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Date	May 2021	Marks available	2	Reference code	21M.2.HL.TZ1.1
Level	Higher level	Paper	Paper 2	Time zone	Time zone 1
Command term	Discuss	Question number	1	Adapted from	N/A

Question

The mechanism of action of vinblastine, an anticancer drug, was investigated over a range of concentrations. Vinblastine is an alkaloid isolated from the periwinkle plant (Catharansus roseus). The percentage of cells in mitosis and ratio of anaphase to metaphase in cells exposed to this drug in vitro for a fixed time were recorded. The data are displayed in two graphs.

[Source: Republished with permission of American Society for Pharmacology and Experimental Therapeutics, from Mechanism of Mitotic Block and Inhibition of Cell Proliferation by the Semisynthetic Vinca Alkaloids Vinorelbine and Its Newer Derivative Vinflunine, Molecular Pharmacology, Vivian K. Ngan, Krista Bellman, Bridget T. Hill, Leslie Wilson and Mary Ann Jordan, Volume 60 , Issue 1, 2001; permission conveyed through Copyright Clearance Center, Inc.]

By referring to both graphs, evaluate the hypothesis that vinblastine targets cells in mitosis and prevents them from completing the process.

[3]

Some anticancer drugs inhibit mitosis by blocking the formation of the spindle. Suggest one other way in which vinblastine could block mitosis.

[1]

Discuss one advantage and one disadvantage of using plant tissue to investigate drugs intended to treat cancer in humans.

Advantage:

Disadvantage:

[2]

Markscheme

Evaluation of evidence in graph on left

a. increase in (percentage of) cells in mitosis (as vinblastine concentration rises) ✔

b. supports hypothesis that cells get stuck in/cannot complete mitosis ✔

Evaluation of evidence in graph on right

c. drop in anaphase-metaphase ratio due to fewer cells in anaphase/more cells in metaphase ✔

d. cells not progressing from metaphase to anaphase/get stuck in metaphase ✔

Do not allow mpa if the candidate is arguing that the hypothesis is not supported.

a. causes microtubules/spindle fibres to break up / tubulin molecules to depolymerize ✔

b. prevents contraction of spindle microtubules/fibres ✔

c. disrupts/damages kinetochores/centromeres/microtubule motors/centrioles/centrosomes ✔

d. prevents separation/pulling apart of (sister) chromatids/chromosomes/centromeres ✔

e. prevents microtubules/spindle binding to chromatids/chromosomes/centromeres/DNA ✔

Mark the first suggestion only in the answer.

Do not allow answers about DNA replication or other processes that precede mitosis.

Advantage:

avoids risks for humans/harm to humans / more ethical (than with human patients/volunteers) ✔

Disadvantage:

differences between plant and human cells so humans may not respond in same way
OR
plants have cell wall/no centrioles/other relevant difference between plant and human cells ✔

Not enough for mpa to say ‘not using humans’.

For mpb there must be either a statement that differences between cells may cause a different response, or a specific example of a cell difference.

Examiners report

This was another challenging data-analysis task with marks spread across the whole range, but full marks were rarely awarded. Many candidates thought that a rise in mitotic index had to mean that more cells were entering mitosis and they failed to deduce from the drop to zero of the anaphase-metaphase ratio that cells were getting stuck in metaphase. As in (d), it was necessary both to understand how cells progress through the cell cycle and be able to make deductions from trends in quantitative data.

A wide range of possible ways in which mitosis could be blocked was accepted and about a third of candidates suggested one of these. Methods of blocking DNA replication or cytokinesis were not accepted as the question specifically referred to mitosis.

Most candidates were able to suggest an advantage of using plant tissue and any reasonable answer was accepted. Fewer gave a disadvantage that was credited. Vague answers such as plants are different from humans were not rewarded with a mark.

Syllabus sections

Core » Topic 1: Cell biology » 1.6 Cell division

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