Question
DNA methylation has a critical role in gene regulation by affecting transcription. Samples were taken from two colon cancer tumours (T1 and T2) and two normal colon samples (N1 and N2). A particular gene was implicated as a possible cause of cancer. The promoter of this gene was cloned (A–J). The data show the DNA methylation patterns from these samples. The numbers (32–269) represent different markers in the promoter.
a.i. Identify the stage of mitosis labelled X in the image, giving a reason.
[1]
a.ii. Outline what is indicated by the mitotic index of tissue taken from a tumour. [2]
a.iii. DNA has regions that do not code for proteins. State two functions of these regions.
1.
2.[2]
b.i. Outline the difference in methylation pattern between tumorous and normal tissue samples. [2]
b.ii. Suggest a way methylation may affect tumour cell genes. [1]
▶️Answer/Explanation
a.i. Stage of mitosis labelled X: Late Anaphase or Early Telophase
- In the cell labelled X, we see the chromatids separated and clearly moving or already at opposite poles.
- The “tails” of some chromatids can still be seen stretching between the poles, which is characteristic of late anaphase.
- However, since many chromatids have already reached the poles, and the cell is nearing the end of division, it can also be interpreted as early telophase.
a.ii.
- The mitotic index shows the proportion of cells undergoing mitosis in a given tissue sample.
- A high mitotic index in tumour cells suggests rapid cell division, which is a sign of aggressive or fast-growing cancer.
a.iii.
- Promoter regions help regulate gene expression by controlling where transcription starts.
- Telomeres protect the ends of chromosomes from damage and prevent the loss of coding DNA during replication.
b.i.
- There is much more DNA methylation in the tumour samples than in the normal samples, especially across most promoter markers.
- Some markers, like 258 and 269, have similar methylation levels in both normal and tumour tissues, showing not all sites are affected.
b.ii.
- DNA methylation can silence tumour suppressor genes, preventing them from stopping abnormal cell division and leading to cancer growth.