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Stability, as one of the… (Class 12 Biology 2024, Q32a)

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BiologyCBSE Class 12 · Board exam 2024, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2024: question 32(a)

Set in the CBSE Class 12 board exam of 2024, paper 57/1/1, section E, question 32(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(a) · 5 marks · Long answerPrinciples of Inheritance and Variation

Stability, as one of the properties of genetic material, was very evident in one of the very early experiments in genetics. Name the scientist and describe his experiment. State the conclusion he arrived at.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/1/1.

Hints

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

Hint 1

Which experiment showed that heat-killed bacteria could still pass on a trait to living bacteria?

Hint 2

List the four combinations Griffith injected into mice and what happened to the mice each time.

Hint 3

What does it say about the genetic material if heat killed the bacteria but did not destroy its transforming ability?

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. The scientist was Frederick Griffith (1928).

    ½ mark

  2. He used two strains of Streptococcus pneumoniae and injected them into mice.

    ½ mark

  3. S strain: smooth, shiny colonies because of a mucous (polysaccharide) coat; virulent, causing pneumonia.

    ½ mark

  4. R strain: rough colonies without the mucous coat; non-virulent.

    ½ mark

  5. Experiment:

    • Live S strain injected → mice die.
    • Live R strain injected → mice live.
    ½ mark
    • Heat-killed S strain injected → mice live.
    ½ mark
    • Heat-killed S strain + live R strain injected → mice die, and live S-strain bacteria are recovered from the dead mice.
    ½ mark

  8. Conclusion 1: the R-strain bacteria had been transformed by the heat-killed S-strain bacteria.

    ½ mark

  9. Conclusion 2: some 'transforming principle' passed from the heat-killed S strain to the R strain and enabled it to make a smooth polysaccharide coat and become virulent. This must be due to the transfer of the genetic material.

    ½ mark

  10. Conclusion 3 (stability): the heat that killed the S bacteria did not destroy the properties of their genetic material. The genetic material is therefore stable and kept its ability to transform the R cells.

    ½ mark

Answer: Frederick Griffith – transformation experiment with S (virulent) and R (non-virulent) Streptococcus pneumoniae in mice; heat-killed S + live R killed mice; conclusion: transforming principle (genetic material) passed from heat-killed S to R and survived heat, showing its stability

Where the marks are

  • Names Frederick Griffith (½ mark)
  • Streptococcus pneumoniae injected into mice (½ mark)
  • S strain smooth, virulent; R strain rough, non-virulent (½ + ½ marks)
  • Results of the four injections (½ × 3 marks)
  • R strain transformed by heat-killed S (½ mark)
  • Transforming principle = genetic material transferred (½ mark)
  • Genetic material not destroyed by heat, showing stability (½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Principles of Inheritance and Variation, one at a time, marked against the answer key and marking points.Back to the full paper2024 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.