Question 1

Which DNA repair mechanism is most likely responsible for the loss of genetic information between direct repeats, leading to diseases such as non-insulin dependent diabetes and Fabry disease?

Accepted Answer

Single-strand annealing (SSA)

Question 2

During homologous recombination, what is the primary function of the 5' resection step?

Accepted Answer

To produce single-stranded 3' overhangs for strand invasion

Question 3

Which protein in eukaryotes is functionally analogous to RecA in bacteria, facilitating strand invasion during homologous recombination?

Accepted Answer

Rad51

Question 4

A patient is found to have a mutation in a gene encoding a DNA repair protein. Which of the following consequences is most likely if the mutation affects a gene involved in double-strand break (DSB) repair?

Accepted Answer

Increased sensitivity to UV irradiation and higher mutation rates

Question 5

Which of the following best describes a Holliday junction?

Accepted Answer

A four-stranded DNA structure formed during homologous recombination

Question 6

Site-specific recombination typically involves which of the following features?

Accepted Answer

Recombination at very short homologous sequences (20-30 bp) recognized by specific recombinases

Question 7

Which equation best represents the probability $P$ of a mutation being inherited by offspring, assuming the mutation occurs in a gamete and is not repaired?

Accepted Answer

$P = 0.5$

Question 8

A thymine dimer is formed in DNA due to UV irradiation. Which repair mechanism is most likely to correct this lesion?

Accepted Answer

Excision repair

Question 9

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

Accepted Answer

Loss of an adenine or guanine base from the DNA backbone

Question 10

In the context of homologous recombination, what is the role of branch migration?

Accepted Answer

To move the crossover point along the DNA duplex, increasing the length of heteroduplex DNA

Question 11

Which of the following statements about DNA repair systems is correct?

Accepted Answer

Multiple repair systems can be triggered by a single type of DNA damage

Question 12

A mutation in a DNA repair gene increases the risk of cancer primarily because:

Accepted Answer

It leads to accumulation of mutations in proto-oncogenes and tumor suppressor genes

Question 13

Which of the following is a key difference between homologous recombination and site-specific recombination?

Accepted Answer

Homologous recombination requires long stretches of sequence homology, while site-specific recombination uses short, defined sequences

Question 14

Which protein complex is responsible for holding double-strand breaks together and preparing them for resection in eukaryotic cells?

Accepted Answer

MRN complex (Mre11-Rad50-Nbs1)

Question 15

If a DNA repair gene is mutated, which of the following is the most likely outcome?

Accepted Answer

Increased mutation rate and higher risk of genetic diseases

Question 16

Which of the following best explains why mutations appear rare despite frequent DNA damage?

Accepted Answer

Efficient DNA repair systems correct most damage before it becomes a mutation

Question 17

Which of the following is a real-world example of a chemical mutagen that can cause bulky DNA adducts, leading to distortion of the DNA structure?

Accepted Answer

Polycyclic aromatic hydrocarbons from cigarette smoke or BBQ

Question 18

Which repair system is most likely to correct a cytosine that has been converted to uracil in DNA?

Accepted Answer

Base excision repair

Question 19

Which of the following steps in homologous recombination directly leads to the formation of heteroduplex DNA?

Accepted Answer

Strand invasion by a single-stranded 3' overhang into a homologous duplex

Question 20

A cell defective in RuvC endonuclease would most likely show defects in which process?

Accepted Answer

Resolution of Holliday junctions during homologous recombination

DNA Recombination II: Mechanisms and Models in Genetics

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