with A-T are predisposed to lymphoid malignancies due to defective DNA repair.
Why Others Are Incorrect:
(A) Base Excision Repair (BER) – Incorrect. BER repairs small base modifications (e.g., oxidative damage, alkylation), not double-strand breaks.
(C) Mismatch Repair (MMR) – Incorrect. MMR corrects replication errors (mismatched bases, small indels) but does not repair DSBs.
(D) Nucleotide Excision Repair (NER) – Incorrect. NER fixes bulky lesions (e.g., thymine dimers, chemical adducts) but is not involved in double-strand break repair.
(E) Non-Homologous End Joining (NHEJ) – Incorrect. While NHEJ repairs double-strand breaks, ATM is specifically linked to HR regulation.
