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A. Explain the… (Class 12 Biology SQP 2025-26, Q31B)

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BiologyCBSE Class 12 · Sample paper, session 2025-26 · 5 marks · Long answer

CBSE Class 12 Biology sample paper 2025-26: question 31(B)

From the official CBSE sample question paper for 2025-26, section E, question 31(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(B) · 5 marks · Long answerMolecular Basis of Inheritance

A. Explain the process of aminoacylation of tRNA. Mention its role in translation.
B. How do ribosomes in the cells act as factories for protein synthesis?
C. Given below is a strand of mRNA undergoing the process of translation, what will be the sequence of Amino acids that will be translated? Name the triplet codons that should be added to bring to the end of translation at X.

(For visually impaired students: C. Explain the phenomenon that forms the genetic basis to prove that codon is a triplet and it is read in a contiguous manner.)

Figure in the paper: mRNA strand with codons CGA, ACA (in ribosome, with tRNA UGU carrying Threonine and Arginine), CAC, CUU, CCG, GGA, CCG, CCG, CCG, CUU, AGU; region marked X at the end. Free tRNAs shown with anticodons and amino acids: Serine (UCA), Leucine (GAA), Glycine (CCU), Proline (GGC), Histidine (GUG), Leucine (GAG), Arginine (GCU). See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Charging of tRNA needs ATP and a specific enzyme.

Hint 2

Which rRNA in bacteria works as an enzyme?

Hint 3

Decode each triplet using the given tRNAs or the genetic code; translation ends at a codon that codes for no amino acid.

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. Aminoacylation (charging) of tRNA: Each amino acid is activated by ATP and attached to its specific tRNA. The enzyme aminoacyl-tRNA synthetase catalyses this, forming aminoacyl-tRNA.
    Role: The charged tRNA carries the correct amino acid to the matching codon. When two charged tRNAs come close on the ribosome, peptide bond formation between them becomes energetically favourable.

    1 mark

  2. B. The ribosome is the cell's protein-synthesising factory. When inactive, it exists as separate large and small subunits. Translation begins when the small subunit meets the mRNA and the large subunit then joins.

    1 mark

  3. The large subunit has two sites where successive aminoacyl-tRNAs bind, holding the amino acids close enough for a peptide bond. The ribosome moves codon by codon along the mRNA. Its rRNA (23S rRNA in bacteria) acts as a ribozyme that catalyses peptide bond formation.

    1 mark

  4. C. Reading the codons CGA–ACA–CAC–CUU–CCG–GGA–CCG–CCG–CCG–CUU–AGU gives the amino acid sequence:
    Arginine – Threonine – Histidine – Leucine – Proline – Glycine – Proline – Proline – Proline – Leucine – Serine.

    1 mark

  5. To end translation at X, a stop (termination) codon must be added: UAA, UAG or UGA. A release factor binds the stop codon and releases the polypeptide.

    1 mark

  6. (For visually impaired students, C) Frameshift mutations: Inserting or deleting one or two bases changes the reading frame from that point onward. Inserting or deleting three bases (or multiples of three) inserts or removes a whole codon and leaves the rest of the frame intact. This proves the codon is a triplet read contiguously, without punctuation.

Answer: A. Amino acids activated by ATP are linked to tRNA by aminoacyl-tRNA synthetase; makes peptide bond formation favourable. B. Ribosome subunits assemble on mRNA; large subunit holds two aminoacyl-tRNAs; 23S rRNA ribozyme forms peptide bonds. C. Arg-Thr-His-Leu-Pro-Gly-Pro-Pro-Pro-Leu-Ser; stop codon UAA/UAG/UGA at X.

Where the marks are

  • Aminoacylation by aminoacyl-tRNA synthetase with ATP and its role in peptide bond formation (1 mark)
  • Ribosome subunits assemble on mRNA; two binding sites in the large subunit (1 mark)
  • rRNA (23S) acts as ribozyme catalysing peptide bonds (1 mark)
  • Correct amino acid sequence (1 mark)
  • Stop codon UAA/UAG/UGA at X (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 2025-26: 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.