DNA repair Transitional Reasoning Download PDF

Questions

1. 1.

   A laboratory analyzes a mutant bacterial strain exposed to ultraviolet (UV) radiation that exhibits stalled replication forks and accumulated cyclobutane pyrimidine dimers. Biochemical profiling reveals that while DNA polymerase I and DNA ligase activities are normal, the cells cannot excise the damaged multi-nucleotide segment flanking the dimer. Based on these observations, which DNA repair pathway is dysfunctional in this strain, and what specific enzymatic step is defective?

   • A) Nucleotide excision repair, because lesion-specific endonucleases fail to introduce dual incisions into the phosphodiester backbone of the damaged strand.
   • B) Base excision repair, because a specific DNA glycosylase fails to cleave the N-glycosidic bond to generate an apurinic or apyrimidinic site.
   • C) Mismatch repair, because exonucleases fail to selectively recognize and degrade non-Watson-Crick base pairings on the unmethylated strand.
   • D) Homologous recombination repair, because end-processing nucleases fail to generate 3' single-stranded overhangs for strand invasion.