Question

Explain Operons Detailly

a. Describe the purpose of the essential operon components (promoter, operator, genes, regulatory protein). Keep your answers general for all operons.
​​​​b.Contrast repressible and inducible operons. Include descriptions of differences at the level of both function and molecular interaction.

c. Metabolically why does it make sense that the Trp Operon is a repressible operon and the Lac Operon is an inducible operon?

Answer 1

a) Operon is smallest regulatory unit of gene expression. Operon consisits of different components which includes promoter, operator, regulatory and structural genes.

Promoter is site on which RNA polymerase binds and act to turn on and off gene transcription. Operator gene is located downstream of promoter gene, to which repressor binds and block RNA polymerase from transcribing gene.

Regulatory gene code for regulatory protein which regulate gene expression. Regulatory protein can be activator or repressor. Activator bind to region near promoter site to enhance binding of RNA polymerase and to increase rate of transcription. This is called as positive regulation. Repressor on the other hand bind to operator region blocking RNA polymerase from transcribing structural genes and hence prevents gene expression. This is called negative regulation.

b) Inducible operon is the operon which is expressed in presence of a inducer. Binding of a inducer to regulatory protein triggers gene transcription to occur. These are of two types- negative control inducible operon and positive control inducible operon.

Repressible operon is operon in which normally operon is inhibited in presence of effector molecule. Below image show all four possible types of operon

c) Trp (tryptophan operon) is a repressible operon i.e it is negatively regulated repressible operon. This is because when the inactive aporepressor produced by regulatory protein it doesn't alone able to bind to operator gene and gene expression occurs. But when effector molecule i.e corepressor is present in this case tryptophan, repressor become active and bind to operator gene and repress operon. So it is a repressible operon because presence of repressor that is tryptophan repress expression.

In case of lac operon it is an negative regulated inducible operon. In this operon repressor produced by regulatory gene is active and alone bind to operator gene to turn off operon. But when inducer is present that is lactose it bind to repressor and make it inactive, so it cannot bind to operator and operon is induced means gene expression occurs. So it is inducible as prsence of inducer lead to induced gene expression.