A lac operator mutation in a partial diploid
An E. coli strain carries two copies of the lac operon. Copy 1 has a constitutive operator mutation (Oc) and a nonfunctional lacZ. Copy 2 has a normal operator and a normal lacZ. Both copies make a normal repressor. When is functional β-galactosidase produced?
Answer D: explanation for every choice
A
Constitutive expression occurs only from the Oc copy, whose lacZ is defective.
B
Copy 2 can be induced by lactose and has a working lacZ.
C
Copy 2 is fully inducible, not leaky.
D
Correct. The Oc mutation acts only in cis, on genes on the same DNA. That copy's lacZ is nonfunctional, so working enzyme comes only from copy 2, which is still regulated normally.
TakeawayOperator mutations act in cis. Repressor mutations can act in trans.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb152.