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How is the rate of… (Class 12 Biology SQP 2025-26, Q18)

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BiologyCBSE Class 12 · Sample paper, session 2025-26 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2025-26: question 18

From the official CBSE sample question paper for 2025-26, section B, question 18, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18 · 2 marks · Very short answerMolecular Basis of Inheritance

How is the rate of initiation of RNA polymerase at a given promoter in a transcriptional unit of prokaryotes regulated?

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

Think of the lac operon and what binds next to the promoter.

Hint 2

Regulatory proteins can switch the gene on or off by acting at the operator.

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 ability of RNA polymerase to start transcription at a promoter depends on accessory (regulatory) proteins. These proteins help the polymerase recognise the start site.

    1 mark

  2. The regulatory proteins can act positively (activators) or negatively (repressors). In an operon, they work by interacting with the operator, which lies next to the promoter. A repressor bound to the operator blocks RNA polymerase, as in the lac operon.

    1 mark

Answer: By accessory regulatory proteins (activators/repressors) that act on the operator adjacent to the promoter and control recognition of start sites by RNA polymerase.

Where the marks are

  • Accessory proteins regulate the recognition of start sites by RNA polymerase (1 mark)
  • Positive (activator) and negative (repressor) regulation through the operator next to the promoter (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.