Multicellular organisms are composed of many different types of cells that cooperate and communicate to form organized systems. In humans there are over 10 trillion cells comprising 200 types of tissues. Each cell originates from a single fertilized egg cell and differentiates through activation of certain genes while others remain inactive, allowing cells to perform specialized functions. Bacteria also regulate genes in response to environmental signals, turning groups on and off. The lac operon in E. coli is a well-studied example of genetic control, where a repressor protein either allows or prevents transcription of genes involved in lactose metabolism depending on the presence of lactose.

6. The lac operon In an E. Coli cell growing in the absence of lactose , a repressor protein binds to the operator, preventing RNA polymerase from transcribing the lac operon's genes. The operon is OFF When the inducer, lactose , is added, it binds to the repressor and changes the repressor's shape so as to eliminate binding to the operator. As long as the operator remains free of the repressor, RNA polymerase that recognizes the promoter can transcribe the operon's structural genes into mRNA. The operon is ON

7. The lac operon Lactose absent :

8. The lac operon Lactose present :