Regulation of Gene Expression – Lac Operon

Francois Jacob and Jacques Monod proposed the model of gene regulation known as the operon model/lac operon. Alec Jeffreys – DNA fingerprinting technique. Matthew Meselson and F. Stahl – semi-conservative DNA replication in $E.\ coli$. Alfred Hershey and Martha Chase – proved that DNA, not protein, is the genetic material.

The $lac$ operon consists of one regulatory gene and three structural genes needed to metabolise lactose. It is induced by lactose (inducible operon), so lactose is the inducer.

The lac operon has three structural genes: z, y and a.
The z gene codes for $\beta$-galactosidase, which hydrolyses lactose into glucose and galactose.
The y gene codes for permease, which increases the permeability of the cell to lactose, and the a gene codes for transacetylase.
If the z gene is mutated, the cells cannot synthesise a functional $\beta$-galactosidase.
Even if lactose enters the cell (permease is still made by the y gene), it cannot be broken down into glucose and galactose and cannot be used as a source of energy.
So the cells cannot grow on lactose alone because they cannot synthesise functional beta galactosidase.

The lac operon of E. coli, described by Jacob and Monod, is a set of genes that control the use of lactose (milk sugar).
It contains a promoter, an operator and three structural genes: z (beta-galactosidase), y (permease) and a (transacetylase).
Lactose (as allolactose) acts as the inducer: it binds to the repressor and switches the operon on, so the enzymes are made only when lactose is present.
So 'lac' refers to lactose.

In the lac operon, the regulator (i) gene produces a repressor protein.
In the absence of lactose, the repressor binds to the operator gene, which lies between the promoter and the structural genes. This blocks RNA polymerase and switches off transcription of the structural genes (z, y, a).
When lactose (allolactose, the inducer) is present, it binds to the repressor and makes it inactive, so it can no longer bind the operator, and the genes are transcribed.
So the repressor binds to the operator gene.