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Study the diagram given below… (Class 12 Biology 2025, Q25)

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BiologyCBSE Class 12 · Board exam 2025, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2025: question 25

Set in the CBSE Class 12 board exam of 2025, paper 57/1/1, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerCompetency-basedMolecular Basis of Inheritance

Study the diagram given below and answer the questions that follows :

  1. (a)

    Identify the structure shown in the above figure.

    0 marks
  2. (b)

    Identify the labels P and Q.

    0 marks
  3. (c)

    Write the nature of histone proteins.

    0 marks
  4. (d)

    Distinguish between Euchromatin and Heterochromatin.

    0 marks

Figure in the paper: A spherical protein core (label Q points to it) with a thread-like strand (label P) wrapped around it, with a flat disc-like structure alongside; the strand continues out on both sides. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/1/1.

Hints

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

Hint 1

The structure is the 'beads on string' unit of chromatin.

Hint 2

The DNA is negatively charged. What charge must the core proteins carry for DNA to wrap around them?

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) Nucleosome.

    ½ mark

  2. (b) P = DNA (the negatively charged helix wrapped around the core).

    ½ mark

  3. Q = histone octamer (the core of eight histone molecules).

    ½ mark

  4. (c) Histones are basic, positively charged proteins, rich in the amino acids lysine and arginine.

    ½ mark

  5. (d)

    EuchromatinHeterochromatin
    Loosely packedDensely packed
    Stains lightlyStains darkly
    Transcriptionally activeTranscriptionally inactive
    1 mark

Answer: (a) Nucleosome; (b) P = DNA, Q = histone octamer; (c) basic/positively charged; (d) euchromatin is loosely packed, light-staining and active; heterochromatin is dense, dark-staining and inactive.

Where the marks are

  • Nucleosome (½ mark)
  • P = DNA (½ mark)
  • Q = histone octamer (½ mark)
  • Histones are basic/positively charged (½ mark)
  • Any one correct difference between euchromatin and heterochromatin (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 paper2025 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.