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 cellcell: Basic unit of life. Unicellular organisms only have one cell. Multicellular organisms have many cells. has a complete set of the organism's genesgene: The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production., only those needed for its specialised functions are switched on. All other genes are switched off.

Gene activity in different specialised cells

A good example of the mechanism by which genes can be switched on and off is found in bacteriabacteria: Single-celled micro-organisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium..

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

The gene which codes for ß-galactosidase is called the structural genestructural gene: A structural gene is a gene whose product is an enzyme, or a protein that is involved in structural functions and is part of a DNADNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. chain which includes a regulator generegulator gene: A gene that is involved in turning on or off the transcription of structural genes and a sequence of bases called the operator.

The Jacob-Monod hypothesis - second stage

The regulator gene is continually transcribed into messenger RNARNA: (Ribonucleic acid) - it converts the genetic information stored in DNA into proteins. 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

If moleculesmolecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds 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.

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