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The schematic… (Class 12 Biology SQP 2024-25, Q24)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2024-25: question 24

From the official CBSE sample question paper for 2024-25, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerMolecular Basis of Inheritance

The schematic representation given below shows the concept of Central Dogma.

A. During the process of replication and transcription the pairing of nitrogenous bases is not similar. Explain.
B. How is the above process modified in a retrovirus? Name the process.
C. Justify why during the process of transcription only a segment of DNA is copied into RNA.

Figure in the paper: Central dogma scheme: DNA with a circular arrow labelled replication; DNA → mRNA labelled transcription; mRNA → protein labelled translation. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Which base in RNA replaces thymine?

Hint 2

In a retrovirus the information flows opposite to the usual DNA → RNA direction.

Hint 3

Think about energy cost and whether a cell needs all its proteins at once.

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. In replication (DNA → DNA), adenine pairs with thymine (A=T), and G pairs with C.

    ½ mark

  2. A. In transcription (DNA → RNA), adenine of the DNA template pairs with uracil (A=U), because RNA has uracil in place of thymine. G still pairs with C.

    ½ mark

  3. B. In retroviruses the genetic material is RNA, which is used as a template to synthesise DNA (RNA → DNA) by the enzyme reverse transcriptase.

    ½ mark

  4. B. The process is called reverse transcription.

    ½ mark

  5. C. Copying the whole DNA would waste a great deal of energy, since transcription is an energy-expensive process. Only the gene (transcription unit) coding for the protein needed at that time is transcribed. Also, if both strands were copied, two complementary RNAs would form a double strand and could not be translated.

    1 mark

Answer: A. A pairs with T in replication but with U in transcription; B. RNA → DNA by reverse transcriptase – reverse transcription; C. Transcribing all DNA wastes energy; only the required gene is transcribed.

Where the marks are

  • A pairs with T during replication (0.5 mark)
  • A pairs with U during transcription (0.5 mark)
  • In retroviruses RNA is used to make DNA (0.5 mark)
  • Names reverse transcription (0.5 mark)
  • Justifies on energy cost or need-based transcription of only specific genes (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 2024-25: 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.