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Explain the processing of… (Class 12 Biology 2024, Q27)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 3 marks · Short answer

CBSE Class 12 Biology 2024: question 27

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerMolecular Basis of Inheritance

Explain the processing of heterogeneous nuclear RNA (hnRNA) into a fully functional mRNA in eukaryotes. Where does this processing occur in the cell ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

Eukaryotic genes are split genes. What must be removed from the transcript?

Hint 2

Something is added at each end of the hnRNA. What is added at the 5' end and what at the 3' end?

Hint 3

Where in the cell is RNA transcribed?

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. The primary transcript (hnRNA) made by RNA polymerase II contains both coding exons and non-coding introns. It is non-functional until processed.

  2. Capping: an unusual nucleotide, methyl guanosine triphosphate (m⁷Gppp), is added to the 5'-end of hnRNA.

    ½ mark

  3. Tailing: adenylate residues (200–300) are added to the 3'-end in a template-independent manner to form the poly-A tail.

    ½ mark

  4. Splicing: the introns are removed.

    ½ mark

  5. The exons are then joined together in a defined order.

    ½ mark

  6. The fully processed hnRNA is the mature, functional mRNA, which is transported to the cytoplasm for translation.

    ½ mark

  7. Site: this processing occurs in the nucleus.

    ½ mark

Answer: hnRNA undergoes 5' capping (m⁷Gppp), 3' tailing (poly-A) and splicing (introns removed, exons joined) to form mRNA; this takes place in the nucleus.

Where the marks are

  • Capping at the 5' end with methyl guanosine triphosphate (½ mark)
  • Tailing at the 3' end with poly-A (adenylate residues) (½ mark)
  • Splicing: removal of introns (½ mark)
  • Joining of exons in a defined order (½ mark)
  • Formation of functional mRNA (½ mark)
  • Site of processing: nucleus (½ 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 paper2024 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.