Skip to content

(i) Why did Hershey and… (Class 12 Biology 2024, Q33b)

questionbuddy.in/q/cbse-12-biology-2024-57-5-1-q33b-did-hershey-chase-use-experiment

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

CBSE Class 12 Biology 2024: question 33(b)

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section E, question 33(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(b) · 5 marks · Long answerMolecular Basis of Inheritance

(i) Why did Hershey and Chase use 'S' and 'P' in their experiment ? Explain. (1)
(ii) State the importance of (1) blending and (2) centrifugation in their experiment. (2)
(iii) Write the conclusion they arrived at the end of their experiment. (2)

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

Which of the two elements is found only in proteins, and which only in nucleic acids?

Hint 2

After infection, the viral coats are still stuck to the bacteria. Think about what is needed first to remove them, and then to separate them.

Hint 3

Ask where the radioactivity was found, in the bacterial pellet or in the supernatant, for each label.

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. (i) Sulphur is present in protein but not in DNA, so S labelled the viral protein coat. Phosphorus is present in DNA but not in protein, so P labelled the viral DNA.

    ½ mark

  2. Labelling each molecule separately let them trace which one, protein or DNA, entered the bacteria (E. coli) on infection.

    ½ mark

  3. (ii)(1) Blending: shaking (agitating) the culture in a blender detached the empty viral coats from the surface of the bacteria after infection.

    1 mark

  4. (ii)(2) Centrifugation: spinning separated the heavier bacteria, which formed a pellet, from the lighter free viral coats, which stayed in the supernatant. The radioactivity of each fraction could then be checked.

    1 mark

  5. (iii) Observation: bacteria infected with P-labelled phages were radioactive, while the S label stayed in the supernatant and not in the bacteria. So only the viral DNA entered the bacteria.

    1 mark

  6. Conclusion: DNA, not protein, is the genetic material that passes from the virus to the bacterium.

    1 mark

Answer: (i) S labels protein and P labels DNA, to trace which enters the bacteria; (ii) blending removes viral coats from bacteria, and centrifugation separates bacteria from the viral coats; (iii) DNA is the genetic material.

Where the marks are

  • States that S labels protein (it is absent from DNA) and P labels DNA (it is absent from protein) (½ mark)
  • Explains that the labels trace which molecule passes into the bacteria (½ mark)
  • Blending detaches viral coats from the bacteria (1 mark)
  • Centrifugation separates bacteria (pellet) from viral particles or coats (supernatant) (1 mark)
  • Concludes that DNA is the genetic material, supported by the radioactivity observation (2 marks)

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 paper2024 board paper, set 57/5/1: 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.