Explanation : In bacterial genetics, the lac operon is an operon (a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter) required for the transport and metabolism of lactose in E. coli and some other enteric bacteria. When lactose is present, it acts as an inducer for the lac operon. This means that lactose, when present, triggers the activation of the lac operon, allowing the bacteria to produce the necessary enzymes (beta-galactosidase, lactose permease, and thiogalactoside transacetylase) to metabolize lactose. The lac operon is a classic example of a gene system subject to both positive and negative control.