Question

Describe how the lac operon functions, and explain how it permits E. coli to produce the enzymes needed to break down lactose when it is present in the bacterium.

Answer 1

The lac operon is basically present in the purpose of \( {\color{red}{\textrm{transcriptional regulation}}} \). The mechanism is that a protein (called the repressor) is binding to binding to an operator which is placed after the promoter in the transcriptional direction. When the repressor is bound to the operon the RNA-polymerase can't execute transcription as it is virtually being blocked by the repressor.
However, when E. coli is growing on a lactose medium, lactose can bind to the repressor, from which the repressor lose affinity for the operator and the RNA-polymerase can execute transcription of the lac genes (except the repressor protein) which produce mRNA translated into lactose catalytic enzymes.

When there no longer is lactose present in the medium, the repressor will again bind to the operon and the lac genes are no longer expressed.