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Regulation of Gene Expression: Lac Operon

The lac operon is an inducible gene-regulation system in bacteria where lactose or its isomer acts as an inducer, allowing transcription of genes needed for lactose metabolism.

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Student-friendly explanation

The lac operon, proposed by Jacob and Monod, includes regulatory and structural components. The structural genes z, y, and a code for enzymes involved in lactose metabolism. A promoter provides the RNA polymerase binding site, while the operator controls access to transcription. In absence of lactose, the repressor binds the operator and blocks transcription. When lactose is present, the inducer binds the repressor and changes its shape, so it cannot bind the operator. RNA polymerase then transcribes structural genes. This shows how cells conserve energy by expressing genes only when their products are needed.

How to write this in exams

  1. 1

    Start with the exact idea

    The lac operon is an inducible gene-regulation system in bacteria where lactose or its isomer acts as an inducer, allowing transcription of genes needed for lactose metabolism.

  2. 2

    Then show how to use it

    Name operon components, describe repressor formation, explain operator blocking without lactose, show inducer binding when lactose is present, state RNA polymerase transcription of structural genes, link enzyme production to lactose metabolism.

  3. 3

    Add one concrete example

    When E. coli grows in a medium containing lactose, beta-galactosidase is produced because the inducer prevents the repressor from blocking the operator.

  4. 4

    Avoid this incomplete answer

    Writing that the operator produces repressor protein.

Definition

The lac operon is an inducible gene-regulation system in bacteria where lactose or its isomer acts as an inducer, allowing transcription of genes needed for lactose metabolism.

Example

When E. coli grows in a medium containing lactose, beta-galactosidase is produced because the inducer prevents the repressor from blocking the operator.

Rule to remember

Regulation rule: no inducer means repressor active and operon off; inducer present means repressor inactive and operon on.

Memory hook

Lactose unlocks the block by disabling the repressor.

Examples and method

Worked example

If a question states that inducer is added to bacterial culture, explain that repressor becomes inactive, operator is free, and structural genes are transcribed.

Method to apply

Name operon components, describe repressor formation, explain operator blocking without lactose, show inducer binding when lactose is present, state RNA polymerase transcription of structural genes, link enzyme production to lactose metabolism.

Diagram support

Label regulatory gene, promoter, operator, structural genes lac z, lac y, lac a, repressor, inducer, RNA polymerase, and mRNA.

How CBSE asks it

Asked as labelled operon diagram, on-off condition explanation, assertion-reason on inducer, or role of z, y, and a genes.

Avoid common mistakes

Common confusion

Students often say lactose directly binds RNA polymerase. The inducer binds the repressor, which then allows RNA polymerase to transcribe.

Common wrong answer

Writing that the operator produces repressor protein.

Exam tip

Draw the operon in order and explain separately what happens in absence and presence of inducer.

Quick check

What happens to the lac operon when lactose is absent?

When lactose is absent, the repressor binds to the operator and blocks RNA polymerase movement. Therefore, the structural genes are not transcribed effectively.

Answer writing and exam use

1-mark answer

Define lac operon as inducible, draw component order, explain off state without lactose, explain on state with inducer, and mention enzyme production from structural genes.

2-mark answer

Define lac operon as inducible, draw component order, explain off state without lactose, explain on state with inducer, and mention enzyme production from structural genes.

3-mark answer

Define lac operon as inducible, draw component order, explain off state without lactose, explain on state with inducer, and mention enzyme production from structural genes.
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