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    In my book it says: Unit consisting of genes that work under the control of an operator gene. Im confused how can a gene be controlled by a gene?
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    If by operator gene you mean operon, it's a segment of DNA when an inhibiting factor binds to and doesn't allow mRNA transcription to occur.

    These genes are usually in the following order: promoter -> operator -> structural (actual)

    So if a RNA polymerase binds to the promoter and attempts to transcribe mRNA, it won't be able to do so if something else is bound to the operator. Kinda like sticking something in a zipper (repressor) and trying to unzip. Removal of the inhibiting force allows for transcription to occur. Another example would be trying to open a locked door - have to unlock it first (the repressor is the lock).

    The classic example is the lac operon, which controls lactose transport and metabolism in E. coli. Like the previously mentioned example, the lac operon gene structure is promoter -> operator -> structural (lacZ, lacY, lacA). A repressor protein is bound to the operator section. So RNA pol binds to the promoter and can't proceed because the lac repressor is in place. Lactose is partially metabolised to allolactose, This metabolite can bind to the lac repressor. By doing so, the lac repressoris inhibited, and unbinds itself from the operator segment, allowing for transcription of lacZ, lacY and lacA. When all of the lactose has been metabolised, then the repressor protein is capable of binding to the operator segment again, shutting down lacZ, lacY and lacA transcription.
 
 
 
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