Question 1

Which DNA repair mechanism is most likely to introduce mutations during the repair of double-strand breaks in mammalian cells?

Accepted Answer

Non-homologous end joining (NHEJ)

Question 2

A patient with Xeroderma Pigmentosum (XP) is highly sensitive to sunlight and prone to skin cancers. Which DNA repair pathway is defective in this condition?

Accepted Answer

Nucleotide excision repair

Question 3

Which enzyme is responsible for photoreactivation repair of pyrimidine dimers in bacteria, but is absent in mammals?

Accepted Answer

Photolyase

Question 4

During mismatch repair in eukaryotes, how is the newly synthesized DNA strand distinguished from the template strand?

Accepted Answer

By the presence of nicks in the new strand

Question 5

Which repair pathway allows DNA polymerase to synthesize DNA across a damaged template without using a template, and is considered error-prone?

Accepted Answer

Translesion synthesis

Question 6

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

Accepted Answer

Homologous recombination repair, because a sister chromatid is present as a template

Question 7

Which of the following is a common feature of excision repair systems?

Accepted Answer

They remove the damaged region and resynthesize DNA using the complementary strand as a template

Question 8

Which step is NOT part of the nucleotide excision repair (NER) process in E. coli?

Accepted Answer

Direct reversal of thymine dimers by photolyase

Question 9

A mutation causes a base conversion in DNA, but does not grossly distort the overall structure. Which repair system is most likely to detect and correct this error?

Accepted Answer

Mismatch excision repair

Question 10

Which formula best represents the probability $P$ of a mutation being passed to daughter cells if a single-strand DNA lesion is not repaired before replication?

Accepted Answer

$P = 0.5$

Question 11

Which repair pathway is most likely to be used for the removal of uracil from DNA, and what is the first step?

Accepted Answer

Base excision repair; removal of the damaged base by a glycosylase

Question 12

Which of the following statements about homologous recombination repair (HRR) is TRUE?

Accepted Answer

HRR uses a homologous chromosome or sister chromatid as a template for repair

Question 13

Which repair system is most likely to be activated when RNA polymerase stalls at a lesion during transcription?

Accepted Answer

Transcription-coupled nucleotide excision repair

Question 14

Which of the following is NOT a step in the general excision repair pathway?

Accepted Answer

Synthesis of RNA using the damaged strand as a template

Question 15

Which repair pathway is most likely to be used for the removal of bulky DNA adducts caused by UV irradiation?

Accepted Answer

Nucleotide excision repair

Question 16

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

Accepted Answer

BER removes a single base, while NER removes a stretch of nucleotides

Question 17

Which repair system is most likely to be used when DNA damage does not block replication or transcription, but results in a mispairing?

Accepted Answer

Mismatch excision repair

Question 18

Which protein complex is responsible for the initial recognition of DNA damage in the nucleotide excision repair pathway in humans?

Accepted Answer

XPC complex

Question 19

Which repair pathway is most likely to be used for the repair of single-strand breaks in DNA?

Accepted Answer

Direct ligation

Question 20

Which equation best describes the number of possible repair outcomes $N$ for a double-strand break (DSB) in a diploid cell, given two homologous chromosomes and two possible repair pathways (HRR and NHEJ)?

Accepted Answer

$N = 4$

DNA Repair Mechanisms: Structure, Types, and Pathways

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