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Jacob - Monod model

model of repressors and activators that explains how operons work. (Structure and function)



a cluster of genes transcribed as a single mRNA


Structure gene

codes for the protein of interest

Upstream of the structural gene is the operator site, a non transcribable region of DNA that is capable of binding a prepress protein.

Further upstream is the promotor site, which is similar in function to promotes in eukaryotes: it provides a place for RNA polymerase to bind.

Furthest upstream is the regulator gene, which codes for a protein known as the repressor.


There are two types of operons:

inducible systems
repressible systems


Inducible systems

(such as the lac operon) are bound by a repressor under normal conditions; they can be turned on by an inducer pulling the repressor from the operator site

The repressor is bound tightly to the operator system and thereby acts as a roadblock. RNA polymerase is unable to get from the promotor to the structural gene because the repressor is in the way. To remove that block, an inducer must bind the repressor protein so that RNA polymerase can move down the gene.

(positive control mechanisms)


Repressible systems

(such as the trp operon) are transcribed under normal conditions; they can be turned off by a corepressor with the repressor and the binding of this complex to the operator site.

allows constant production of a protein product. The repressor made by the regulator gene is inactive until it binds to a corepressor. The complex then binds the operator site to prevent further transcription. Serves as negative feedback.

(negative control mechanism)


As the concentration of inducer increases,

it will pull more copies of the repressor off of the operator region, freeing up those genes for transcription


lac operon

example of an inducible system
contains the gene for lactase
bacteria only want to use this option if lactose is HIGH and glucose is LOW

is assisted by binding of the catabolic activator protein (CAP)
Falling levels of glucose cause an increase in the signaling molecule cyclic AMP (cAMP) which binds to CAP.


As the levels of repressible systems increase,

it can bind the repressor and the complex will attach to the operator region to prevent further transcription of the same gene.


trp operon

when tryptophan is high in the local environment, it acts as a corepressor.