In order to express genes, others must be turned off while others are turned on. In every cell of an organism lies DNA. This DNA is generally identical throughout the organism; for example, a stem cell contains genes for hair and everything else but becomes a bone cell.

But sometimes it is necessary for cells to have mRNA replication inhibited so the cell isn’t producing excess enzymes and proteins, wasting valuable energy and potentially overwhelming the cell.  So yeah, things need to be shut off and on, sort of like a light switch. The genes that need to be turned on or off are on cells.

When an enzyme needs to be made for a certain situation and the

The regulatory gene is a gene before the operon that codes for a protein which controls the transcription of a gene or group of genes. This helps the RNA polymerase attach to the DNA strand. 

. The operon is the cluster of genes that needs to be regulated. It contains a promoter, an operator and the genes. Promoters

The operator is like a key’s hole near the steering wheel to start the engine of a vehicle. In order to start or stop it, a key is needed. The key is the repressor or activation protein that fits into the operator. In order to remove the key, an inducer (or, think of it like a hand) is needed to attach to it and change the shape so that the repressor will not work and the genetic information on an operon can transcribed.

Citations:

http://www2.estrellamountain.edu/faculty/farabee/BIOBK/operon.gif