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Gene Regulation

in Prokaryotes

Lac Operon

Lac Operon allows bacteria too break down lactose. However these genes are only active when lactose is present.

This process involves:

Bellow is an animation of Lac Operon,

add lactose when you are ready

NOTE: There is a step missing, for simplicity sake, I skipped the step of making the mRNA into proteins.

As you can see from the animation above, when no lactose is added, the Repressor molecule stays attached to the operator. Preventing RNA polymerase from attaching.

However, when Lactose is added it attaches to the Repressor, changing its shape. The repressor releases, allowing RNA polymerase to read the 3 Genes, Lac Z, Lac Y and Lac A.

The proteins produced breakdown the lactose until there is none left. Once there is no more lactose left the repressor re-attaches to the operator, preventing RNA polymerase from attaching and reading the genes.

This is an example of Positive control.

The majority of what we know about gene regulation, comes from bacteria. Before we start there are a few differences between Pro and Eukaryotic gene regulation systems.  The main difference is that Prokaryotes, such as bacteria, have an Operator and Repressor.

An example of an organisms ability to turn off and turn on genes, is the Trp Operon in bacteria.

Trp Operon allows bacteria too produce tryptophan (an amino acid) when there is none present in its environment. Humans do not have the ability to synthesis Tryptophan. Thus it is an essential amino acid. Meaning we must obtain it from our diet.

Trp Operon varies from the Lac Operon in two ways.

Trp Operon


This is a section of DNA, which sits just after the promoter. Acting as an ON and OFF switch for gene transcription.


This molecule attaches to the operator setting in the OFF position.

While the repressor is attached to the Operator it prevents RNA polymerase from attaching.

Bacteria will have two types of controls. Positive and negative controls. They both require an Operator and Repressor, however react differently depending on their environment.

This is better explained by the animations bellow.

Lac Operon is a Positive control

Trp Operon is a Negative control

As you can see by the animation above, when tryptophan is present, the repressor is changed to fit on the operator, preventing RNA polymerase from attaching.

When tryptophan is in low supply, the repressor changes shape, detaching from the operator allowing RNA polymerase to read the gene. These genes synthesise tryptophan.

Once again this is an example of a negative control.

Click here for gene regulation in Eukaryotic cells.

Click here for questions

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