Some genes arc normally switched off and the presence of certain substances, switch on the genes. Such type of gene regulation is called induction. Synthesis of an enzyme in response to the presence of its substrate is called induction and the gene is called inducible gene, and, the substrate itself is known as inducer, generally the enzymes of catabolic pathways are inducible.
One should bear in mind that this induction allows the cell to conserve its resource and avoid wastage. When the substrate is not available, the enzyme has no role to play and the cell does not synthesise the enzyme, there by conserve the huge resource that would have been utilized in the transcription and translation of the enzyme gene.
Conversely when the expression of a gene is turned off in response to a substance, the process is called repression and the gene is called repressible gene. In this case, the genes are normally on and producing their proteins (enzymes) responsible for synthesis of some other products. But when the end product itself is available to the cell the gene expression is switched off.
The repressing substance is known as co-rcpressor and is usually the end product of an enzymatic pathway. Usually the enzymes of anabolic pathways are repressible. The Operon concept / model
Francois Jacob and Jacques Monod in 1961 on the basis of their study on the inducible system for the synthesis of (3-galactosidase enzyme in E.coli proposed the operon model to explain the induction and repression. They were awarded with Nobel prize in physiology and medicine ini.965.
An operon is a unit of coordinated control of gene expression in bacteria (prokaryotc) including the structural genes and the controlling sequences on DNA recognized by regulator gene product. An operon thus consists of, (i) an operator gene / scquentc which control the activities of a number of contiguous structural genes that take part in synthesis of protein (s). The structural genes synthesize mRNA molecules under the operational control of operator. The operator is under the control of a repressor molecule synthesized by regulator gene which is not a part of opcron.
In the inducible system the regulator synthesize aetivc repressor protein which as an active dimer binds to the operator inhibitng the binding of RXA polymerase to the promoter, there by inhibiting the expression (transcription) of all the contiguously present structural gene.
In the presence of inducer the repressor instead of forming an active dimmer forms iducer- repressor complex which does not bind to the operator. As a result, RXA polymerase binds to the promoter and all the structural genes are transcribed into a common mRXA (polycistronic).
In the repressible system, the regulator synthesizes inactive repressor that cannot bind to the operator, so that the operon is turned on normally. When the corepressor is present, the inactive repressor forms a complex with the co-repressor which binds to the operator and prevents gene expression.