Question 1

Which of the following DNA lesions occurs most frequently in a diploid mammalian cell in 24 hours, according to the provided data?

Accepted Answer

Depurination

Question 2

A patient with extreme sensitivity to UV light and neurological abnormalities is most likely to have a defect in which DNA repair gene(s)?

Accepted Answer

Xeroderma pigmentosum (XP) groups A-G

Question 3

Depurination of DNA results in which of the following outcomes if not repaired before DNA replication?

Accepted Answer

A deletion of a base pair in the new DNA strand

Question 4

Which of the following best describes the chemical change that occurs during cytosine deamination?

Accepted Answer

Cytosine loses an amino group and is converted to uracil

Question 5

A mutation in the Brca2 gene is most strongly associated with which of the following phenotypes?

Accepted Answer

Breast, ovarian, prostate, and pancreatic cancer

Question 6

Which DNA repair mechanism is most accurate for repairing double-strand breaks and uses a sister chromatid as a template?

Accepted Answer

Homologous recombination

Question 7

If a cytosine in DNA is deaminated and not repaired before replication, what is the most likely base pair change in the daughter DNA?

Accepted Answer

A G-C pair is replaced by an A-T pair

Question 8

Which of the following is a direct consequence of UV radiation on DNA structure?

Accepted Answer

Formation of thymine dimers

Question 9

A defect in which DNA repair pathway would most likely result in increased sensitivity to ionizing radiation and chromosomal instability?

Accepted Answer

Non-homologous end joining

Question 10

Which of the following equations best represents the hydrolysis reaction that leads to depurination in DNA?

Accepted Answer

$$\mathrm{Guanine{-}deoxyribose} + H_2O \rightarrow \mathrm{Guanine} + \mathrm{deoxyribose}$$

Question 11

Which of the following best explains why spontaneous DNA damage must be rapidly repaired in cells?

Accepted Answer

To prevent the accumulation of mutations that can lead to diseases such as cancer

Question 12

A patient is diagnosed with a syndrome characterized by premature aging and cancer at several sites. Which DNA repair defect is most likely responsible?

Accepted Answer

Werner syndrome

Question 13

Which of the following is a common feature of homologous recombination in all organisms?

Accepted Answer

Pairing of homologous DNA sequences to guide repair

Question 14

Which of the following DNA repair mechanisms is most likely to introduce small insertions or deletions at the site of repair?

Accepted Answer

Non-homologous end joining

Question 15

A cell is exposed to a chemical that increases the rate of 7-methylguanine formation. Which endogenous process is most likely responsible for repairing this lesion?

Accepted Answer

Base excision repair

Question 16

Which of the following best describes the role of DNA polymerase in DNA repair?

Accepted Answer

Synthesizes new DNA to fill in gaps after damaged DNA is removed

Question 17

A defect in which of the following genes is most likely to result in colon cancer due to mismatch repair deficiency?

Accepted Answer

Msh2, Msh3, Msh6, Mlh1, Pms2

Question 18

Which of the following best explains why individuals with xeroderma pigmentosum are highly sensitive to sunlight?

Accepted Answer

They have a defect in nucleotide excision repair, preventing removal of UV-induced thymine dimers

Question 19

Which of the following is the correct sequence of events in base excision repair?

Accepted Answer

Recognition of damaged base, removal by glycosylase, incision by endonuclease, DNA synthesis, ligation

Question 20

A cell with a defect in homologous recombination is most likely to experience problems during which phase of the cell cycle?

Accepted Answer

S and G2 phases, when sister chromatids are available as templates

DNA Repair: Mechanisms, Lesions, and Human Syndromes

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