how is lac operon switches on and off .how does it sfunctions are activated and inhibited

Operon – An arrangement where a polycistronic gene is regulated by a common promoter and regulatory genes

The elucidation of lac operon as a transcriptionally active system was first done by geneticist Jacob and biochemist Monod.

Genes constituting lac operon:





i gene


It codes for repressor of lac operon.

z gene


It codes for β-galactosidase.

y gene


It codes for permease, which increases the permeability of cell to β-galactosidase.

a gene


It codes for transacetylase.


All genes involved in lac operon are required for metabolism of lactose.

Inducer – Lactose acts as an inducer for lac operon since it regulates the switching on and off of the operon.

If lactose is provided to the growth media of bacteria in absence of any other carbon source, then it is transported inside the cells by permease.

For permease to be present and lactose to enter inside the cells, low level of expression of lac operon must be present all the time.

Regulation in Absence of Inducer

  •  In absence of inducer, i gene transcribes to synthesise repressor mRNA, which translates to form repressor.
  • This repressor binds with the operator region of operon and prevents RNA polymerase to transcribe genes – z, y, and a (negative regulation).
  •  Therefore, in absence of the products of these genes, metabolism of lactose ceases.


Regulation in Presence of Inducer

  • Inducer binds with the protein product of gene i (repressor) and inactivates it.
  • This inactivated repressor is unable to inactivate RNA polymerase enzyme and z, y, and a genes synthesise their respective mRNA, which in turn gets translated to form β-galactosidase, permease, and transacetylase.
  • In presence of all these enzymes, the metabolism of lactose proceeds in a normal manner.

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