Most genes (~75%) are constitutive, meaning that they are expressed continuously by a cell.  Others are regulated so that they are expressed only when needed.

Some gene regulation occurs at the level of translation, so while mRNA is produced, it may not be used to produce proteins.  Some mRNA molecules act as riboswitches, altering their shape in response to changes of pH, temperature or nutrient concentration to either allow or block the binding of ribosomes.

Short interference RNA (siRNA) or antisense RNA is an RNA molecule that is complementary to mRNA, tRNA or DNA.  siRNA functions by binding to its complementary nucleotide base to silence it (make it become inactive).

The Operon

Prokaryotic cells have linear sequences of DNA called operons.  The operon is composed of a promoter sequence, followed by an operator gene, followed by one or more structural genes that act as blueprints for proteins.  The operon is controlled by a regulator gene found elsewhere on the chromosome.

Operons can be inducible (turned on by a substrate) or repressible (turned off by a product).

In the inducible lactose (lac) operon above, the operator gene is blocked by a repressor protein when the cell is not in the presence of lactose.  When lactose is present, it acts as an inducer by binding to the repressor protein, thus preventing it from attaching to the operator.  RNA polymerase can then bind to the operator and transcribe the mRNA molecule.  Three different proteins are synthesized.  When all of the lactose in the cell has been catabolized, the repressor protein binds to the operator and shuts down the operon.

In the repressible tryptophan (trp) operon above, the structural genes involved in tryptophan biosynthesis are transcribed continuously as long as the cell needs the amino acid.  When a sufficient concentration of tryptophan has been produced, the amino acid will bind to a repressor protein, causing it to bind to the operator gene and shut the trp operon down.