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Date	May 2019	Marks available	1	Reference code	19M.2.HL.TZ2.2
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Identify	Question number	2	Adapted from	N/A

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.

[Source: Philipp Schatz, Dimo Dietrich & Matthias Schuster. Rapid analysis of CpG methylation patterns using RNase T1
cleavage and MALDI-TOF. Nucleic Acids Research (2004) 32 (21): e167, doi:10.1093/nar/gnh165.
Reproduced by permission of Oxford University Press]

Identify the stage of mitosis labelled X in the image, giving a reason.

[1]

a.i.

Outline what is indicated by the mitotic index of tissue taken from a tumour.

[2]

a.ii.

DNA has regions that do not code for proteins. State two functions of these regions.

[2]

a.iii.

Outline the difference in methylation pattern between tumorous and normal tissue samples.

[2]

b.i.

Suggest a way methylation may affect tumour cell genes.

[1]

b.ii.

Markscheme

telophase because the chromosomes/chromatids have reached the poles
OR
«late» anaphase as some chromosomes/chromatids are still moving/tails visible ✔

OWTTE

a.i.

a. mitotic index is an indication of the ratio/percentage of cells undergoing mitosis/cell division ✔

b. cancer cells «generally» divide much more than normal «somatic» cells ✔

c. a high/elevated mitotic index in tumours / possible diagnosis of cancer /measure of how aggressive/fast growing the tumour is ✔

a.ii.

a. promoters / operators / regulation of gene expression/transcription ✔

b. telomeres/give protection to the end of chromosomes «during cell division» ✔

c. genes for tRNA/rRNA production ✔

d. other valid function for non-coding sequence ✔

Do not accept stop codon, accept centromeres (connecting sister chromatids).

a.iii.

a. «overall» much more methylation in the colon tumour samples than normal ✔

b. tumour and normal samples the markers 258 and 269 similar degree of methylation/fewer differences ✔

c. degree of methylation on certain markers may correlate with the presence of cancer / correct example of a marker only methylated in tumour cells eg marker 32 ✔

b.i.

a. «DNA» methylation may inhibit transcription of genes that would prevent cancer/tumor formation ✔

b. «DNA» methylation may increase mitosis/cell division leading to tumor formation ✔

Do not accept discussion of histone methylation.

b.ii.

Examiners report

Both anaphase and telophase were accepted as answers, but students had difficulty providing a reason that was an appropriate justification; for example, students might say telophase and then support this with the statement because chromatids are seen moving to opposite poles. Students sometimes referred to chromatids as homologous chromosomes. As this is an image of plant cells, students would be mistaken by referring to a cleavage furrow. The mechanism of cytokinesis involves the formation of a cell plate.

a.i.

Many students could accurately define the term mitotic index. An area of misunderstanding was to categorize the mitotic index as a rate rather than a ratio.

a.ii.

This question was well answered. The occasional answer made reference to processes associated with translation.

a.iii.

Many students detected that greater degrees of methylation in tumour samples. Fewer could accurately summarize the specific differences between methylation in the two samples.

b.i.

Students lacked the understanding of epigenetics necessary to make reasonable suggestions.

b.ii.

Syllabus sections

Core » Topic 1: Cell biology » 1.6 Cell division

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