Only a certain subset of genes are used at any given time. Other genes are only transcribed when needed or at a certain time. Glucose is a better source of energy than lactose. Normally, lac operon is blocked, rna polymerase binds to promoter but cannot transcribe. In prokaryotes, genes are regulated at transcription (when it needs to) 3 genes for lactose metabolism in e. coli on dna. If no lactose = cell does not make enzymes. If lactose is present = enzymes are made. Operon = a group of genes controlled by a single regulatory signal. All genes work together (transcribed or not at all) Structure genes (lac y, lac z, lac a) Promoter and operator are binding sites on dna = not transcribed. Usually off transcription needs to be turned on. To turn on, an inducer molecule inactivates the repressor. Repressor prevents transcription (repressor is a constitutive gene) Allolactose (inducer) binds to repressor => change its shape!