3. 3. A geneticist wants to isolate mutants in mice that are defective in nucleo

Question

3. 3. A geneticist wants to isolate mutants in mice that are defective in nucleotide excision repair (NER) but is unable to do biochemical analysis on the NER system so must use some other method to identify putative NER mutations. Which of the following would most likely be useful in identifying NER mutations in mice?

1.Identifying possible NER mutants by their progeny consisting of a reduced number of females with a normal number of males when they are crossed to normal mice

2. Identifying possible NER mutants by their high spontaneous mutation rates for base-pair substitution and frameshift mutations

3.Identifying possible NER mutants by their increased susceptibility to UV irradiation compared to normal mice

4.Identifying possible NER mutants by their high rate of cytosine deaminations compared to normal mice

5. All of the above would be characteristic of NER nutants

4.Which of the following is NOT a step in bacterial nucleotide excision repair (NER)?
1. The UvrA(2)-UvrB complex creates a single-stranded bubble at the damage site to which UvrA subunits are tightly bound and the UvrB subunit dissociates.
2.The UvrA(2)-UvrB complex scanned the DNA for damage.
3.Pol I fills the gap created by the removal of the damaged strand and the DNA is sealed by ligase.
4.UvrC (exonuclease) nicks the damaged strand on one side of the damage randomly.
5.UvrD (helicase) unwinds and releases the damaged strand.

Explanation / Answer

Ans. 3 NER system repairs damage caused by thymidine dimer or addition of a bulky chemical adduct on base as they result in distortion of double helix. This type of damage is caused by UV rays of sun. An organism defective in enzymes required for DNA repair is sensitive to ultraviolet light and cannot remove thymine–thymine and thymine–cytosine adducts. In humans, a disease called xeroderma pigmentosum occurs if an organism is unable to repair damage caused by UV.

So, the correct option will be 3. Identifying possible NER mutants by their increased susceptibility to UV irradiation compared to normal mice.

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Ans. 4 Option 1. The UvrA(2)-UvrB complex creates a single-stranded bubble at the damage site to which UvrA subunits are tightly bound and the UvrB subunit dissociates is incorrect. In bacterial NER system, UvrB subunits are tightly bound and ATP hydrolysis dissociate UvrA subunits. This allows UvrC to bind DNA.

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