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Read the following passage… (Class 12 Biology 2026, Q29)

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BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 4 marks · Case-based

CBSE Class 12 Biology 2026: question 29

Set in the CBSE Class 12 board exam of 2026, paper 57/5/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedMolecular Basis of Inheritance

Read the following passage and answer the questions that follow :

The process of copying genetic information from template strand of DNA into RNA is called transcription. It is mediated by RNA polymerase. Transcription takes place in the nucleus of eukaryotic cells. In transcription, only a segment of DNA and only one of the strands is copied into RNA.

  1. (a)

    Why is the strand of DNA with polarity transcribed and not the other strand of polarity ?

    1 mark
  2. (b)(i)

    Why is hnRNA required to undergo splicing ?

    1 mark

  3. OR

    (b)(ii)

    Mention the two additional processes which hnRNA needs to undergo after splicing to become functional.

    1 mark
  4. (c)

    Why is only one strand of the DNA transcribed ? Give two reasons.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

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

Hint 1

Recall the direction in which every nucleic-acid polymerase adds nucleotides.

Hint 2

hnRNA has non-coding pieces and needs modifications at both of its ends.

Hint 3

Imagine both strands were copied: what would the two RNAs do to each other, and how many proteins would one gene make?

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) RNA polymerase can polymerise nucleotides only in the 5'→3' direction. The RNA can be built 5'→3' only on the template strand with 3'→5' polarity, so this strand is transcribed.

    1 mark

  2. (b)(i) hnRNA is the primary transcript. It contains both coding sequences (exons) and non-coding sequences (introns). Splicing removes the introns and joins the exons in a defined order, so that only coding sequence remains for translation.

    1 mark

  3. (b)(ii) Capping: an unusual nucleotide, methyl guanosine triphosphate (mGppp), is added to the 5' end of hnRNA. Tailing: about 200–300 adenylate residues (poly-A tail) are added at the 3' end in a template-independent manner.

    1 mark

  4. (c) Reason 1: If both strands were transcribed, one DNA segment would code for two different RNAs and hence two different polypeptides. This would complicate the genetic information transfer machinery, and one product might be useless to the cell.

    1 mark

  5. (c) Reason 2: The two RNA molecules produced at the same time would be complementary to each other. They would pair to form double-stranded RNA, which cannot be translated into protein, so transcription would be futile.

    1 mark

Answer: (a) RNA polymerase works only 5'→3'; (b)(i) to remove introns and join exons; (b)(ii) capping and tailing; (c) two different proteins would complicate the machinery; complementary RNAs would form non-translatable dsRNA.

Where the marks are

  • (a) RNA polymerase polymerises only in the 5'→3' direction (1 mark)
  • (b)(i) Splicing removes introns from hnRNA, which has both exons and introns (1 mark)
  • (b)(ii) Capping at the 5' end (½ mark) and poly-A tailing at the 3' end (½ mark)
  • (c) Two polypeptides would complicate the genetic machinery (1 mark)
  • (c) Complementary RNAs would form dsRNA that cannot be translated (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 paper2026 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.