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Observe the given picture… (Class 12 Biology 2026, Q28)

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BiologyCBSE Class 12 · Board exam 2026, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2026: question 28

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, section C, question 28, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 3 marks · Short answerCompetency-basedBiotechnology: Principles and Processes

Observe the given picture carefully. A mixture of DNA with fragments ranging from 100 base pairs to 1800 base pairs were separated by electrophoresis on agarose gel with the following arrangement :

  1. (a)

    What result will be obtained in staining with ethidium bromide ? Explain with reasons.

    1 mark
  2. (b)

    The above setup was modified as shown below and a band with 100 base pairs was obtained at X.
    What changes were made to the previous design to get a band at 'X'. Why did the band appear at 'X' ?

    2 marks

Figure in the paper: First figure: agarose gel tank with wells at the top near the positive (+) electrode at the top-left and negative (−) electrode at the bottom; arrow marks direction from + towards −; an arrow points to one well. Second figure: tank with negative (−) electrode at top-left near the wells and positive (+) at the bottom; several bands shown below the wells in lanes; the lowest band, marked X, is the farthest-migrated band in one lane. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/1/1.

Hints

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

Hint 1

What charge does DNA carry, and which electrode attracts it?

Hint 2

Compare the electrode next to the wells in the two set-ups.

Hint 3

In a gel, fragment size decides how far a fragment travels.

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) No bands will be seen in the gel after ethidium bromide staining.

    ½ mark

  2. Reason: DNA fragments are negatively charged, so they move only towards the anode (+). Here the wells are at the positive end, so the DNA does not move through the gel towards the cathode (−). No fragments are separated within the gel.

    ½ mark

  3. (b) Change made: the positions of the electrodes were interchanged, so the cathode (−) is now near the wells and the anode (+) is at the far end.

    1 mark

  4. Why the band appears at X: the negatively charged DNA now moves through the gel towards the anode. Agarose acts as a sieve, so smaller fragments move faster and farther. The smallest fragment (100 bp) therefore travels farthest and appears at X.

    1 mark

Answer: (a) No bands, because negatively charged DNA stays at or moves towards the anode near the wells and does not migrate through the gel; (b) anode and cathode were interchanged; the 100 bp fragment, being smallest, moved farthest towards the anode to X

Where the marks are

  • States that no bands will be seen (½ mark)
  • Reason: negatively charged DNA does not move towards the cathode (½ mark)
  • Electrodes interchanged (1 mark)
  • Smallest (100 bp) fragment moves farthest towards the anode due to the sieving effect (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Biotechnology: Principles and Processes, one at a time, marked against the answer key and marking points.Back to the full paper2026 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.