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Molecular Basis of Inheritance - The Lac Operon

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Lac Operon is a polycistronic structural gene regulated by a common promoter and regulatory genes, as elucidated by Jacob and Monod.

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The operon consists of one regulatory gene (ii gene) and three structural genes (zz, yy, and aa).

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The ii gene codes for the repressor protein, which is synthesized constitutively.

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The zz gene codes for β\beta-galactosidase, responsible for the hydrolysis of lactose into glucose and galactose (C12H22O11+H2O→C6H12O6+C6H12O6C_{12}H_{22}O_{11} + H_{2}O \rightarrow C_{6}H_{12}O_{6} + C_{6}H_{12}O_{6}).

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The yy gene codes for permease, which increases the permeability of the cell to β\beta-galactosides.

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The aa gene codes for transacetylase, which transfers an acetyl group from acetyl-CoA to β\beta-galactosides.

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Lactose (or allolactose) acts as an inducer. When present, it binds to the repressor and inactivates it, allowing RNA polymerase access to the promoter.

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Regulation of the Lac Operon by a repressor is termed 'negative regulation', though it is also under 'positive regulation' control (via CAP-cAMP complex).

📐Formulae

Lactose+H2O→β-galactosidaseGlucose+Galactose\text{Lactose} + \text{H}_{2}\text{O} \xrightarrow{\beta\text{-galactosidase}} \text{Glucose} + \text{Galactose}

Repressor+Inducer→Inactive Repressor Complex\text{Repressor} + \text{Inducer} \rightarrow \text{Inactive Repressor Complex}

Transcription∝1[Active Repressor]\text{Transcription} \propto \frac{1}{[\text{Active Repressor}]}

💡Examples

Problem 1:

In an experimental setup of E. coliE.\ coli, the medium contains high levels of lactose but very low levels of glucose. Explain the status of the lac operon.

Solution:

The lac operon will be 'Switched On'.

Explanation:

Lactose acts as an inducer and binds to the repressor protein produced by the ii gene. This prevents the repressor from binding to the operator (OO). Consequently, RNA polymerase can bind to the promoter and transcribe the structural genes z,y,z, y, and aa. Low glucose levels also increase cAMP levels, facilitating the binding of the CAP-cAMP complex to the promoter, further enhancing transcription.

Problem 2:

What would be the result of a mutation in the ii gene that makes the repressor protein unable to bind to the operator (OO)?

Solution:

Constitutive expression of the z,y,z, y, and aa genes.

Explanation:

If the repressor protein cannot bind to the operator, there is no 'brake' on the system. RNA polymerase will continuously transcribe the structural genes regardless of whether the inducer (lactose) is present or absent.