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Date May 2022 Marks available 2 Reference code 22M.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

The onion (Allium cepa) root cells shown in the micrograph are in different stages of mitosis.

[Source: Reischig, J., 2014. Mitosis (261 13) [Pressed; root meristem of onion]. [image online] Available at https://commons.wikimedia.org/wiki/File:Mitosis_(261_13)_Pressed;_root_meristem_of_onion_(cells_in_prophase,_metaphase,_anaphase,_telophase).jpg   This file is licensed under the Creative Commons Attribution -ShareAlike 3.0 Unported (CC BY-SA 3.0) https://creativecommons.org/licenses/by-sa/3.0/deed.en [Accessed 3 December 2019].]

Identify, with a reason, the stage shown at X.

[2]
a.i.

Calculate the length of the entire cell labelled Y, showing your working.

[1]
a.ii.

State the role of cyclins in the cell cycle.

[1]
a.iii.

Distinguish between the structure of chromosomes in prokaryotes and eukaryotes.

[2]
b.i.

Explain Cairns’s technique to measure the length of the DNA molecule.

[2]
b.ii.

Markscheme

  1. anaphase;
  2. the (replicated) chromosomes/chromatids are separating/moving to opposite poles of the cell;

OWTTE

a.i.

50 μm = 27/28/29 mm, Y = 8/9/10 mm
OR
50 x 9 /27
OR
16.7 μm (accept answers in the range of 14.8 μm to 17.2 μm)

Award [1] for correct ratios not precise measurements in the work or [1] for correct answer with correct unit.

a.ii.
  1. (group of regulatory proteins that) control/regulate the cell cycle;
  2. activate cyclin-dependent kinases (which control cell cycle processes);
a.iii.
  1. prokaryotes (usually) have one chromosome while eukaryotes have numerous chromosomes;
  2. prokaryotes have a circular chromosome while eukaryotes have linear ones;
  3. eukaryotes’ chromosomes are associated with histones/proteins but prokaryotes/Eubacteria have naked DNA vs eukaryote DNA associated to proteins/histones;

Accept only differences.

Differentiating terms expected;

b.i.
  1. Cairns grew prokaryotes/E. coli in radioactive thymidine/thymine/thymine containing tritium;
  2. contents of cell put on photographic film/surface (for several weeks) / used autoradiography and electron microscopes;
  3. measured the length of the DNA molecule and photographed it / produced image of DNA;
  4. could show the new strands were all labelled with thymidine/thymine;
b.ii.

Examiners report

This question was generally well done; a few identified the incorrect phase of mitosis, but the observations about cellular events being observed was often correct.

a.i.

Most answers were correct, but some calculations were difficult to follow; some errors in unit conversions led to incorrect decimal places.

a.ii.
[N/A]
a.iii.

This question led to a large number of correct answers. Some answers focused on the difference between prokaryotes and eukaryotes instead of their chromosomes.

b.i.

Asking students to outline Cairn’s technique led to a number of blank answers. That there was a role for autoradiography was better known than how this was used to obtain the length estimate.

b.ii.

Syllabus sections

Core » Topic 1: Cell biology » 1.6 Cell division
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