This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
[{'content': 'Number of structural genes in Lac operon is \nSelect and explain. \nA. 3, B. 4, C. 5, D. 6', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The structural gene for b-galactosidase (lacZ) is clustered with the genes responsible for the permeation of lactose into the cell (lacY) and for thiogalactoside transacetylase (lacA). The structural genes for these three enzymes, along with the lac promoter and lac operator (a regulatory region), are physically associated to constitute the lac operonRef: Harper 27e, page: 430', 'role': 'assistant'}]