Explanation of the lac operon, based on 9700/41/M/J/18 Question 3

It is made up of 3 genes: lacZ, lacY and lacA which encode for B-galactosidase (LacZ), lactose permease (LacY), and another enzyme (not studied at A level) respectively.The functions of LacZ (the enzyme) is cleavage of lactose to galactose and glucose.LacY is a symporter which pumps lactose along with H+ into the cell.
Gene I is not part of the operon, it encodes for a repressor protein (this means that it negatively influences transcription by binding at the operator region). It is constitutively expressed which means that it is produced all the time. Because of this the lac operon is only expressed when lactose is present because lactose (actually a product of lactose) binds to LacI so that it cannot bind the DNA, as a result RNAPol can bind to the promoter region start transcription