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The E. coli lac operon is an example of an inducible set of genes.

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These genes are responsible for the breakdown of lactose into sugars used for cellular metabolism.

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This inducible system also involves bacterial DNA, a repressor, mRNA, and the sugar molecule lactose.

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This animation will only focus on two of the three proteins encoded by the lac operon,

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beta-galactosidase and permease.

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Gene expression can be induced or turned on when a specific inducer molecule appears in a cell.

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For inducible systems, a repressor molecule prevents gene expression

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by binding to the upstream controlling region.

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Lactose is the lac operon inducer molecule.

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After first appearing in the cellular environment,

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Lactose passively enters the E. coli cell and binds to the repressor molecule.

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This binding releases the repressor from the controlling region.

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At this point, RNA polymerase can begin transcription of the operon.

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Here we show two of the three lac operon genes being transcribed into mRNA.

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Ribosomes then bind to the mRNA and the two proteins are translated.

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The first protein is beta-galactosidase, which breaks down lactose into two simple sugars.

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The second protein is permease, a membrane-bound protein.

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When embedded in the cell membrane, permease functions to provide a direct route for the lactose outside the cell to be imported into the cell.

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This import occurs at a much greater rate than the passive transfer we first observed.

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Because translation continues inside the cell, other permease proteins become embedded in the membrane.

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This further increases the rate at which lactose enters the cell.

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Beta-galactosidase breaks the cellular lactose into the simple sugars glucose and galactose.

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Once its concentration is greatly reduced, the lactose bound to the repressor are released.

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At this point, the repressor again binds to the controlling region and gene expression is halted.

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For all inducible systems like the lac operon,

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it is the interaction of the repressor and inducer molecules that mediate gene expression.

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Thank you.