The operon model

after Jacob and Monod ...

Prokaryotic cells must have the opportunity to control protein biosynthesis in order to use them ecologically. Otherwise, either too many or too few proteins would be formed.
Jacob (* 1920) and Monod (1910-1976) researched E. coli bacteria and found gene regulation through substrate induction and end-product repression.
Before going into detail, a few concepts have to be clarified:
The operon is a DNA segment that RNA polymerase uses as a starting point in transcription. Promoter, operator and structural genes make up this sector:
promoter: Serves as a starting point and starting point for RNA polymerase
operator: At this point, the repressor or activator docks
structural genes: contain the information on which proteins should be syntethised
As long as no repressor is seated on the operator, the RNA polymerase, including the associated transcription, runs smoothly. The structural genes are read and new proteins are syntethisiert.
If a repressor now binds to the operator, it alters its structure and prevents the RNA polymerase from draining off. Thus, the transcription is stopped -> The protein biosynthesis comes to a standstill.
This scheme also works the other way around, ie with activators that dock to the operator and thus enable transcription by RNA polymerase.
In the operon model, a distinction can be made between two different possibilities of gene regulation. The "substrate induction" produces the enzyme during the presence of the substrate to be processed. An example of substrate induction is provided by the lactose operon in E. coli. In contrast, the second possibility of gene regulation is the "end product inhibition". Initially, an inactive repressor exists until an excess of the metabolic product arises. At this point, the repressor activates and the enzyme in question is no longer expressed.