Cell Biology MCAT Practice Question
A 28-year-old woman presents with a history of early-onset breast cancer and a family history of cancer. Genetic testing reveals a mutation in RAD51. Her fibroblasts are obtained for research and demonstrate marked hypersensitivity to ionizing radiation with increased chromosomal breaks and rearrangements. Despite this sensitivity, the cells maintain normal capacity for nucleotide excision repair as demonstrated by normal removal of UV-induced thymine dimers. When exposed to ionizing radiation, the patient's cells accumulate Holliday junction intermediates that persist abnormally long. Which of the following best explains the primary defect in this patient's cells?
Answer choices
- ADefective endonuclease activity preventing resolution of Holliday junctions
- BInability to remove thymine dimers from damaged DNA
- CLoss of mismatch recognition complex function
- DImpaired homology recognition and strand invasion during homologous recombinationCorrect answer
- EDefective non-homologous end joining ligase activity
- FReduced base excision repair glycosylase function
See the full explanation
Get the correct-answer rationale, why each distractor is wrong, the underlying mechanism, and high-yield associations — plus adaptive practice that targets your weak areas — with a free MedBoardPRO account.