Question

What turns the lac operon off ?

Answer 1

Operon:

1. A cluster of genes with related functions that are clustered together with DNA sequences that allow only certain genes to express when required and turn off the expression of genes when not required.
2. The operon is an example of gene regulation that is commonly observed in prokaryotes and eukaryotes.
3. An operon is composed of structural (genes that code for protein) and regulatory genes (genes involved in regulating the expression of structural genes).

Lac operon:

1. Two French scientists Francois Jacob and Jacques Monod examined this operon which is switched on when the bacterial cells encounter lactose.
2. It is an example of the inducible operon( which turns on the expression of genes by encountering a small molecule called an inducer) which is found in Escherichia coli.
3. The lac operon explains the metabolism of lactose, in the presence and absence of lactose.
4. In the presence of lactose, the operon expresses three genes, namely lacZ, lacY, and lacA.
5. A repressor protein R synthesized by the lac I gene of operon inhibits the expression of three genes by binding to the promoter site of the operon, then an inducer allolactose binds to the repressor which allows the operon to synthesize the enzymes required for the metabolism of lactose.
6. The structural gene LacZ encodes for Beta-galactosidase, which breaks down lactose into glucose and galactose.
7. The Structural gene LacY encodes for beta-galactosidase permease, transporting lactose into the cell.
8. The Structural gene LacA encodes for galactoside acetyltransferase, which protects from the toxic effect of galactoside by acetylating it.

Lac operon turns off:

1. When the cells have an appropriate amount of glucose, then the cells need to synthesize glucose from lactose.
2. The lac operon turns off when the repressor protein binds to the operator site, thereby inhibiting RNA polymerase to bind the to operator and express the genes which regulate the metabolism of lactose