Final answer:
The correct answer is B. allolactose, which is the effector molecule for the Lac repressor protein, binding to it and causing it to release from the DNA, thereby allowing the transcription of the lac operon genes involved in lactose metabolism in bacteria.
Step-by-step explanation:
The effector for the Lac repressor protein is allolactose. In bacteria like E. coli, when glucose is not present, lactose is used as a carbon source. Lactose is converted into allolactose by the enzyme β-galactosidase. Allolactose binds to the repressor protein, causing it to change shape and release from the operator site on the DNA. This action frees RNA polymerase to transcribe the genes for lactose metabolism.
Without allolactose, the repressor protein binds to the operator and prevents transcription of the lac operon genes. When glucose levels are low and lactose is available, the conversion of lactose to allolactose triggers the expression of genes necessary for lactose breakdown. If a mutation occurred in the repressor protein that prevented it from binding lactose or its isomer allolactose, it would result in a constitutive expression of the operon, leading to the continuous production of enzymes for lactose digestion regardless of its presence.