Final answer:
True, the lac operon does transcribe polycistronic mRNA when lactose is present in the cell, with the absence of glucose allowing for the optimal transcription of the operon.
Step-by-step explanation:
The statement that polycistronic mRNA is transcribed from the lac operon when lactose is present in the cell is true. The lac operon is an inducible operon that is normally off. It requires specific conditions to be activated: the absence of glucose and the presence of lactose. When lactose is available, it is converted to allolactose, which then binds to the lac repressor, changing its shape and preventing it from binding to the operator site.
This allows RNA polymerase to move from the promoter site to the operator site and transcribe the lac operon's genes. Additionally, the presence of cyclic AMP (cAMP), which increases when glucose levels are low, leads to the binding of cAMP to the catabolite activator protein (CAP), further facilitating transcription of the lac operon.