Q.1 Which of the following is a type of point mutation where a single base is replaced by another?
Insertion
Deletion
Missense
Frameshift
Explanation - Missense mutation changes one DNA base pair, leading to a different amino acid in the protein.
Correct answer is: Missense
Q.2 What does DNA repair help prevent?
Muscle cramps
Genetic diseases
Eye infections
Heartburn
Explanation - DNA repair fixes mistakes in DNA, reducing the chance of mutations that can cause diseases.
Correct answer is: Genetic diseases
Q.3 Which enzyme copies DNA during cell division?
DNA polymerase
RNA polymerase
Ligase
Helicase
Explanation - DNA polymerase is the main enzyme that synthesizes new DNA strands.
Correct answer is: DNA polymerase
Q.4 A silent mutation does NOT change the?
DNA sequence
Protein function
RNA sequence
Chromosome number
Explanation - Silent mutations alter a codon but still code for the same amino acid, so the protein stays unchanged.
Correct answer is: Protein function
Q.5 Which type of mutation adds extra letters to the DNA sequence?
Deletion
Insertion
Missense
Nonsense
Explanation - Insertion adds extra nucleotides, potentially changing the reading frame.
Correct answer is: Insertion
Q.6 Which process repairs broken DNA strands by cutting the strand and filling in the missing part using a template?
Base excision repair
Non‑homologous end joining
Homologous recombination
Photoreactivation
Explanation - Homologous recombination uses a matching DNA sequence to accurately repair breaks.
Correct answer is: Homologous recombination
Q.7 What is a common cause of mutations in DNA?
Sunlight
Water
Wind
Dust
Explanation - UV light from the sun can damage DNA and cause mutations such as thymine dimers.
Correct answer is: Sunlight
Q.8 Which repair system fixes small, non‑severe errors in DNA mismatched base pairs?
Mismatch repair
Nucleotide excision repair
Base excision repair
Homologous recombination
Explanation - Mismatch repair detects and corrects base‑pair mismatches that escape proofreading.
Correct answer is: Mismatch repair
Q.9 A nonsense mutation changes a codon so it becomes a?
Start codon
Stop codon
Amino‑acid codon
Silence codon
Explanation - A nonsense mutation creates a premature stop codon, ending protein synthesis early.
Correct answer is: Stop codon
Q.10 Which enzyme helps seal nicks in DNA strands after repair?
DNA ligase
DNA polymerase
DNA helicase
DNA primase
Explanation - DNA ligase joins broken ends by forming phosphodiester bonds.
Correct answer is: DNA ligase
Q.11 The DNA repair process that removes damaged bases and fills the gap is called?
Photoreactivation
Base excision repair
Non‑homologous end joining
Mismatch repair
Explanation - Base excision repair removes altered bases and restores the correct sequence.
Correct answer is: Base excision repair
Q.12 Which type of mutation deletes a few nucleotides causing a shift in the reading frame?
Missense
Deletion
Insertion
Silent
Explanation - Deletion removes nucleotides, shifting the reading frame and changing downstream proteins.
Correct answer is: Deletion
Q.13 Which repair mechanism uses light energy to fix DNA damage caused by UV light?
Photoreactivation
Non‑homologous end joining
Homologous recombination
Base excision repair
Explanation - Photoreactivation employs an enzyme that uses visible light to repair thymine dimers.
Correct answer is: Photoreactivation
Q.14 The presence of two identical DNA strands that can serve as templates for repair is known as?
Homologous recombination
Nucleotide excision repair
Mismatch repair
Non‑homologous end joining
Explanation - Homologous recombination uses an identical sister chromatid to accurately repair breaks.
Correct answer is: Homologous recombination
Q.15 Which of the following is NOT a typical result of a DNA mismatch?
Incorrect base pairing
Increased mutation rate
Enhanced repair
Protein malfunction
Explanation - Mismatch errors normally increase mutation risk until repaired, not enhance it.
Correct answer is: Enhanced repair
Q.16 Which protein is part of the bacterial mismatch repair system that recognizes mismatches?
MutS
MutL
UvrA
Rad51
Explanation - MutS binds mismatched bases and initiates the bacterial mismatch repair cascade.
Correct answer is: MutS
Q.17 The enzyme that removes a damaged base and cuts the phosphodiester bond on either side is?
DNA glycosylase
DNA polymerase
DNA ligase
DNA helicase
Explanation - DNA glycosylase excises the damaged base, creating an abasic site for further processing.
Correct answer is: DNA glycosylase
Q.18 Which repair pathway is primarily used during DNA replication to fix errors before the next cell cycle?
Proofreading by DNA polymerase
Mismatch repair
Photoreactivation
Non‑homologous end joining
Explanation - DNA polymerase has an exonuclease proofreading activity that corrects mistakes during synthesis.
Correct answer is: Proofreading by DNA polymerase
Q.19 What is the consequence of a frameshift mutation for the resulting protein?
Same protein
Shortened protein
Random amino acids
No change
Explanation - Frameshift changes every amino acid downstream, often producing a non‑functional protein.
Correct answer is: Random amino acids
Q.20 Which of these is a factor that can increase mutation rates in cells?
Low temperature
DNA repair proteins
UV radiation
Calcium ions
Explanation - UV light can damage DNA, leading to higher mutation frequencies if not repaired.
Correct answer is: UV radiation
Q.21 Which type of DNA damage is repaired by nucleotide excision repair?
Single‑base mismatches
Large helix‑distorting lesions
Broken phosphodiester bonds
Methylated bases
Explanation - NER removes bulky distortions such as thymine dimers that block replication.
Correct answer is: Large helix‑distorting lesions
Q.22 What role does the protein Rad51 play in DNA repair?
Ligates DNA ends
Helices unwinding
Facilitates homologous recombination
Recognizes mismatches
Explanation - Rad51 promotes strand invasion during homologous recombination repair.
Correct answer is: Facilitates homologous recombination
Q.23 A mutation that does not alter the protein but changes a single nucleotide in a non‑coding region is called?
Silent
Missense
Nonsense
Frameshift
Explanation - Silent mutations alter DNA but still encode the same amino acid or affect non‑coding DNA.
Correct answer is: Silent
Q.24 Which DNA repair enzyme directly cuts the damaged DNA strand at the site of damage?
DNA glycosylase
DNA endonuclease
DNA polymerase
DNA ligase
Explanation - Endonucleases make cuts on both strands to remove damaged sections.
Correct answer is: DNA endonuclease
Q.25 Why is DNA repair essential for healthy organisms?
To grow faster
To prevent cancer
To change color
To taste sweet
Explanation - Repair mechanisms reduce harmful mutations that can lead to uncontrolled cell growth.
Correct answer is: To prevent cancer
Q.26 Which of the following best describes a point mutation?
A large DNA segment is duplicated
A single base pair is altered
Multiple bases are deleted
The entire chromosome is lost
Explanation - Point mutations involve changes at a single nucleotide location.
Correct answer is: A single base pair is altered
Q.27 Which DNA repair pathway is primarily responsible for correcting UV‑induced thymine dimers?
Mismatch repair
Photoreactivation
Base excision repair
Non‑homologous end joining
Explanation - Photolyase repairs thymine dimers using light energy.
Correct answer is: Photoreactivation
Q.28 During homologous recombination, which protein is essential for strand invasion?
Ku70/80
Rad51
MutL
Ligase IV
Explanation - Rad51 facilitates the search for homology and strand pairing in HR.
Correct answer is: Rad51
Q.29 What is the primary difference between Nucleotide Excision Repair (NER) and Base Excision Repair (BER)?
NER repairs single base mismatches; BER repairs bulky lesions
NER removes large helix distortions; BER removes small base modifications
NER operates only in prokaryotes; BER in eukaryotes
NER uses helicase; BER uses polymerase
Explanation - NER deals with bulky lesions; BER targets small, non‑distorting base changes.
Correct answer is: NER removes large helix distortions; BER removes small base modifications
Q.30 Which enzyme initiates the Base Excision Repair pathway?
DNA polymerase β
DNA glycosylase
Ligase
Endonuclease III
Explanation - DNA glycosylase removes the damaged base, creating an AP site.
Correct answer is: DNA glycosylase
Q.31 A nonsense mutation leads to a truncated protein because it introduces a?
Start codon
Stop codon
Amino‑acid codon
Non‑coding region
Explanation - A premature stop codon halts translation, producing a shortened protein.
Correct answer is: Stop codon
Q.32 Which protein complex is involved in the initial recognition of double‑strand breaks for non‑homologous end joining?
Ku70/80
Rad51
MutS
Ligase IV
Explanation - Ku proteins bind broken DNA ends to initiate NHEJ.
Correct answer is: Ku70/80
Q.33 Which of these is a characteristic of frameshift mutations?
They always cause a premature stop codon
They change the reading frame for all downstream codons
They are always silent
They do not affect protein function
Explanation - Frameshifts alter the codon reading frame, often changing the entire protein sequence.
Correct answer is: They change the reading frame for all downstream codons
Q.34 In mismatch repair, which enzyme in bacteria binds mismatched base pairs to trigger repair?
MutS
MutL
UvrA
Rad51
Explanation - MutS recognizes mismatches and initiates the repair pathway.
Correct answer is: MutS
Q.35 Which of the following best explains why DNA polymerase has proofreading activity?
To add extra nucleotides
To remove mismatched bases after incorporation
To create DNA loops
To separate strands
Explanation - Proofreading uses the 3’→5’ exonuclease activity to correct errors during synthesis.
Correct answer is: To remove mismatched bases after incorporation
Q.36 Which type of mutation can be most easily corrected by the mismatch repair system?
Large deletions
Single base pair mispairings
Insertions of entire genes
Chromosomal translocations
Explanation - Mismatch repair targets small base mismatches and single‑nucleotide loops.
Correct answer is: Single base pair mispairings
Q.37 What is the role of Ligase IV in DNA repair?
Helicase unwinding DNA
Repairing thymine dimers
Sealing DNA breaks after NHEJ
Recognizing mismatches
Explanation - Ligase IV joins DNA ends during non‑homologous end joining.
Correct answer is: Sealing DNA breaks after NHEJ
Q.38 Which environmental factor can lead to increased point mutations in DNA?
High temperature
UV radiation
Low pH
High oxygen concentration
Explanation - UV light forms thymine dimers and other lesions that become point mutations if not repaired.
Correct answer is: UV radiation
Q.39 In the BER pathway, what enzyme fills the gap after the damaged base is removed?
DNA polymerase β
Ligase III
Ku70
Rad51
Explanation - Pol β inserts the correct nucleotides during base excision repair.
Correct answer is: DNA polymerase β
Q.40 Which mutation type is most likely to result in a silent change in the protein?
Missense
Nonsense
Silent
Frameshift
Explanation - Silent mutations change DNA but preserve the amino acid sequence.
Correct answer is: Silent
Q.41 Which DNA repair system is crucial for repairing inter‑strand crosslinks?
NER
Homologous recombination
Mismatch repair
BER
Explanation - HR uses a sister chromatid to repair crosslinks that block replication.
Correct answer is: Homologous recombination
Q.42 What is the primary consequence of a frameshift mutation during protein synthesis?
Alteration of downstream amino acids
Creation of a new start codon
Stabilization of mRNA
No effect
Explanation - Frameshift changes the reading frame, altering all subsequent codons.
Correct answer is: Alteration of downstream amino acids
Q.43 Which of the following best describes the function of the DNA glycosylase enzyme?
Adds nucleotides during replication
Removes damaged bases from DNA
Seals DNA strands
Unwinds DNA helix
Explanation - DNA glycosylase initiates BER by cleaving the N‑glycosidic bond of a damaged base.
Correct answer is: Removes damaged bases from DNA
Q.44 In non‑homologous end joining, what is the role of the protein XRCC4?
Ligates DNA ends
Helicase activity
Recognition of mismatches
Unwinds DNA
Explanation - XRCC4 works with Ligase IV to join broken DNA ends.
Correct answer is: Ligates DNA ends
Q.45 Which mutation type changes a codon to encode a different amino acid?
Missense
Nonsense
Silent
Frameshift
Explanation - Missense mutations alter the codon, leading to a different amino acid in the protein.
Correct answer is: Missense
Q.46 Why do organisms have multiple DNA repair pathways?
To increase mutation rate
To repair different types of DNA damage
To produce more DNA
To slow down replication
Explanation - Different pathways target specific lesions to maintain genomic integrity.
Correct answer is: To repair different types of DNA damage
Q.47 Which DNA repair process requires a homologous DNA template to restore accuracy?
Non‑homologous end joining
Base excision repair
Homologous recombination
Photoreactivation
Explanation - HR uses an identical DNA sequence to guide accurate repair.
Correct answer is: Homologous recombination
Q.48 Which enzyme is responsible for cutting the damaged DNA strand in the NER pathway?
UvrABC endonuclease
DNA polymerase
Ligase
Ku70/80
Explanation - UvrABC complex introduces double‑strand cuts flanking the lesion.
Correct answer is: UvrABC endonuclease
Q.49 Which mutation is most likely to produce a protein with a completely new amino acid sequence after the point of mutation?
Silent
Missense
Frameshift
Nonsense
Explanation - Frameshift changes the downstream sequence for all codons, drastically altering the protein.
Correct answer is: Frameshift
Q.50 Which protein is essential for the recognition of single‑base mismatches during bacterial mismatch repair?
MutS
MutL
UvrA
Ligase IV
Explanation - MutS is the mismatch sensor in prokaryotic cells.
Correct answer is: MutS
Q.51 What is the main function of the DNA polymerase in BER?
Insert the correct nucleotide after excision
Seal DNA ends
Unwind DNA strands
Recognize mismatches
Explanation - Pol β fills the gap created by excision of a damaged base.
Correct answer is: Insert the correct nucleotide after excision
Q.52 Which of the following best describes a nonsense mutation?
A silent change
An error that stops protein synthesis prematurely
An insertion of bases
A deletion of a gene
Explanation - Nonsense mutations introduce a premature stop codon, truncating the protein.
Correct answer is: An error that stops protein synthesis prematurely
Q.53 What is the consequence of a failure in the mismatch repair system in humans?
Increased susceptibility to cancers
Decreased cell division speed
Improved DNA replication fidelity
Lower mutation rates
Explanation - Defective MMR leads to microsatellite instability and higher cancer risk.
Correct answer is: Increased susceptibility to cancers
Q.54 Which repair pathway specifically repairs double‑strand breaks using a sister chromatid as a template?
Non‑homologous end joining
Homologous recombination
Base excision repair
Photoreactivation
Explanation - HR uses an identical DNA sequence to accurately restore the break.
Correct answer is: Homologous recombination
Q.55 In which DNA repair mechanism is the enzyme UvrABC involved?
Nucleotide excision repair
Base excision repair
Mismatch repair
Non‑homologous end joining
Explanation - UvrABC recognizes and excises bulky DNA lesions in the NER pathway.
Correct answer is: Nucleotide excision repair
Q.56 Which protein complex initiates the recognition of DNA double‑strand breaks in eukaryotes?
Ku70/Ku80
MutS/MutL
UvrA/B/C
Rad51
Explanation - Ku binds DNA ends to recruit other NHEJ factors.
Correct answer is: Ku70/Ku80
Q.57 Which of the following best explains the role of DNA polymerase η in translesion synthesis?
It stalls replication at lesions
It bypasses lesions by inserting nucleotides opposite damaged bases
It repairs double‑strand breaks
It excises mismatched bases
Explanation - Pol η allows replication to continue past UV lesions, though often error‑prone.
Correct answer is: It bypasses lesions by inserting nucleotides opposite damaged bases
Q.58 Which type of mutation is most likely to be lethal during early development?
Missense
Nonsense
Silent
Frameshift
Explanation - Nonsense mutations can truncate essential proteins, leading to developmental failures.
Correct answer is: Nonsense
Q.59 Which DNA repair pathway involves the removal of a damaged base, followed by DNA synthesis and ligation?
Base excision repair
Mismatch repair
Non‑homologous end joining
Photoreactivation
Explanation - BER removes a single base and fills in the gap with correct nucleotides.
Correct answer is: Base excision repair
Q.60 What is the primary function of the DNA glycosylase in BER?
To bind mismatched base pairs
To excise damaged bases
To seal nicks
To unwind DNA
Explanation - Glycosylase initiates BER by cleaving the damaged base from the sugar‑phosphate backbone.
Correct answer is: To excise damaged bases
Q.61 Which of the following enzymes is critical for the ligation step of non‑homologous end joining?
DNA polymerase β
Ligase IV
Ku70/80
Rad51
Explanation - Ligase IV seals the DNA ends during NHEJ.
Correct answer is: Ligase IV
Q.62 Which repair pathway is most active in the S phase of the cell cycle?
Homologous recombination
Nucleotide excision repair
Base excision repair
Mismatch repair
Explanation - HR uses a sister chromatid available during S and G2 phases.
Correct answer is: Homologous recombination
Q.63 The protein XRCC1 is most closely associated with which DNA repair process?
Base excision repair
Homologous recombination
Non‑homologous end joining
Mismatch repair
Explanation - XRCC1 acts as a scaffold in BER, coordinating repair enzymes.
Correct answer is: Base excision repair
Q.64 Which mutation leads to an amino acid substitution that can affect protein folding?
Nonsense
Missense
Silent
Insertion
Explanation - Missense mutations change a codon, potentially altering the protein's structure.
Correct answer is: Missense
Q.65 Which of the following is an example of an environmental mutagen?
UV light
Water
Heat
Oxygen
Explanation - UV light causes DNA lesions that can become mutations if unrepaired.
Correct answer is: UV light
Q.66 What is the consequence of a defect in the DNA polymerase η gene?
Increased UV sensitivity
Reduced replication speed
Improved DNA repair
Enhanced mismatch repair
Explanation - Pol η is involved in translesion synthesis; defects lead to UV‑sensitive skin cancers.
Correct answer is: Increased UV sensitivity
Q.67 Which protein complex recognizes and binds to double‑strand breaks for NHEJ?
Ku70/Ku80
Rad51
MutS
UvrA
Explanation - Ku proteins bind DNA ends and recruit the NHEJ machinery.
Correct answer is: Ku70/Ku80
Q.68 Which type of DNA damage is repaired by the photolyase enzyme?
Oxidative base damage
Thymine dimers
Double‑strand breaks
Single‑strand nicks
Explanation - Photolyase repairs UV‑induced thymine dimers via photoreactivation.
Correct answer is: Thymine dimers
Q.69 Which repair pathway uses a short DNA oligonucleotide as a template to fill a gap?
Homologous recombination
Non‑homologous end joining
Base excision repair
Nucleotide excision repair
Explanation - BER uses a short stretch of DNA to replace the excised base.
Correct answer is: Base excision repair
Q.70 What is the hallmark of a frameshift mutation?
Change in the reading frame from the mutation point onward
No change in protein sequence
Replacement of one amino acid
Premature stop codon
Explanation - A frameshift alters the codon reading frame, affecting all downstream codons.
Correct answer is: Change in the reading frame from the mutation point onward
Q.71 Which enzyme removes the sugar‑phosphate backbone from the AP site during BER?
AP endonuclease
DNA polymerase β
Ligase
MutS
Explanation - AP endonuclease cleaves the backbone 5′ to the AP site, creating a nick.
Correct answer is: AP endonuclease
Q.72 Which of the following best describes the role of Rad51 in homologous recombination?
Unwinds DNA helices
Seals DNA breaks
Promotes strand invasion using a homologous template
Recognizes mismatched base pairs
Explanation - Rad51 facilitates the exchange of strands during HR repair.
Correct answer is: Promotes strand invasion using a homologous template
Q.73 Which repair mechanism is primarily responsible for correcting oxidative base lesions such as 8‑oxoG?
Base excision repair
Nucleotide excision repair
Mismatch repair
Non‑homologous end joining
Explanation - BER removes oxidative base lesions using glycosylases specific for such damage.
Correct answer is: Base excision repair
Q.74 In the context of DNA repair, what does the term 'error‐free repair' mean?
Repair that introduces no new mutations
Repair that removes all DNA
Repair that slows replication
Repair that increases mutation rate
Explanation - Error‑free repair restores the original sequence without introducing new errors.
Correct answer is: Repair that introduces no new mutations
Q.75 Which protein is a key component of the eukaryotic NHEJ pathway and assists in the alignment of DNA ends?
Ku70/Ku80
XRCC4
Rad51
MutS
Explanation - XRCC4, with Ligase IV, aligns and ligates broken ends during NHEJ.
Correct answer is: XRCC4
Q.76 What is the typical outcome when mismatch repair fails in microsatellite regions?
Microsatellite stability
Microsatellite instability
Increased DNA replication speed
Reduced mutation rate
Explanation - Failure leads to expansions or contractions of repetitive sequences.
Correct answer is: Microsatellite instability
Q.77 Which enzyme initiates the base excision repair of oxidized guanine (8‑oxoG)?
Ogg1
MutS
UvrA
Ligase IV
Explanation - Ogg1 glycosylase removes 8‑oxoG from DNA.
Correct answer is: Ogg1
Q.78 Which of the following best describes the role of the DNA polymerase ε in eukaryotic DNA repair?
It synthesizes leading strand during replication
It repairs mismatched bases
It ligates DNA ends
It excises damaged bases
Explanation - Pol ε is involved in leading‑strand synthesis and participates in repair during replication.
Correct answer is: It synthesizes leading strand during replication
Q.79 In homologous recombination, which protein is responsible for the search of homologous DNA sequence?
Rad51
Ku70/80
MutS
Ligase IV
Explanation - Rad51 forms nucleoprotein filaments that scan for homologous sequences.
Correct answer is: Rad51
Q.80 Which DNA repair pathway is most error‑prone during the processing of lesions that block replication fork progression?
Translesion synthesis
Homologous recombination
Base excision repair
Nucleotide excision repair
Explanation - TTS allows replication across lesions but often introduces errors.
Correct answer is: Translesion synthesis
Q.81 Which of the following statements is true regarding the non‑homologous end joining (NHEJ) pathway?
It requires a homologous template
It is highly accurate
It often results in small insertions/deletions
It repairs only single‑strand breaks
Explanation - NHEJ can be error‑prone, leading to indels at repair sites.
Correct answer is: It often results in small insertions/deletions
Q.82 Which DNA repair mechanism is primarily active in prokaryotes to fix mismatches after DNA replication?
Mismatch repair
Non‑homologous end joining
Base excision repair
Photoreactivation
Explanation - Bacterial mismatch repair (MutS/MutL) corrects replication errors.
Correct answer is: Mismatch repair
Q.83 Which enzyme is responsible for removing UV‑induced thymine dimers in bacteria?
UvrABC complex
Ligase IV
Rad51
MutS
Explanation - UvrABC excises thymine dimers in the bacterial NER pathway.
Correct answer is: UvrABC complex
Q.84 What is the primary outcome of a defective DNA polymerase proofreading activity?
Reduced mutation rate
Increased mismatch repair
Higher frequency of replication errors
Improved DNA repair fidelity
Explanation - Loss of proofreading increases the error rate during DNA synthesis.
Correct answer is: Higher frequency of replication errors
Q.85 Which DNA repair protein is specifically involved in the recognition of base damage in eukaryotic cells?
XPC
Ku70
MutS
Rad51
Explanation - XPC recognizes helix distortion in the NER pathway.
Correct answer is: XPC
Q.86 Which type of DNA repair mechanism would most likely correct a single base mutation caused by cytosine deamination?
Base excision repair
Mismatch repair
Nucleotide excision repair
Non‑homologous end joining
Explanation - BER corrects deamination (C to U) by glycosylases like UDG.
Correct answer is: Base excision repair
Q.87 Which repair pathway is utilized to fix DNA lesions that cause replication fork stalling during the S phase?
Translesion synthesis
Homologous recombination
Base excision repair
Non‑homologous end joining
Explanation - TTS allows DNA polymerases to bypass lesions that stall replication.
Correct answer is: Translesion synthesis
Q.88 In the context of cancer genetics, which condition is characterized by defective nucleotide excision repair?
Xeroderma pigmentosum
Cystic fibrosis
Sickle cell anemia
Hemophilia
Explanation - XP patients lack functional NER, leading to high UV sensitivity and cancer risk.
Correct answer is: Xeroderma pigmentosum
Q.89 Which of the following best describes the 'error‑free' nature of homologous recombination?
It introduces no new mutations during repair
It always introduces small insertions
It repairs only single‑strand nicks
It increases mutation rate
Explanation - HR uses a homologous template, restoring the original sequence accurately.
Correct answer is: It introduces no new mutations during repair
Q.90 Which protein complex is responsible for detecting UV-induced DNA damage in yeast?
Rad1-Rad10
UvrABC
XPC
Ku70/80
Explanation - Rad1-Rad10 recognizes and processes damaged sites in eukaryotic NER.
Correct answer is: Rad1-Rad10
Q.91 What is the effect of a missense mutation on a DNA polymerase’s exonuclease domain?
Enhanced proofreading
Loss of proofreading ability
No change
Increased DNA binding
Explanation - Missense in the exonuclease domain can impair error correction during replication.
Correct answer is: Loss of proofreading ability
Q.92 Which DNA repair pathway requires a DNA endonuclease that cleaves 5′ to the damaged site?
Base excision repair
Nucleotide excision repair
Mismatch repair
Non‑homologous end joining
Explanation - BER uses an AP endonuclease to cut 5′ to the abasic site.
Correct answer is: Base excision repair
Q.93 In eukaryotes, which protein is primarily responsible for the recruitment of the NHEJ machinery to DNA double‑strand breaks?
Ku70/80
MutS
XRCC4
Rad51
Explanation - Ku proteins bind ends and recruit other NHEJ factors.
Correct answer is: Ku70/80
Q.94 What is the primary consequence of a defective OGG1 glycosylase in humans?
Increased susceptibility to UV‑induced skin cancers
Reduced risk of colorectal cancer
Improved DNA repair fidelity
Decreased oxidative damage
Explanation - OGG1 removes 8‑oxoG; its deficiency leads to oxidative mutagenesis and cancer.
Correct answer is: Increased susceptibility to UV‑induced skin cancers
Q.95 Which DNA repair process is responsible for repairing crosslinks between DNA strands caused by chemotherapy agents like cisplatin?
Homologous recombination
Nucleotide excision repair
Mismatch repair
Base excision repair
Explanation - HR repairs inter‑strand crosslinks using a sister chromatid.
Correct answer is: Homologous recombination
Q.96 Which of the following is a hallmark of xeroderma pigmentosum patients?
Increased UV‑induced DNA damage repair
Low mutation rate
Resistance to chemotherapy
Inability to repair DNA via NER
Explanation - XP patients have defective NER, leading to high UV sensitivity.
Correct answer is: Inability to repair DNA via NER
Q.97 Which DNA repair system is predominantly error‑free and involves the synthesis of a new DNA strand using a homologous template?
Homologous recombination
Non‑homologous end joining
Base excision repair
Mismatch repair
Explanation - HR uses a homologous DNA sequence to guide accurate repair.
Correct answer is: Homologous recombination
Q.98 What is the consequence of a missense mutation that changes a highly conserved cysteine residue in the active site of DNA polymerase?
Enhanced enzyme activity
Loss of enzymatic activity
No effect
Increased DNA binding affinity
Explanation - Conserved residues are critical for catalytic function; their alteration often abolishes activity.
Correct answer is: Loss of enzymatic activity
Q.99 Which of the following proteins is NOT typically involved in the mismatch repair pathway in eukaryotes?
MSH2
MLH1
RPA
PCNA
Explanation - RPA binds single‑stranded DNA during replication; it is not a core MMR protein.
Correct answer is: RPA
Q.100 Which DNA repair pathway is characterized by the rapid removal of a 5–20 bp DNA fragment surrounding a lesion?
Nucleotide excision repair
Base excision repair
Mismatch repair
Non‑homologous end joining
Explanation - NER excises a short oligonucleotide flanking the damage.
Correct answer is: Nucleotide excision repair
Q.101 In the context of genetic stability, what is the main difference between NHEJ and HR?
NHEJ uses a template; HR does not
HR is error‑prone; NHEJ is accurate
NHEJ can introduce indels; HR is error‑free
HR repairs single‑strand nicks; NHEJ repairs double‑strand breaks
Explanation - HR uses a homologous template; NHEJ ligates ends directly, often causing small indels.
Correct answer is: NHEJ can introduce indels; HR is error‑free
Q.102 Which DNA repair pathway is primarily responsible for repairing UV-induced lesions in mammalian cells?
Nucleotide excision repair
Mismatch repair
Base excision repair
Non‑homologous end joining
Explanation - NER removes bulky UV lesions such as thymine dimers from the DNA.
Correct answer is: Nucleotide excision repair
Q.103 Which enzyme complex is responsible for the initial recognition of DNA lesions in the eukaryotic NER pathway?
XPC-RAD23B
Ku70/80
Rad51
MutS
Explanation - XPC-RAD23B detects helix distortions characteristic of NER substrates.
Correct answer is: XPC-RAD23B
Q.104 What is the main function of the DNA polymerase ζ in translesion synthesis?
Proofreading during replication
Bypassing DNA lesions in a low‑fidelity manner
Ligating DNA ends
Initiating base excision repair
Explanation - Pol ζ is a Y‑family polymerase that tolerates lesions but often introduces mutations.
Correct answer is: Bypassing DNA lesions in a low‑fidelity manner
Q.105 Which of the following proteins is essential for the accurate repair of double‑strand breaks via homologous recombination in eukaryotes?
Rad51
Ku70
Ligase IV
MutS
Explanation - Rad51 mediates strand invasion and exchange during HR repair.
Correct answer is: Rad51
Q.106 Which DNA repair pathway is typically active during the G1 phase of the eukaryotic cell cycle?
Homologous recombination
Non‑homologous end joining
Base excision repair
Mismatch repair
Explanation - NHEJ repairs breaks during G1 when no sister chromatid is available.
Correct answer is: Non‑homologous end joining
Q.107 Which of the following is a hallmark of a defective NER pathway in humans?
Decreased UV sensitivity
Increased mutation rate at repetitive sequences
Increased repair of oxidative base damage
Improved cell survival after UV exposure
Explanation - NER deficiency leads to accumulation of UV‑induced mutations, especially in repetitive DNA.
Correct answer is: Increased mutation rate at repetitive sequences
Q.108 Which protein is involved in the recruitment of DNA polymerase β during base excision repair?
XRCC1
Ku70
Rad51
MutL
Explanation - XRCC1 scaffolds BER enzymes including Pol β.
Correct answer is: XRCC1
Q.109 Which enzyme is responsible for the removal of the 3′–phosphate group after DNA break resection in homologous recombination?
TDP1
Pol β
Ligase IV
Ku80
Explanation - TDP1 removes obstructing 3′–phosphates to allow DNA synthesis.
Correct answer is: TDP1
Q.110 Which of the following best describes the 'error‑free' nature of the base excision repair pathway?
It introduces new mutations during repair
It restores the original base accurately
It only repairs mismatches
It removes entire DNA strands
Explanation - BER excises and replaces damaged bases without altering the rest of the DNA sequence.
Correct answer is: It restores the original base accurately
Q.111 Which protein complex recognizes DNA double‑strand breaks and initiates the non‑homologous end joining pathway?
Ku70/Ku80
Rad51
MutS
Ligase IV
Explanation - Ku proteins bind broken DNA ends and recruit other NHEJ factors.
Correct answer is: Ku70/Ku80
Q.112 Which DNA repair enzyme is directly responsible for cutting the DNA strand 5′ to the AP site in BER?
AP endonuclease 1 (APE1)
DNA polymerase β
Ligase III
Ku80
Explanation - APE1 creates a nick at the 5′ side of the abasic site.
Correct answer is: AP endonuclease 1 (APE1)
Q.113 Which of the following is NOT typically a component of the mismatch repair machinery in human cells?
MSH2
MLH1
PCNA
XRCC1
Explanation - XRCC1 is involved in BER, not MMR.
Correct answer is: XRCC1
Q.114 Which DNA repair pathway is primarily responsible for removing oxidized purines such as 8‑oxoG?
Base excision repair
Nucleotide excision repair
Mismatch repair
Non‑homologous end joining
Explanation - BER uses glycosylases like OGG1 to excise oxidized bases.
Correct answer is: Base excision repair
Q.115 Which protein is involved in the final ligation step of the NHEJ repair pathway?
Ligase IV
Rad51
Ku70
XRCC1
Explanation - Ligase IV seals DNA ends during NHEJ.
Correct answer is: Ligase IV
Q.116 Which type of DNA repair is known to be error‑prone but can rescue replication in the presence of lesions?
Translesion synthesis
Homologous recombination
Base excision repair
Nucleotide excision repair
Explanation - TTS allows replication past lesions but often introduces mutations.
Correct answer is: Translesion synthesis
Q.117 Which of the following proteins is a key component of the human NER complex that recognizes helix distortion?
XPA
XPC
XPG
XPB
Explanation - XPC recognizes bulky helix distortions in the DNA during NER.
Correct answer is: XPC
Q.118 Which DNA repair pathway is specifically used to repair inter‑strand crosslinks in S‑phase?
Homologous recombination
Non‑homologous end joining
Base excision repair
Mismatch repair
Explanation - HR repairs crosslinks using a sister chromatid as a template.
Correct answer is: Homologous recombination
Q.119 What is the consequence of a mutation in the BRCA1 gene concerning DNA repair?
Defective mismatch repair
Defective homologous recombination
Defective base excision repair
Defective non‑homologous end joining
Explanation - BRCA1 is essential for HR; mutations increase breast and ovarian cancer risk.
Correct answer is: Defective homologous recombination
Q.120 Which DNA repair mechanism directly removes 8‑oxoguanine from the genome?
Base excision repair
Nucleotide excision repair
Mismatch repair
Non‑homologous end joining
Explanation - OGG1 glycosylase excises 8‑oxoguanine in BER.
Correct answer is: Base excision repair
Q.121 Which of the following proteins is involved in the synapsis step of homologous recombination?
Rad51
Ku70
Ligase IV
MutS
Explanation - Rad51 mediates strand pairing during HR.
Correct answer is: Rad51
Q.122 In eukaryotic cells, which protein complex initiates the NER pathway by binding to DNA lesions?
TFIIH
Rad51
Ku70/80
MutS
Explanation - TFIIH helicase unwinds DNA during NER to allow lesion excision.
Correct answer is: TFIIH
Q.123 Which DNA repair pathway is responsible for excising bulky adducts induced by chemotherapeutic agents like cisplatin?
Nucleotide excision repair
Base excision repair
Homologous recombination
Non‑homologous end joining
Explanation - NER removes bulky chemical adducts from the DNA strand.
Correct answer is: Nucleotide excision repair
Q.124 Which of the following best describes the primary advantage of the homologous recombination repair pathway?
It is fast but error‑prone
It uses a template for high fidelity
It repairs single‑strand nicks without a template
It does not require a helicase
Explanation - HR uses a homologous sequence, ensuring accurate repair.
Correct answer is: It uses a template for high fidelity
Q.125 Which protein is essential for the initial recognition of single‑strand breaks in the base excision repair pathway?
AP endonuclease 1
DNA polymerase β
Ligase III
XRCC1
Explanation - XRCC1 coordinates BER proteins and recognizes single‑strand breaks.
Correct answer is: XRCC1
Q.126 Which of the following best explains why non‑homologous end joining can lead to small insertions or deletions?
It uses a homologous template
It directly ligates ends without processing
It repairs only single‑strand breaks
It is error‑free
Explanation - NHEJ directly joins ends, which can result in small indels.
Correct answer is: It directly ligates ends without processing