Genetic control of differentiation
As an organism develops, tissues such as muscle, liver or blood become differentiated from each other. Each tissue consists of specialised cells which only carry out certain functions. Although a specialised cell has a complete set of the organism's genes, only those needed for its specialised functions are switched on. All other genes are switched off.

The bacterium Escherichia coli produces the enzyme ß-galactosidase to digest lactose. The enzyme is only made if lactose is present. When no lactose is present the gene that codes for the synthesis of ß-galactosidase is switched off. This ensures that resources are not wasted in the production of an enzyme when it is not needed.

The way this gene is controlled is described by the Jacob-Monod hypothesis:

The Jacob-Monod hypothesis - first stage

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The Jacob-Monod hypothesis - first stage
The regulator gene is continually transcribed into messenger RNA. This is used to synthesise a repressor protein. When there is no lactose present the repressor protein becomes attached to the operator. This prevents the structural gene being transcribed, and no enzyme is made.

The Jacob-Monod hypothesis - third stage

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The Jacob-Monod hypothesis - third stage



If molecules of lactose are introduced, they bind to the repressor protein molecules. This prevents the repressor protein binding to the operator. Because the operator is free the structural gene is transcribed and the enzyme is made.

Lactose is called the inducer because its presence induces synthesis of the enzyme.

The ß-galactosidase breaks down any lactose which is present and when there is none left the repressor protein once again binds to the operator and switches off the structural gene.