Final answer:
The correct word to complete the sentence is 'dissociates from the operator.' In the presence of lactose, it turns into allolactose, which binds to the repressor, causing it to dissociate from the operator, allowing gene expression for lactose digestion in E. coli.
Step-by-step explanation:
Place the correct word in the missing position to complete the sentence: If the repressor dissociates from the operator, the gene will be expressed.
In prokaryotic cells like E. coli, the regulation of gene expression is crucial for adapting to different environmental conditions. Specifically, in the lac operon system, a repressor protein can bind to the operator region, preventing transcription of genes necessary for lactose digestion. When lactose is present, it is converted into allolactose, which then binds to the repressor protein. This binding causes a structural change in the repressor, leading to its dissociation from the operator, thus allowing RNA polymerase to access the DNA and transcribe the genes required for lactose metabolism. This efficient regulatory system ensures that the organism only produces enzymes when the substrate (lactose) is available, conserving energy and resources.