and the sugar molecule lactose.

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

Gene expression can be induced (or turned on)

when a specific inducer molecule appears in a cell.

For inducible systems,

a repressor molecule prevents gene expression

by binding to the upstream controlling region.

Lactose is the lac operon inducer molecule.

After first appearing in the cellular environment,

lactose passively enters the E. coli cell

and binds to the repressor molecule.

This binding releases the repressor from the controlling region.

At this point,

RNA polymerase can begin transcription of the operon.

Here we show two of the three lac operon genes

being transcribed into mRNA.

Ribosomes then bind to the mRNA,

and the two proteins are translated.

The first protein is ß-galactosidase

which breaks down lactose into two simple sugars.

The second protein is permease,

a membrane bound protein.

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.

This import occurs at a much greater rate

than the passive transfer we first observed.

Because translation continues inside the cell,

other permease proteins become embedded in the membrane.

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

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

Once its concentration is greatly reduced,

the lactose bound to the repressor are released.

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

For all inducible systems, like the lac operon,

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