DP Biology (last assessment 2024)

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Question 22M.2.HL.TZ2.2

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Date May 2022 Marks available [Maximum mark: 8] Reference code 22M.2.HL.TZ2.2
Level HL Paper 2 Time zone TZ2
Command term Calculate, Distinguish, Explain, Identify, State Question number 2 Adapted from N/A
2.
[Maximum mark: 8]
22M.2.HL.TZ2.2

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].]

(a.i)

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

[2]

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

OWTTE

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.ii)

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

[1]

Markscheme

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.

Examiners report

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

(a.iii)

State the role of cyclins in the cell cycle.

[1]

Markscheme
  1. (group of regulatory proteins that) control/regulate the cell cycle;
  2. activate cyclin-dependent kinases (which control cell cycle processes);
(b.i)

Distinguish between the structure of chromosomes in prokaryotes and eukaryotes.

[2]

Markscheme
  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;

Examiners report

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.ii)

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

[2]

Markscheme
  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;
Examiners report

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.

Syllabus sections