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Meselson and Stahl carried… (Class 12 Biology 2023, Q31a)

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BiologyCBSE Class 12 · Board exam 2023, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2023: question 31(a)

Set in the CBSE Class 12 board exam of 2023, paper 57/1/1, section E, question 31(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(a) · 5 marks · Long answerMolecular Basis of Inheritance

Meselson and Stahl carried out an experiment to prove the nature of DNA replication. Recall the experiment and answer the following questions.
(i) Which two types of nitrogen were used by them in their experiment and why ?
(ii) Why did they take samples of E. coli at definite time intervals for their observation ?
(iii) State the role of caesium chloride density gradient in their experiment.
(iv) Write the conclusions they arrived at.

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

Two isotopes of an element found in DNA bases were used.

Hint 2

How long does E. coli take to divide?

Hint 3

The bands after one and two generations tell you whether parental strands stay together.

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) They used **normal (light) nitrogen, ** (as )

    ½ mark

  2. and **heavy nitrogen, ** (as ).

    ½ mark

  3. Why: nitrogen is incorporated into DNA bases, so the two isotopes make DNA of two different densities, heavy () and light (). Parental and newly made strands can then be told apart.

    1 mark

  4. (ii) E. coli divides about every 20 minutes. The -labelled cells were shifted to a medium and sampled at 20-minute intervals, so each sample showed the DNA after one more round of replication. This revealed the mode of replication generation by generation.

    1 mark

  5. (iii) Caesium chloride density gradient centrifugation separates DNA molecules by density. It distinguishes heavy (), hybrid () and light () DNA as separate bands.

    1 mark

  6. (iv) After one generation (20 min), all DNA was of hybrid/intermediate density. After two generations (40 min), DNA was equal amounts of hybrid and light. So they concluded that DNA replicates semiconservatively: each daughter DNA has one parental strand and one new strand.

    1 mark

Answer: (i) ¹⁴N and ¹⁵N, to make light and heavy DNA; (ii) generation time 20 min, so each sample = one replication round; (iii) separates DNA by density; (iv) DNA replication is semiconservative

Where the marks are

  • Names ¹⁴N and ¹⁵N (½ + ½ mark)
  • Reason: to produce DNA of distinguishable (light/heavy) densities (1 mark)
  • Sampling linked to the 20 min generation time (1 mark)
  • CsCl gradient separates heavy, hybrid and light DNA by density (1 mark)
  • Conclusion: semiconservative replication (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.