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Gene expresses itself in a… (Class 12 Biology 2023, Q29)

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BiologyCBSE Class 12 · Board exam 2023, set 57/1/1 · 4 marks · Case-based

CBSE Class 12 Biology 2023: question 29

Set in the CBSE Class 12 board exam of 2023, paper 57/1/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedMolecular Basis of Inheritance

Gene expresses itself in a cell system as a protein/enzyme. How does an expression of gene occur in a cell system and when does it need to occur, and how the gene expression is regulated in a prokaryote cell system was studied by the combined efforts of Jacque Monod, the biochemist and Francois Jacob, the geneticist. For their work on lactose metabolism in E. coli and introducing the concept of "lac operon" they were awarded the Nobel Prize in 1965.

  1. (a)

    Why is lac operon said to be a transcriptionally regulated system ?

    1 mark
  2. (b)

    It is said that "the lac operon has to be operational at a very low level in the bacterial cell all the time." Justify.

    1 mark
  3. (c)

    Why is the regulator gene in lac operon marked as 'i' gene ?

    1 mark
  4. (d)

    Draw a schematic diagram of lac operon in absence of inducer in the culture medium of the bacteria.

    1 mark

  5. OR

    (d)

    Draw a schematic diagram of lac operon in the presence of inducer in the culture medium of the bacteria.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Whether the genes are expressed depends on whether RNA polymerase can move past the operator.

Hint 2

How does lactose get into the cell in the first place? Which gene product helps?

Hint 3

'i' is the first letter of the role of the protein it codes for.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) The lac operon is controlled at the level of transcription, that is, by whether RNA polymerase is allowed to transcribe the structural genes. In the absence of lactose, the repressor binds the operator and blocks RNA polymerase, so no transcription occurs. In the presence of lactose (inducer), the repressor is inactivated and cannot bind the operator, so RNA polymerase transcribes the operon.

    1 mark

  2. (b) Lactose cannot enter the cell on its own; it needs permease, coded by the y gene of the operon. So the operon must always work at a very low level to keep some permease present. This lets lactose enter the cell and act as the inducer that switches the operon on.

    1 mark

  3. (c) The regulator gene is called 'i' because it codes for the inhibitor (repressor) protein. The repressor binds the operator and switches off the operon.

    1 mark

  4. (d) In absence of inducer (diagram described): draw DNA showing genes in order i – p – o – z – y – a. The i gene is transcribed to mRNA and translated into the repressor. The repressor binds the operator (o), so RNA polymerase at the promoter (p) is blocked. Label: 'no transcription of z, y, a'.

    1 mark

  5. (d) In presence of inducer (alternative, diagram described): draw the same gene order i – p – o – z – y – a. The repressor made by i is bound by the inducer (lactose/allolactose), becomes inactive and cannot bind the operator. RNA polymerase transcribes z, y, a into a polycistronic mRNA, which is translated into β-galactosidase, permease and transacetylase.

Answer: (a) Regulated by repressor allowing/blocking RNA polymerase transcription; (b) basal permease (y gene) needed for lactose entry; (c) i codes for inhibitor/repressor; (d) diagrams: repressor on operator (no transcription) / inducer–repressor complex, z, y, a transcribed

Where the marks are

  • Explains control at transcription via repressor–operator binding in absence/presence of lactose (1 mark)
  • Permease from y gene needed for lactose to enter the cell (1 mark)
  • i = inhibitor gene coding the repressor that binds the operator (1 mark)
  • Correctly labelled schematic of lac operon in either condition (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Molecular Basis of Inheritance, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 57/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.