Question 1

Which DNA repair mechanism is primarily responsible for correcting UV-induced thymine dimers in E. coli, and what is the key enzyme involved?

Accepted Answer

Direct repair; photolyase

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, and what type of DNA damage accumulates as a result?

Accepted Answer

Nucleotide excision repair (NER); bulky adducts and thymine dimers

Question 3

Which of the following best describes the molecular steps of base excision repair (BER)?

Accepted Answer

A DNA glycosylase removes the damaged base, an AP endonuclease cuts the backbone, DNA polymerase adds the correct nucleotide, and ligase seals the nick.

Question 4

Which repair mechanism is most likely to introduce mutations during the repair process due to its error-prone nature?

Accepted Answer

Translesion synthesis

Question 5

A double-strand break occurs in a human cell during the G1 phase. Which repair pathway is most likely to be used, and what is a potential consequence?

Accepted Answer

Non-homologous end joining (NHEJ); possible loss of nucleotides at the break site

Question 6

Which of the following is TRUE regarding the recognition of the newly synthesized DNA strand during mismatch repair in E. coli?

Accepted Answer

The new strand is recognized by the absence of methylation at GATC sequences.

Question 7

Which enzyme is responsible for the direct removal of a methyl group from O6-methylguanine in DNA, and what is unique about its mechanism?

Accepted Answer

O6-methylguanine methyltransferase (MGMT); it transfers the methyl group to itself and is inactivated (suicide enzyme)

Question 8

A bulky DNA adduct caused by benzo[a]pyrene is detected in a lung cell. Which repair pathway will most likely remove this lesion, and what is the first enzymatic step?

Accepted Answer

Nucleotide excision repair (NER); a nuclease excises a segment of the damaged strand

Question 9

Which of the following best explains why homologous recombination repair (HRR) is considered an error-free mechanism for double-strand break repair?

Accepted Answer

It uses the undamaged sister chromatid as a template for accurate repair.

Question 10

Which of the following scenarios would most likely result in a crossover event during homologous recombination repair?

Accepted Answer

Both Holliday junctions are resolved in opposite orientations.

Question 11

A DNA polymerase incorporates an incorrect base during replication. Which enzymatic activity allows the polymerase to remove this base and replace it with the correct one?

Accepted Answer

3' to 5' exonuclease activity

Question 12

Which of the following types of DNA damage is most efficiently repaired by base excision repair (BER)?

Accepted Answer

Oxidized bases

Question 13

In eukaryotic cells, how does the mismatch repair system distinguish the newly synthesized strand from the parental strand?

Accepted Answer

By the presence of nicks and discontinuities in the new strand

Question 14

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

Accepted Answer

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

Question 15

A scientist exposes bacterial cells to UV light, causing the formation of cyclobutane pyrimidine dimers. Which repair mechanism can directly reverse this damage, and what is the energy source required?

Accepted Answer

Direct repair by photolyase; visible light (300–600 nm)

Question 16

Which of the following is a possible outcome if a base mismatch is not repaired before the next round of DNA replication?

Accepted Answer

A permanent mutation is introduced into one of the daughter DNA molecules.

Question 17

Which of the following best describes the difference between non-homologous end joining (NHEJ) and homologous recombination repair (HRR) for double-strand break repair?

Accepted Answer

NHEJ can introduce small insertions or deletions, while HRR is generally error-free.

Question 18

Which of the following equations correctly represents the energy required to break a phosphodiester bond during DNA repair?

Accepted Answer

$$\Delta G = -30.5\ \mathrm{kJ/mol}$$

DNA Repair Mechanisms in Genetics

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