There is a spectrum of syndromes in humans known as Xeroderma pigmentosum.
The individuals that have XD most commonly have mutations in their nucleotide excision DNA repair mechanisms that make them particularly susceptible to environmental mutagens such as UV light. Individuals have to be careful with how much sunlight they are exposed to since they have an extremely elevated chance for developing skin cancer.. There have been several cell lines that have been established from XD patients that can be studied in tissue culture. In an experiment several different cell lines of unknown origin were tested for their ability to undergo unscheduled DNA synthesis (UDS), an assay for DNA repair. In this assay the amount of radioactive nucleotides that are incorporated into DNA after the cell sustains a mutagenic event are measured. The amount of radioactivity incorporated is measured by the number of counts per minute (CPM). Below is a table from such an experiment. Which cell line is most likely from XD patient(s)?
Cell line CPM in UDS assay CPM if no mutagenic event
A 2,500 2,750
B 300,000 5,000
C 45,000 25,000
D 275,000 50,000
Wild-type control 750,000 75,000
A. A
B. B
C. C
D. D
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