[Biology Class Notes] on LAC Operon Pdf for EXAM

Gene regulation in prokaryotes is often explained with the assistance of the Lac Operon model. Here the alteration in physiological and environmental conditions is often observed resulting in an alteration in expression in prokaryotes. It was observed by Jacob and Monod. The lac operon can be defined as an Operon or group of genes with a single promoter. The genes present in the operon encode the proteins that allow the bacteria to process lactose as an energy source.

Gene regulation is defined as any change in a gene’s expression that may result in a change in the produced amino acid sequence. Gene expression is the process of making the polypeptide chain that a gene encodes. As a result, we may state that the gene’s expression can be measured in terms of the amount of protein produced by the genes.

Regulation Of Gene Expression

We can deduce that gene regulation occurs at several stages of gene expression, including the following:

• At the replication stage, every mistake in copying the DNA might lead to a change in expression.

• At the transcriptional level, any error in polymerization during transcription can result in a shift in gene expression.

• Post-transcriptional level, there may be some alterations during post-transcriptional modification, such as RNA splicing.

• At the translational level, if a mistake in the attachment of mRNA to tRNA molecules occurs during translation, various modifications may occur.

An enzyme, such as peroxidase in bacteria, can be used to illustrate how the expression of a gene is regulated. Hydrolysis of hydrogen peroxide to water and oxygen is catalyzed by this enzyme. If the bacteria are relocated to a new environment where they are unable to produce hydrogen peroxide, the enzyme will no longer function. In this case, the bacteria will cease to manufacture the enzyme. As a result, we may conclude that the expression of genes is regulated by environmental, metabolic, and physiological factors.

Lac Operon Notes

• The lac operon contains the genes that are involved in metabolism.

• The genes are expressed when lactose is present but glucose is not.

• Catabolite activator proteins and lac repressor operons are turned on and off depending on glucose and lactose levels.

• The lac repressor suppresses transcription of the operon. In the presence of lactose, it ceases to function as a repressor.

• When glucose levels are low, the catabolite activator protein stimulates the operon’s transcription.

Structure of Lac Operon

The structural genes in the lac operon are lacZ, lacY, and lace, which encode galactosidase, permease, and transacetylase, respectively.

A single promoter (Plac) that resides upstream of these structural genes and binds RNA polymerase is responsible for the transcription.

There is an operator site (Olac) between the promoter and the structural gene and the lacI gene, which encodes the lac repressor protein. 

Laci has its promoter (PlacI), which binds RNA polymerase and causes lac repressor mRNA to be transcribed, resulting in lac repressor protein monomers being produced.

The active tetramer is composed of four identical repressor monomers that can be tightly bound to the lac operator site Olac.

Lac operon contains genes involved in metabolism. The genes are expressed only when lactose is present and glucose is absent. The operon is put in on and off mode in response to the glucose and lactose levels: catabolite activator protein and lac repressor. The lac repressor comes in the way of transcription of the operon. In the presence of lactose, it stops acting as a repressor. Catabolite activator protein activates the transcription of the operon, only glucose levels are low.