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7.E : Mutation and Repair of DNA (Exercises)

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  • Q7.12

    If the top strand of the segment of DNA GGTCGTT were targeted for reaction with nitrous acid, and then it underwent two rounds of replication, what are the likely products?


    Are the following statements about nucleotide excision repair in E. coli true or false?

    1. UvrA and UvrB recognize structural distortions resulting from damage in the DNA helix.
    2. In a complex with UvrB, UvrC cleaves the damaged strand on each side of the lesion.
    3. The helicase UvrD unwinds the DNA, thereby dissociating the damaged patch.


    Are the following statements about mismatch repair in E. coli true or false?

    1. MutS will recognize a mismatch.
    2. MutL, in a complex with ATP, will bind to the MutS (bound to the mismatched region) and activate MutH.
    3. MutH will cleave 5' to the G of the nearest methylated GATC motif (GmeATC).
    4. The mismatch repair system can discriminate between old versus newly synthesized strands of DNA.

    For the next 6 problems, consider the following DNA sequence, from the first exon of the HRAS gene. A transversion of G to T at position 24 confers anchorage independence and tumorigenicity to NIH 3T3 cells (fibroblasts). This mutation is one step in tumorigenic transformation of bladder cells, and it likely plays a role in other cancers.

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    hat would the sequence be if the G at position 14 (top strand) were alkylated at the O6 position by MNNG and then went through 2 rounds of replication?


    hat would the sequence be if the C at position 24 (bottom strand) were oxidized by HNO2 and then went through 2 rounds of replication?


    What would happen if this sequence were irradiated with UV at a wavelength of 260 nm?


    If you were in charge of maintaining this DNA sequence, and you had the enzymatic tools known in E. coli, how would you repair the damage from question 7.15? Consider what would happen if the damage were corrected before or after replication.


    How could

    1. the oxidative damage in problem 7.16 or
    2. the UV products in problem 7.17 be repaired?

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