Lac Operon - Concept, Diagram, Notes, Gene Regulation

The lac operon is an operon utilised for the transport and metabolism of lactose in E.coli and many other enteric bacteria. As glucose is the preferred carbon source, the person allows for digestion of lactose when glucose is unavailable. Gene regulation of the lac operon was the first regulatory mechanism to be understood as an example of prokaryotic gene regulation.

This Story also Contains

1. What is Lac Operon?
2. Gene Expression Regulation
3. Gene Regulation in Prokaryotes
4. Concept of Lac Operon
5. Gene Regulation in Eukaryotes
6. MCQs on Lac Operon

The lactose metabolism system was used by Fracis Jacob and Jacques Monod to determine how the biological cell knows which enzyme to synthesise. Their work on lac operon got them the Nobel Prize in Physiology in 1965. The lac operon is a topic of the chapter Molecular Basis of Inheritance. It is an important chapter in the Biology subject.

What is Lac Operon?

It is a classic example of gene regulation in prokaryotes, although first described as a lactose operon in E. coli, where proteins that utilise this sugar are expressed. This system has been the premier model for understanding how gene expression is regulated in transcription in prokaryotes.

François Jacob and Jacques Monod first proposed the concept of the lac operon in the 1960s. Their work on the mechanism of the lac operon revealed how genes are turned on or off in response to environmental changes and was awarded the Nobel Prize in Physiology or Medicine in 1965. The lac operon model explained by the roles of the promoter, operator, structural genes, and the regulatory “i” gene in the lac operon forms the basic concept of Class 12 biology.

This operon clearly shows the concept of an operon and how prokaryotes have efficiently regulated gene expression. Knowing what the lac operon is and how it functions enhances our knowledge of prokaryotic as well as eukaryotic gene regulation.

Gene Expression Regulation

Gene regulation and gene expression are required to monitor cellular functioning and several pre-and post-transcriptional modifications. Regulation of gene expression is important because:

• It keeps a check on normal cellular functioning.

• Enables satisfaction of cell requirements successfully from the environment.

• It is a process that incorporates several mechanisms, whose controls are at transcription, post-transcriptional modifications, and translational control levels as is shown below.

Gene Regulation in Prokaryotes

Gene regulation in prokaryotes means with the help of activator and repressor proteins that bind to DNA. The processes control the transcription of a gene either positively or negatively. This is especially efficient in a prokaryotic system that very often has groups of genes in clusters called operons, whose process of transcription is driven by a single promoter into one mRNA for coordinated expression of these genes.

The lac operon is a classic example of the concept of an operon, which describes how bacteria, such as E. coli, manage their genomes as they respond to environmental stimuli. The lac operon model allows for the coordinated expression of several genes encoding proteins required to metabolise lactose in the presence of lactose. Lac operon consists of a promoter, operator, structural genes, and the regulatory i gene in the lac operon, whose structure provides a tight control mechanism.

Diagram: Lac Operon

The given diagram shows the structure and components of Lac Operon

Concept of Lac Operon

The lac operon regulation unit consists of many different genes that help in glucose metabolism in prokaryotes. The structure of Lac Operon has the following components:

• Promoter (P): The site where the RNA polymerase binds to initiate transcription.

• Operator (O): A region in the DNA that the repressor protein can bind to and, in so doing, inhibits transcription.

• Structural Genes: Genes that code for lacZ, lacY and lacA.

• lacZ: Coats for β-galactosidase, an enzyme that degrades lactose to glucose and galactose.

• lacY: Codes for protein permease, which permits passage of lactose into the cell.

• lacA: It's the gene for transacetylase acting in the metabolism of lactose.

• Repressor Protein (LacI): In the absence of the inducer, lactose, it remains bound over the operator and hinders transcription.

Mechanism of Action

Lactose acts as an inducer if present and a repressor of the operons when unavailable.

Lactose Absent: The repressor protein binds to the operator and hence will prevent access by the transcribing RNA polymerase to the structural genes.

Lactose Present: Lactose is metabolised into allolactose, which is an enzymatic product of the activity of β-galactosidase. Allolactose binds with the repressor protein, resulting in a change in its shape and releasing the operator.

Under such conditions, the structural genes in the operational network are now exposed and will be transcribed by RNA polymerase. The eventual results are the production of enzymes that metabolise lactose.

Lac Operon Mutations

Some of the mutations of the lac operon lead to very dramatic effects, for example:

• lacZ: The system can no longer digest lactose since β-galactosidase will not be synthesised.

• lacY: Lactose intake will be lowered drastically since permease is missing.

• lacI: There is no functional repressor. Thus, in the case of the absence of lactose, the operon would be induced.

Such types of mutations serve in the biochemical function of the operon based on the metabolic effects of lactose.

Experimental Evidence

The experiments conducted by Jacob and Monod described the lac operon as follows:

• PaJaMo Experiment: Demonstrated how the enzyme β-galactosidase is inducible with lactose, and this, in turn, provided the hypothesis of a system that would be an inducible operon.

• Merodiploid Analysis: Partial diploids were employed to reach up to the functions of several parts or sub-components of the lac operon, such as the operator or the repressor.

These experiments proved gene regulation to be a model that regulated the lac operon.

Gene Regulation in Eukaryotes

While prokaryotes have relatively simple regulation of gene expression, regulation in eukaryotes is much more complicated due to the structure of chromatin and the multifactorial regulatory levels. Eukaryotes use epigenetic changes, transcription factors, and interference RNA to regulate gene expression. Unlike what has happened in operon systems, such as lac, eukaryotic regulation must cross signals from different cell areas and developmental stages.

In eukaryotes, the existence of enhancers and silencers, coupled with a very large diversity of transcription factors, is essential for tight control. Prokaryotic operons, such as the lac operon, operate in a highly effective fashion because their genes share promoters. However, eukaryotic genes are much more complex. For example, RNA polymerase activity at a promoter is modulated by several elements to produce the accuracy required to create a multicellular organism.

While far removed from systems such as the lac operon model, the study of eukaryotic regulation compared with prokaryotic mechanisms tells much about complexity and adaptability in life. These concepts are generally explained in Class 12 biology along with the lac and trp operons. They show diversity in gene regulation.

MCQs on Lac Operon

Q1. Directions: In the following questions, a statement of Assertion (A) is followed by a statement of reason (R).

Assertion: The β-galactosidase enzyme is synthesised by the lacZ gene.

Reason: b- galactosidase converts lactose into glucose and galactose.

Mark the correct choice as:

Option 1: If both assertion and reason are true and the reason is the correct explanation of the assertion.

Option 2: If both assertion and reason are true but the reason is not the correct explanation of the assertion.

Option 3: If the assertion is true but the reason is false.

Option 4: If both assertion and reason are false.

Correct answer: (2) If both assertion and reason are true, but the reason is not the correct explanation of the assertion.

Explanation:

The lac operon -In the lac operon, a polycistronic structural gene is regulated by a common promoter and regulatory genes. Such arrangements are very common in bacteria and are referred to as operons. Bacteria prefer glucose as a carbon source. If glucose is absent, only then is lactose consumed by bacteria, which requires B-galactosidase and lactose permease enzymes. B- Galactosidase enzyme is a product of the lac-z gene.

Hence, the correct answer is Option (2) If both assertion and reason are true but the reason is not the correct explanation of the assertion.

Q2. Which enzyme/s will be produced in a cell in which there is a nonsense mutation in the lac gene?

Option 1: Lactose permease and transacetylase

Option 2: -galactosidase

Option 3: Lactose permease

Option 4: Transacetylase

Correct answer: 2) β-galactosidase.

Explanation:

The lac operon in E. coli has three structural genes:

1. lacZ → codes for β-galactosidase (breaks lactose into glucose and galactose).

2. lacY → codes for permease (helps lactose enter the cell).

3. lacA → codes for transacetylase (function not essential for lactose metabolism)

A nonsense mutation in the lacY gene will result in early termination and stop permease from being produced. However, because lacZ (β-galactosidase) lies upstream of lacY in the operon it will still be transcribed and translated.

Therefore, a mutation in lacY does not prevent the cell from producing β-galactosidase; nevertheless, the absence of permease will result in a decrease in lactose uptake.

Hence, the correct answer is option 2) β-galactosidase.

Q3. Match column I with column II and select the correct option from the given codes.

Column I Column II

1. Operator site i)Binding site for RNA polymerase

2. Promoter site ii) Binding site for repressor molecule

3. Regulator gene iii) Codes for protein / enzyme

4. Structural gene iv) Codes for repressor molecule

Option 1: A-(ii), B-(i), C-(iii), D- (iv)

Option 2: A-(ii), B-(i), C-(iv), D- (iii)

Option 3: A-(iv), B-(iii), C-(i), D- (ii)

Option 4: A-(ii), B-(iii), C-(i), D- (iv)

Correct answer: 2) A-(ii), B-(i), C-(iv), D- (iii)

Explanation:

Hence Correct option is 2) A-(ii), B-(i), C-(iv), D- (iii)

Also Read-

• Practice MCQs on Lac Operon

• Structure of RNA

• RNA World - The Genetic Material