Topic: Gene Regulation (lac operon)
The lac operon is a set of genes in bacteria used for lactose metabolism. It is only turned on when lactose is present and glucose is absent.
Steps:
Understanding the Question: What is the actual signal molecule that binds to the repressor to "turn on" the genes?
Key Formulas and Approach: Distinguish between the sugar source (lactose) and the chemical signal (the inducer).
Detailed Solution:
When lactose enters the cell, a small amount is converted by the enzyme $\beta$-galactosidase into an isomer called Allolactose.
Allolactose acts as the **inducer** by binding to the lac repressor protein.
This binding causes the repressor to change shape and release the DNA, allowing RNA polymerase to transcribe the genes.
Thus, while lactose is the "fuel," Allolactose is the actual "key" that unlocks the operon.