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A scientist was… (Class 12 Biology SQP 2026-27, Q7)

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BiologyCBSE Class 12 · Sample paper, session 2026-27 · 1 mark · MCQ

CBSE Class 12 Biology sample paper 2026-27: question 7

From the official CBSE sample question paper for 2026-27, section A, question 7, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q7 · 1 mark · MCQCompetency-basedMolecular Basis of Inheritance

A scientist was culturing in a medium initially, then he shifted to a medium. The DNA was extracted from after 60 minutes of transfer to Medium and performed density gradient centrifugation to check the isotope distribution in newly replicated DNA. What will be the densities of DNA molecules formed?

  1. (A)25% hybrid and 75% light
  2. (B)50% hybrid and 50% light
  3. (C)75% hybrid and 25% light
  4. (D)85% hybrid and 15% light

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

How long does one E. coli generation take?

Hint 2

Only 2 strands of the original heavy DNA remain, however many molecules are made.

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. E. coli divides every 20 min, so 60 min means 3 generations, giving DNA molecules.

  2. Replication is semiconservative, so only 2 of these 8 molecules still carry an old strand (hybrid). Hybrid DNA is and light DNA is . Option (A).

    1 mark

Answer: (A) 25% hybrid, 75% light

Where the marks are

  • Works out 3 generations and applies semiconservative replication to get 25% hybrid and 75% light (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 paperSample paper 2026-27: 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.