DP Biology (first assessment 2025)

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Question 22N.2.HL.TZ0.4d

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Date November 2022 Marks available [Maximum mark: 3] Reference code 22N.2.HL.TZ0.4d
Level HL Paper 2 Time zone TZ0
Command term Explain Question number d Adapted from N/A
d.
[Maximum mark: 3]
22N.2.HL.TZ0.4d
(d)

Explain the role of lactose in the expression of the gene for lactase production.

[3]

Markscheme
  1. lactose binds to repressor protein;
  2. repressor protein (with lactose bound) cannot block/bind to the promoter/Y;
  3. RNA polymerase/X binds to the promoter/transcribes the gene;
  4. lactase produced (if lactose present)/lactase production inhibited if lactose absent;

Accept the converse of mpa to mpd with lactose absent.

Examiners report

Many candidates realised from the diagram that lactose binds to the repressor. Fewer could explain convincingly that this removes the repressor from the promoter, so RNA polymerase can bind and transcribe the gene for lactase. Many candidates earned a mark by giving the last idea on the mark scheme, which is that presence of lactose results in lactase production. It should be emphasized that the lac operon does not need to be taught to future students — the expectation here was that answers would be based on general understanding of transcription and on the information provided in the question. These was no expectation that candidates would already be familiar with the lac operon.

Syllabus sections