This set of Cytogenetics Multiple Choice Questions on “Repair of DNA Damage: MMR, BER – 1”.
1. Which of the following modes of repair is restricted to S phase of the cell cycle?
A. BER
B. MMR
C. NER
D. DSB
Answer: B
Clarification: MMR/mis-match repair system concerns the wrong bases incorporated by polymerase error while replication. It has to distinguish the parent and daughter strand for this repair. Since after replication completion and during division both the stands are indistinguishable, it is restricted to S phase.
2. DNA microsatellites are prone to which type of mutation?
A. Transition
B. Transversion
C. Addition
D. Methylation
Answer: C
Clarification: DNA microsatellites have several repetitive sequences within it, thus these regions are prone to polymerase slippage that can lead to insertion or deletion of bases.
3. In an experiment you make two setups, in 1st you knock down the DAM and in 2nd you over express DAM. What will be the observation for both the setups?
A. In 1st case BER is compromised and in 2nd MMR is compromised
B. In 1st case MMR is compromised, while in 2nd MMR is more accurate
C. In both cases MMR is compromised
D. In both cased BER is compromised
Answer: C
Clarification: DAM knockdown leads to no methylation, and then both strands are indistinguishable as both are not methylated. While in DAM over expression both are methylated at a shorter window giving lesser time for MMR to work. Thus, the MMR in either case is compromised.
4. Which of the following factors in MMR can recognize the mismatch?
A. Mut L
B. Mut S
C. Mut H
D. XRCC1
Answer: B
Clarification: Mut S is responsible for recognizing the mismatch and then it complexed with Mut L which helps it to bind to the nearest Mut H and carry out the repair process.
5. If the nick is at 5’ end of the mismatch which of the following will be recruited?
A. Rec J
B. Ruv D
C. Exonuclease I
D. Exonuclease X
Answer: D
Clarification: Rec J is an exonuclease which is also known as exonuclease VII. It is responsible for chewing the DNA from 5’ to 3’ direction. While exonuclease I and X chew from 3’ to 5’ direction which will not be of much use in this case.
6. Which MMR protein homologue has not yet been discovered in eukaryotes?
A. Mut L
B. Mut H
C. Mut S
D. Rec J
Answer: B
Clarification: While Mut S homologue (MSH) and Mut L homologue (MLH) has been discovered in eukaryotes, the Mut H homologue is not found. The nick in the lagging strand is expected to be provided by the unligated okazaki fragments.
7. Which of the following disease in humans is due to the error in MMR?
A. Lynch syndrome
B. Xeroderma Pigmentosum
C. Cockyne’s syndrome
D. Trichothio dystrophy
Answer: A
Clarification: Lynch syndrome, also known as Hereditary Non-Poyposis Colon Cancer is due to the slippage of replicative machinery in the microsatellite region of the chromosomes where it encounters repetitive sequences. Option b and d are errors in NER.
8. Which of the following mismatches will be ignored by MMR?
A. A—C
B. T—T
C. G—T
D. C—C
Answer: D
Clarification: It has been observed that although MMR can effectively detect and repair most of the mismatches like T—T, A—C, G—T, A—A etc, it can’t detect C—C, this may be due to c—C causing less distortion in the DNA.
9. Mut H acts as ______________
A. Adaptor
B. Exonuclease
C. Endonuclease
D. Helicase
Answer: C
Clarification: Mut H can bind to hemi-methylated DNA and make a nick in the strand with unmethylated GATC sequence. As it makes a nick within the DNA it is an endonuclease.
10. 6, 4 PP are brought about by which type of mutagen?
A. Methylase
B. Phosphatase
C. Radiation
D. Base Analogue
Answer: C
Clarification: 6,4 Pyrimidine photoproducts are formed when the DNA is exposed to strong radiation like UV or X-rays. These can be repaired by BER.
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