The question at hand refers to a gene that is not used in conditions where its use is not needed, which is known as a repressed gene.
A repressed gene is a gene that is being repressed, meaning that its expression is not being allowed by the organism. The E.coli's lac operon is an example of this, it is a regulatory fragment that has:
- Structural genes
- One promoter
- Operator regions.
The enzyme known as RNA polymerase will fuse to the promoter region of this fragment in order to read the genetic code for the lac operon in question. Written in this genetic code, is the rule that the structural genes in the lac operon may only be expressed when in an environment with sugar in the form of lactose.
On the other hand, in environments where lactose is not present, a protein known as Lac Repressor, which interferes in the transcription process, prevents the RNA polymerase from fusing to the promoter region. This serves to repress the lac operon in the E.coli bacteria when lactose is not present in the environment.
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