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10.7. Regulation of gene expression

Regulation of gene expression

There must be mechanisms that regulate gene expression, since otherwise the cells would be constantly synthesizing large amounts of proteins.

The amount of proteins that are synthesized depends on the amount of mRNA in the cytoplasm. As the mRNA only lasts a few minutes, the amount of mRNA synthesized will regulate the amount of enzyme  that is synthesized.

Gene expression can be regulated by regulating mRNA synthesis.

  • In prokaryotes, it depends on the available substrate.
  • In eukaryotes, it depends on the hormones of the internal environment.

Regulation in prokaryotes

Jacob and Monod, discovered in 1961, the model called operon to explain how protein synthesis is regulated in bacteria.

An operon is a functional genetic unit formed by a group or complex of genes capable of regulating their own expression through the substrates with which the proteins encoded by their genes interact.

All these genes are located on the chromosome close enough for the regulation of their expression to occur in a coordinated way.

The regulation of genes can be:

  • Inducible. Genes are only transcribed if a molecule called an inducer is present.
  • Repressible. Genes are expressed but stop if a molecule called a repressor is present.

Components of the operon model

The components of the operon model are:

  • Structural genes. They are those that code for structural proteins andenzymatic proteins .
  • The regulatory gene. They are those that encode the proteins that control the activity of  structuralgenes. These proteins are called repressors, and they control expression.
  • The promoter. It is the area where the RNA-polymerase enzyme binds and the start oftranscriptionbegins.
  • The operator. It is the place where the repressor is fixed, blocking it. This prevents the advancement of RNA polymerase.
  • The inductor. Substrate whose presence induces the expression ofgenes.

The lactose operon

The lac operon of the bacterium Escherichia coli is the best known.

The lactose is a disaccharide that degrades E. coli as a source of energy and carbon, breaking it down intoglucose and galactose. The enzyme β-galactosidase is responsible for the degradation. Regulation is carried out as follows:

  • If there is no lactose in between (do not induce). The regulatory gene synthesizes a repressor protein ( repressor ) and blocks the operator, and thegenes are not transcribed.
  • If there is lactose in the medium (inducer). The lactose acts as inducer and does not block the operator .

Regulation in eukaryotes

The main point of regulation of transcription is, in both prokaryotes and eukaryotes, it is the start of transcription.

The main differences in the regulation of transcription, with respect to prokaryotes are:

  • The activation of transcription occurs by the binding of DNA to certain transcription factors.
  • Positive regulation predominates, although there are also negative regulation elements.
  • Transcription occurs in the nucleus and translation occurs in the cytoplasm.