Question 1

Which DNA repair mechanism is primarily responsible for correcting UV-induced thymine dimers in human cells?

Accepted Answer

Nucleotide excision repair (NER)

Question 2

A patient with xeroderma pigmentosum is highly sensitive to sunlight and prone to skin cancer. Which DNA repair pathway is defective in this disorder?

Accepted Answer

Nucleotide excision repair (NER)

Question 3

Which enzyme is responsible for the direct reversal of O6-methylguanine alkylation in DNA?

Accepted Answer

O6-methylguanine methyltransferase (MGMT)

Question 4

During DNA replication, DNA polymerase incorporates an incorrect base. Which activity allows the enzyme to remove this base and replace it with the correct one?

Accepted Answer

3' exonuclease activity

Question 5

Which of the following best describes the first step in base excision repair (BER)?

Accepted Answer

A DNA glycosylase removes the damaged base

Question 6

A bulky adduct forms on guanine due to exposure to cigarette smoke. Which repair mechanism will most likely remove this lesion?

Accepted Answer

Nucleotide excision repair (NER)

Question 7

Which of the following is TRUE regarding Non-Homologous End Joining (NHEJ)?

Accepted Answer

It can result in the loss or addition of nucleotides at the break site

Question 8

In E. coli, how does the mismatch repair system distinguish the newly synthesized DNA strand from the parental strand?

Accepted Answer

By the absence of methylation on the new strand

Question 9

Which of the following steps is unique to nucleotide excision repair (NER) compared to base excision repair (BER)?

Accepted Answer

Excision of a short single-stranded DNA segment containing the lesion

Question 10

A double-strand break occurs in a human cell during the S phase. Which repair mechanism is most likely to be used, and why?

Accepted Answer

Homologous recombination repair (HRR), because a sister chromatid is available as a template

Question 11

Which of the following is a possible outcome of resolving a Holliday junction during homologous recombination repair?

Accepted Answer

Either crossover or non-crossover products

Question 12

Which repair mechanism is most likely to fix a uracil base that appears in DNA due to cytosine deamination?

Accepted Answer

Base excision repair (BER)

Question 13

Which of the following best describes the role of MutL in the mismatch repair system of E. coli?

Accepted Answer

It acts as a molecular bridge coordinating mismatch recognition and strand excision

Question 14

A patient is found to have a mutation that prevents the formation of single-stranded overhangs during double-strand break repair. Which step of homologous recombination repair is directly affected?

Accepted Answer

End resection

Question 15

Which of the following is a key difference between direct repair and base excision repair (BER)?

Accepted Answer

BER involves cleavage of the DNA backbone, while direct repair does not

Question 16

If mismatch repair (MMR) is nonfunctional in a cell, what is the most likely consequence?

Accepted Answer

Accumulation of base-pair substitutions after replication

Question 17

Which of the following best explains why homologous recombination repair (HRR) is considered more accurate than non-homologous end joining (NHEJ)?

Accepted Answer

HRR uses a homologous sequence as a template, minimizing errors

Question 18

Which repair pathway is most likely to be error-prone and lead to mutations during DNA replication?

Accepted Answer

Translesion synthesis

Question 19

A scientist exposes cells to a chemical that causes depurination. Which repair mechanism will most likely be activated to fix this damage?

Accepted Answer

Base excision repair (BER)

Question 20

Which of the following is the correct sequence of events in nucleotide excision repair (NER)?

Accepted Answer

Nuclease excises lesion → DNA polymerase synthesizes new DNA → DNA ligase seals the gap

DNA Repair Mechanisms in Genetics

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