What is an operon and what effect does the presence of lactose have on the Lac operon?

An operon is a set of genes that function as a single transcription unit.When lactose is present, it binds to the repressor protein LacI to form a repressor-protein complex and causes a change in shape. This conformational change prevents the LacI repressor protein from binding to the operator site on the Lac operon, hence RNA polymerase can bind to the promoter region and transcription of structural genes (Lac Z and Lac Y) can occur in order to metabolise lactose.