Turning Genes On and Off

Turning Genes On and Off We know that each cell in the body carries the blueprint for the whole body. So how is it that only the genes that make a liver cell are turned on in the liver and not the genes in the big toe? There is evidence that we have a great surplus of DNA that is not used.

Jacob and Monod They noted that the enzyme galactosidase was produced in E.coli when the cell needed it to break down lactose, but if no lactose was present no enzyme was produced. Jacob and Monod worked out a theory for how the gene that makes that makes the enzyme was turned on and off.

Operon Theory This is a good example of a biological feedback system. The theory can be summarised as follows: One section of DNA that codes for the enzyme is called the Structural gene (S) Close to this is another section of DNA called the operator gene (O). This activates the structural gene when the enzyme is needed.

Operon Theory The operator gene is controlled by the regulator gene (R), which is found further down the DNA chain. This produces a repressor substance which turns the operator off so no enzyme is made. When the enzyme is needed, the repressor is inhibited, the operator gene can now turn on the structural gene and the enzyme is made.

Operon Theory It has been found that lactose itself can inactivate the repressor by combining with it.

Enzyme (galactosidase) DNA R O S Repressor Enzyme (galactosidase) Inhibitor (Lactose)

The Lac Operon animation

Eukaryotic Control This is a complex and little understood process. There is a promotor region and structural genes, but no operator or regulator regions. RNA binds to certain proteins called transcription factors, which then bind to the promotor region.

Eukaryotic Control Other transcription factors bind to the enhancer region, (this may be far from the promotor region.) The DNA molecule bends like a hairpin bringing the enhancer and promotor regions close to each other. Transcription can occur.