Question

What is operon concept? Use different examples and explain in detail

Answer 1

Operons are group of genes that function to produce proteins needed by the cell. There are two different kinds of genes in operons:

• Structural genes code for proteins needed for the normal operation of the cell. For example, they may be protein needed for the breakdown of sugars. The structural genes are grouped together and a ssingle mRNA molecule is produced during their transcription.
• Regulator genes code for proteins that regulate other genes.

A single mRNA transcript carries the coding information of an entire operon. Operons containing more than one structural gene are multigenic. All structural genes in an operon are co-transcribed and therefore are co-ordinately expressed. In bacteria,genes with related functions frequently occur in co-ordinately regulated units called operons. Each operon contains a set of contiguous structural genes, a promoter which is the binding site for RNA polymerase and an operator which is the binding site for a regulatory protein called a repressor. When a repressor is bound to the operator, RNA polymerase cannot transcribe the structural genes in the operon. When the operator is free of repressor, RNA polymerase can transcribe the operon.

The lac operon:

Lactose is a sugar found in milk. If lactose is present, E. coli needs to produce the necessary enzymes to digest it. Three different enzymes are needed. In the normal condition genes A,B and C bound to the lac operon do not function because a repressor protein is active and bound to the DNA preventing it from transcribing. When the repressor protein is bound to the DNA, RNA polymerase cannot bind to the DNA. The protein must be removed before the genes can be transcribed.

Allolactose, an isomer of lactose, binds with the repressor protein thus inactivating it.

The repressor protein is produced by a regulatory gene. Operons acts like a switch that can turn several genes on or off at the same time. The lac operon is an example of inducibe operon because the structural genes are normally inactive. They are activated when lactose is present.

Further, the lac promoter has two components:

• The RNA polymerase binding site.
• A binding site for Catabolite Activator Protein. Binding of CAP to the promoter activates transcription of the lac operon from being induced when glucose is absent. CAP binds to the romoter only when cAMP is present at sufficient concentrations.

The trp operon:

The trp operon is an example of repressible operon. The structural genes in the tryptophan operon are transcribed only when tryptophan is absent or present in low concentrations. The expression of the genes in the trp operon is regulated by repression of transcriptional initiation and by attenuation i.e premature termination of transcription when tryptophan is prevalent in the environment.