This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Overview of DNA Repair”.
1. What is the correct definition of excision repair?
a) Repair of a single damaged nucleotide
b) Repair of a damaged oligonucleotide
c) Removal of a single damaged nucleotide
d) Removal of a damaged oligonucleotide
View Answer
Explanation: In the excision repair damaged nucleotide is not repaired but removed from the DNA. The other undamaged strand serves as the template for reincorporation of the correct nucleotide by DNA polymerase.
2. How many types of excision repair systems are known?
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: There are two types of excision repair systems. One involves the removal of only the damaged nucleotide. The other involves the removal of a short stretch of single stranded DNA containing the lesion.
3. Why recombinational repair system is called double strand break repair?
a) Both strands are broken
b) One strand is broken
c) No strand is broken
d) Both strand act as template
View Answer
Explanation: Recombinational repair system is used when both strands are damaged such as when the DNA is broken. In such situation one strand cannot serve as a template for the repair of the other.
4. Which enzyme is activated during double stranded break?
a) DNA polymerase
b) Translesional polymerase
c) RNA polymerase
d) Klenow fragment
View Answer
Explanation: In double strand break repair when damaged bases block progression of a replicating DNA polymerase a special translesional polymerase copies the bases across the damaged site. This process does not depend on base pairing between the template and the newly synthesized DNA strand.
5. What is the main enzyme that plays a major role in formation of thymine dimer?
a) DNA glycosylase
b) DNA photolyase
c) DNA gyrase
d) DNA ligase
View Answer
Explanation: In photoreactivation DNA photolyase captures energy from light and uses it to break the covalent bond linking the adjacent pyrimidines. In this process the damaged bases are mended directly.
6. Which two Uvr component molecules scan the DNA during nucleotide excision repair?
a) UvrC, UvrA
b) UvrA, UvrB
c) UvrB, UvrC
d) UvrD, UvrA
View Answer
Explanation: In nucleotide excision repair complex of two UvrA and UvrB molecules scan the DNA with two UvrA subunits being responsible for detecting the distortions to the helix. Upon encountering a distortion, UvrA exits the complex and create a single stranded bubble with UvrB.
7. After creation of the bubble which Uvr component are recruited?
a) Uvr B
b) Uvr A
c) Uvr C
d) Uvr D
View Answer
Explanation: After creation of the single stranded bubble Uvr B recruits Uvr C. Uvr C creates two incisions in the double stranded DNA.
8. Which complex in E. coli performs in translesion synthesis?
a) DNA polymerase IV, DNA polymerase V
b) DNA polymerase II, DNA polymerase III
c) DNA polymerase III, DNA polymerase IV
d) DNA polymerase V, RNA polymerase I
View Answer
Explanation: DNA polymerase IV (Din B) and DNA polymerase V (Umv C) performs translesional synthesis. Din B and Umv C are members of a distinct family of DNA polymerases found in many organisms known as the family of DNA polymerases.
9. In humans which DNA polymerase is involved?
a) DNA polymerase ƞ
b) DNA polymerase α
c) DNA polymerase β
d) DNA polymerase ν
View Answer
Explanation: DNA polymerase ƞ correctly inserts two A residues opposite a thymine dimer. The active site of DNA polymerase ƞ is better at accommodating a thymine dimer than is the active site of another translesional DNA polymerase.
10. The genes encoding the translesional polymerases are expressed as part of a pathway known as the SOS response.
a) True
b) False
View Answer
Explanation: The gene encoding the translesional polymerases are expressed as a part of a repair mechanism known as the SOS pathway. The damage leads to the proteolytic destruction of a transcriptional repressor.
11. In eukaryotes the other name of PCNA is __________
a) Sliding clamp
b) Sliding clamp loader
c) Replicative DNA polymerase
d) Translesional DNA polymerase
View Answer
Explanation: Sliding clamp which is known as PCNA in eukaryotes, anchors the replicative polymerase to the DNA template. The chemical modification is the covalent attachment to the sliding clamp of a peptide known as ubiquitin in a process known as ubiquitinisation.
12. In SOS repair system cleavage of LexA and UmuD is mediated by ___________
a) RecB
b) RecA
c) RecC
d) UvrA
View Answer
Explanation: LexA and UmuD complex is cleaved by a protein called RecA. This process is activated by single stranded DNA resulting from DNA damage.
13. What are the two fundamental components of NHEJ?
a) KU70, KU80
b) KU50, KU70
c) KU40, KU50
d) KU30, KU40
View Answer
Explanation: KU70 and KU80 are the most fundamental components of NHEJ. They constitute heterodimers that binds to the DNA ends.
14. NHEJ is ubiquitin independent in eukaryotic organisms.
a) True
b) False
View Answer
Explanation: NHEJ mechanism is ubiquitin dependent in eukaryotic organisms. The occurrence of NHEJ in prokaryotic cells is less frequent and is predominantly found in eukaryotic cells.
15. In the following compounds which are involved in nucleotide excision repair?
a) UvrA, UvrB, UvrC
b) UvrC, UvrD
c) Uvr C, XPA, XPD
d) XPA, XPC, XPD, XPG
View Answer
Explanation: In the nucleoside excision repair, damage occurs in pyrimidine dimer. In humans XPA, XPC, XPD and XPG are activated to repair the damage.
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