C
CraftExam
medium difficulty6 min

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.

In one line: Lactose unlocks the block by disabling the repressor.

Concept Practice Coming Soon

Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

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 you can reuse

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.

Sketch it in your answer

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What happens to the lac operon when lactose is absent?

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

Common wrong answer

Writing that the operator produces repressor protein.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

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

  3. 3

    Add for 3 marks

    The lac operon, proposed by Jacob and Monod, includes regulatory and structural components.

Practice

Concept practice is coming soon

Login to access concept-level MCQs and weak-concept practice as they become available.

10 MCQs5 MinutesInstant Results
Login

Help improve this page

Found something confusing, incorrect, or missing?

Open Learning Map