True or False: Polycistronic mRNA is transcribed from the lac operon when lactose is present.

• A. False
• B. True
• C. Sometimes
• D. Not applicable

Answer: (B)

Polycistronic mRNA is a single RNA transcript that carries multiple coding sequences. The lac operon in bacteria is arranged as a cluster controlled by one promoter and is transcribed as one mRNA that includes the lacZ, lacY, and lacA genes. When lactose is present, some of it is converted to allolactose, which binds the LacI repressor and releases repression at the operator. With repression lifted, RNA polymerase can transcribe the operon, producing a single polycistronic mRNA that encodes all three proteins. So the statement is true. When lactose is absent, the repressor blocks transcription, and this polycistronic transcript is not produced.