The lactose (lac) operon in E. coli is an example of a repressed gene when lactose is not available in a cell. In 3-5 sentences, explain what this means using correct terminology.

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Explanation:

The lac operon is a unit containing structural genes and a single promoter in bacteria such as E.coli. A repressed gene is a gene in which the expression has been inhibited or prevented from occuring.

In E.coli, the lac operon contains three structural genes viz: lacZ, lacY and lacA, that codes for proteins capable of degrading lactose sugar for energy purposes. In the presence of lactose, the expression (transcription and translation) of these three lac genes occur normally.

However, in a scenario where lactose is absent in the bacterial cell, the expression of the lac gene in the lac operon is inhibited. This inhibition/repression is done by LAC REPRESSOR, which is a transcription factor/protein that binds to the OPERATOR region of the lac operon and prevents RNA polymerase (transcription enzyme) from binding to the PROMOTER region.

Once RNA polymerase is prevented from binding to the promoter, transcription (gene expression) does not occur. Hence, in the absence of lactose sugar, the lac operon is a repressed gene.

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