Question 1

Which DNA repair pathway is defective in individuals with Xeroderma pigmentosum, leading to increased sensitivity to UV-induced DNA damage?

Accepted Answer

Nucleotide excision repair (NER)

Question 2

A patient presents with skin cancer and is found to have mutations in the XPA-XPG genes. Which process is most likely impaired?

Accepted Answer

Repair of UV-induced thymine dimers

Question 3

Which enzyme is responsible for filling in the gap after excision of damaged DNA during nucleotide excision repair?

Accepted Answer

DNA polymerase

Question 4

Replication errors that escape proofreading and mismatch repair are corrected by which process?

Accepted Answer

Postreplication repair

Question 5

Which protein is essential for strand invasion during homologous recombination repair of double-stranded breaks?

Accepted Answer

RecA

Question 6

Double-stranded breaks in DNA can lead to which of the following outcomes if not properly repaired?

Accepted Answer

Chromosomal rearrangements

Question 7

Which repair pathway is most likely to result in loss of genetic information at the break site?

Accepted Answer

Nonhomologous end-joining (NHEJ)

Question 8

During DNA replication, which enzyme provides proofreading activity to correct errors?

Accepted Answer

DNA polymerase III

Question 9

Which of the following best describes the process of photoreactivation?

Accepted Answer

Direct reversal of thymine dimers using visible light

Question 10

A researcher observes that a cell line accumulates double-stranded breaks but fails to repair them using homologous recombination. Which alternative pathway might the cell use?

Accepted Answer

Nonhomologous end-joining (NHEJ)

Question 11

Which of the following equations represents the probability $$P of a $$double-stranded break occurring during DNA replication, given a mutation rate $$\mu$$ and number of replication events $$n$$?

Accepted Answer

$$P = 1 - e$$^{-\mu n}$$

Question 12

Which repair pathway is most likely to be used when a cell is in late S/G2 phase and a double-stranded break occurs?

Accepted Answer

Homologous recombination

Question 13

A mutation in the gene encoding DNA ligase would most directly affect which step in DNA repair?

Accepted Answer

Sealing of nicks in the DNA backbone

Question 14

Which of the following is NOT a consequence of unrepaired double-stranded DNA breaks?

Accepted Answer

Increased DNA methylation

Question 15

Which repair pathway involves the use of a homologous chromosome as a template for repair?

Accepted Answer

Homologous recombination

Question 16

If a cell is deficient in DNA polymerase III proofreading activity, which type of mutation is most likely to increase?

Accepted Answer

Base mismatches

Question 17

Which of the following steps occurs FIRST during homologous recombination repair of a double-stranded break?

Accepted Answer

Resection of DNA ends to produce single-stranded overhangs

Question 18

A cell exposed to UV light accumulates thymine dimers. Which repair mechanism is most likely to be activated?

Accepted Answer

Nucleotide excision repair

Question 19

Which of the following is a direct reversal repair mechanism for DNA damage?

Accepted Answer

Photoreactivation

Question 20

During which phase of the cell cycle are double-stranded breaks most likely to occur naturally?

Accepted Answer

S phase (DNA replication)

DNA Repair Mechanisms and Double-Stranded Break Repair in Genetics

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