The order of bases in the DNA is a code for the order of amino acids in a protein.
Protein synthesis is an energy-requiring process.
However, some enzymes are not required all the time and energy can thus be saved by living organisms if they can control their genes and only switch them on when the enzyme they code for is required. To use lactose the E. coli must first activate the appropriate gene which will then synthesise the enzyme b -galactosidase. Lactose is found in milk but not in many other environments. It would, therefore, be a waste of energy if the gene for b -galactosidase was permanently switched on. It would be more efficient if the E.coli could switch the gene on only when its enzyme was required.