Question 1

Which of the following best describes a tautomeric shift and its consequence during DNA replication?

Accepted Answer

A rare, alternate resonance structure of a nitrogenous base that can result in nonstandard base pairing and incorporation of incorrect nucleotides.

Question 2

A patient is diagnosed with Huntington’s disease, which is associated with trinucleotide repeat expansion. What is the molecular mechanism underlying this disorder?

Accepted Answer

Strand slippage during DNA replication in regions of CAG repeats, resulting in an increased number of repeats.

Question 3

Which chemical mutagen acts by adding ethyl groups to DNA bases, thereby altering their pairing properties?

Accepted Answer

Ethyl methanesulfonate (EMS)

Question 4

What is the primary function of photolyase in DNA repair?

Accepted Answer

It catalyzes the direct reversal of thymine dimers using energy from visible light.

Question 5

During mismatch repair in E. coli, how does the system distinguish the newly synthesized DNA strand from the template strand?

Accepted Answer

By the presence of methylation on the template strand and lack of methylation on the new strand.

Question 6

Which DNA repair pathway is most likely to introduce mutations due to loss of nucleotides at the break site?

Accepted Answer

Nonhomologous end-joining (NHEJ)

Question 7

A scientist exposes cells to UV radiation and observes the formation of covalent bonds between adjacent thymine bases. Which DNA repair mechanism is most likely to correct this damage?

Accepted Answer

Nucleotide excision repair (NER)

Question 8

Which of the following best explains why individuals with xeroderma pigmentosum must avoid sunlight?

Accepted Answer

They have mutations that prevent nucleotide excision repair of UV-induced DNA damage.

Question 9

Which of the following is a direct result of depurination in DNA?

Accepted Answer

Loss of a purine base, creating an abasic (AP) site.

Question 10

A researcher uses 5-bromodeoxyuridine (BrdU) in a cell culture experiment. What is the most likely effect of BrdU incorporation into DNA?

Accepted Answer

BrdU acts as a thymidine analogue and can mispair with guanine, leading to mutations during replication.

Question 11

Which repair pathway involves the removal of a single damaged base followed by replacement with the correct nucleotide?

Accepted Answer

Base excision repair

Question 12

What is the role of the Holliday junction in homologous recombination?

Accepted Answer

It is a crossed structure that allows for branch migration and exchange of genetic material between homologous chromosomes.

Question 13

Which of the following best describes the process of gene conversion during homologous recombination?

Accepted Answer

Repair of DNA without exchange of flanking sequences, resulting in the repaired chromosome having the same sequence as the template.

Question 14

Which enzyme is responsible for sealing nicks in the DNA backbone after repair is completed?

Accepted Answer

DNA ligase

Question 15

A cell is exposed to acridine orange, a known intercalating agent. What type of mutation is most likely to result?

Accepted Answer

Insertions or deletions of nucleotides due to distortion of the DNA double helix.

Question 16

Which repair mechanism is most accurate for repairing double-strand breaks in DNA?

Accepted Answer

Homologous recombination

Question 17

During synthesis-dependent strand annealing (SDSA), what is the source of the template for repair?

Accepted Answer

The sister chromatid provides the homologous sequence for accurate repair.

Question 18

Which of the following is a common consequence of mutations in mismatch repair genes in humans?

Accepted Answer

Increased risk of certain cancers, such as hereditary nonpolyposis colorectal cancer (HNPCC).

Question 19

Which of the following equations best represents the relationship between the number of trinucleotide repeats ($$n) $$and the likelihood of disease onset in trinucleotide repeat disorders?

Accepted Answer

$$P_{disease} \propto n$$

Question 20

Which step in nucleotide excision repair (NER) is responsible for removing the damaged DNA segment?

Accepted Answer

NER endonucleases cut the DNA backbone on either side of the lesion, allowing the damaged segment to be unwound and removed.

DNA Replication, Repair, and Recombination: Mechanisms and Disorders

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