Question 1

Which DNA repair mechanism is primarily responsible for correcting UV-induced thymine dimers in human skin cells, and what is the key enzyme involved?

Accepted Answer

Nucleotide excision repair (NER); UvrABC endonuclease

Question 2

A patient with xeroderma pigmentosum is extremely sensitive to sunlight and develops skin cancers at a young age. Which DNA repair pathway is defective in this disorder?

Accepted Answer

Nucleotide excision repair (NER)

Question 3

During DNA replication, a G-T mismatch occurs. In E. coli, how does the mismatch repair (MMR) system distinguish the newly synthesized strand from the parental strand?

Accepted Answer

By the absence of methylation on the new strand

Question 4

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

Accepted Answer

A DNA glycosylase removes the damaged base, creating an abasic site

Question 5

A double-strand break (DSB) in DNA can be repaired by two major mechanisms. Which statement correctly contrasts Non-Homologous End Joining (NHEJ) and Homologous Recombination Repair (HRR)?

Accepted Answer

NHEJ can occur at any cell cycle phase, while HRR requires a sister chromatid and occurs in S/G2 phase

Question 6

Which type of DNA damage is most likely to be repaired by direct reversal (direct repair) rather than excision and resynthesis?

Accepted Answer

Thymine dimers in bacteria exposed to visible light

Question 7

A researcher exposes cells to a chemical that causes alkylation of guanine, producing O6-methylguanine. Which enzyme directly reverses this damage, and what is unique about its mechanism?

Accepted Answer

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

Question 8

Which of the following is a real-life consequence of defective mismatch repair (MMR) in humans?

Accepted Answer

Increased risk of hereditary nonpolyposis colorectal cancer (HNPCC)

Question 9

In the process of homologous recombination repair (HRR), what is the function of the Holliday junction?

Accepted Answer

It is a cross-shaped DNA structure that allows for strand exchange between homologous DNA molecules

Question 10

A cell is exposed to UV light, resulting in the formation of pyrimidine dimers. If the cell lacks both photolyase and nucleotide excision repair (NER) enzymes, what is the most likely outcome?

Accepted Answer

The dimers will persist, blocking DNA replication and increasing mutation risk

Question 11

Which of the following best explains why translesion synthesis is considered error-prone?

Accepted Answer

It bypasses DNA lesions by inserting random nucleotides opposite the damage

Question 12

A DNA molecule has a bulky adduct attached to a guanine base, distorting the helix. Which repair pathway is most likely to be activated, and what is the first enzymatic step?

Accepted Answer

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

Question 13

Which of the following equations correctly represents the formation of a phosphodiester bond during DNA synthesis?

Accepted Answer

$$(dNMP)_n + dNTP \rightarrow (dNMP)_{n+1} + PP_i$$

Question 14

A patient is found to have a mutation in the gene encoding DNA ligase. Which step in DNA repair would be most directly affected?

Accepted Answer

Sealing of the final phosphodiester bond after gap filling

Question 15

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

Accepted Answer

Resolution of the two Holliday junctions in different orientations

Question 16

Which of the following best describes the role of MutL in the bacterial mismatch repair (MMR) system?

Accepted Answer

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

Question 17

A cell is in the G1 phase of the cell cycle and experiences a double-strand break. Which repair pathway is most likely to be used, and why?

Accepted Answer

Non-homologous end joining (NHEJ), because no sister chromatid is present

Question 18

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

Accepted Answer

BER removes a single nucleotide, while NER removes a short stretch of nucleotides

Question 19

A DNA polymerase with defective 3' to 5' exonuclease activity would most likely result in which of the following?

Accepted Answer

Increased frequency of replication errors due to loss of proofreading

Question 20

Which of the following best explains why the mismatch repair (MMR) system in eukaryotes targets the newly synthesized DNA strand?

Accepted Answer

The new strand contains nicks and discontinuities

DNA Repair Mechanisms: A Comprehensive Study Guide for Genetics Students

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