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IB DP Biology- D2.2 Gene expression- IB Style Questions For HL Paper 2 -FA 2025

IBDP Biology HL Paper 2 -FA 2025- All Chapters

Question

DNA methylation is an important mechanism that influences gene activity by modifying transcription. Samples were collected from two colon cancer tumours (T1 and T2) and two healthy colon tissues (N1 and N2). A gene suspected of contributing to cancer development was investigated. Its promoter region was cloned into sections (A–J), and the methylation patterns at various promoter markers (numbered 32–269) were analysed as shown below.

a.i. Identify the stage of mitosis shown at label X and explain your reasoning. [1]
a.ii. Outline what the mitotic index of a tumour tissue sample indicates. [2]
a.iii. DNA contains regions that do not code for proteins. State two functions of these non-coding regions. [2]
1.
2.
b.i. Describe how the methylation pattern in tumour samples differs from that in normal samples. [2]
b.ii. Suggest one effect that abnormal methylation might have on gene function in tumour cells. [1]
▶️ Answer/Explanation
Detailed solution

a.i. Stage of mitosis labelled X: Late anaphase or early telophase.

  • The chromatids have clearly separated and are positioned near opposite poles of the cell.
  • Some sister chromatids still show stretched “tails,” a feature typical of late anaphase.
  • Since many chromosomes have already reached the poles, it may also represent early telophase.

a.ii.

  • The mitotic index represents the proportion of cells undergoing mitosis in a tissue sample.
  • A high mitotic index in tumour tissue indicates rapid and uncontrolled cell division, typical of aggressive cancers.

a.iii. Functions of non-coding DNA:

  • Promoter or regulatory regions control when and where genes are transcribed.
  • Telomeres protect chromosome ends and prevent loss of essential coding sequences during replication.

b.i.

  • Tumour samples show substantially higher levels of DNA methylation across most promoter markers compared to normal samples.
  • Certain markers (e.g., 258, 269) display similar methylation levels between tumour and normal tissue, indicating that not all sites are altered.

b.ii.

  • Excessive methylation can silence tumour-suppressor genes, allowing uncontrolled cell division and contributing to cancer progression.
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