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What is the lac operon?

Class 12 BiologyNCERT chapter: Molecular Basis of Inheritanceइसे हिंदी में पढ़ें

Exam-ready answer

The lac operon is a polycistronic unit of E. coli with a regulatory gene (i), three structural genes (z, y, a), a promoter and an operator. The i gene codes for a repressor. Without lactose, the repressor binds the operator and blocks transcription; lactose, the inducer, inactivates the repressor and switches the operon on.

Explanation

In bacteria, an operon is an arrangement in which a polycistronic structural gene is regulated by a common promoter and regulatory genes. In the lac operon (lac = lactose), the i gene (from inhibitor) codes for the repressor. The z gene codes for beta-galactosidase, which hydrolyses lactose into galactose and glucose. The y gene codes for permease, which increases the permeability of the cell to beta-galactosides. The a gene codes for transacetylase.

The repressor is made all the time from the i gene. When lactose is absent, the repressor binds to the operator, so RNA polymerase cannot transcribe the structural genes and the operon stays switched off. When lactose is added to the medium, a few molecules enter the cell through permease, because a very low level of expression of the operon is always present. Lactose then acts as the inducer.

The inducer binds the repressor and changes it into an inactive form that cannot bind the operator. RNA polymerase now transcribes z, y and a, the enzymes are made and lactose is broken down. Once lactose is used up, the free repressor binds the operator again and transcription stops. Since control is exerted by a repressor, this is called negative regulation. Jacob and Monod first explained this system.

Points to remember

  1. Operon: a polycistronic structural gene regulated by a common promoter and regulatory genes, found in bacteria.
  2. Regulatory gene i codes for the repressor; z codes for beta-galactosidase, y for permease and a for transacetylase.
  3. Beta-galactosidase hydrolyses the disaccharide lactose into galactose and glucose.
  4. Lactose is the inducer: it inactivates the repressor so that RNA polymerase can transcribe the structural genes.
  5. Regulation of the lac operon by the repressor is called negative regulation.

Exam tip

Draw a labelled diagram of the lac operon in two states, repressor bound to the operator (off) and inducer bound to the repressor (on), and write the enzyme coded by each structural gene.

Questions students also ask

Why is lactose called an inducer?
Lactose is the substrate for beta-galactosidase and it switches the operon on. It binds to the repressor and makes it inactive, so the repressor cannot sit on the operator. RNA polymerase can then transcribe the structural genes, so lactose induces the making of the enzymes that digest it.
What is the role of the operator in the lac operon?
The operator is a DNA segment next to the promoter where the repressor binds. When the repressor sits on the operator, RNA polymerase cannot transcribe the structural genes. When the operator is free, transcription goes on. So the operator works as the on-off switch of the operon.
Why must the lac operon always show a very low level of expression?
Lactose needs permease, the product of the y gene, to enter the cell. A very low level of expression keeps a few permease molecules present, so some lactose can enter when it appears in the medium and act as the inducer. Without this, lactose could never switch the operon on.

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