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Date	May 2019	Marks available	3	Reference code	19M.2.HL.TZ2.1
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Compare and contrast	Question number	1	Adapted from	N/A

Question

Organisms often release chemicals when attacked as part of their defence system. Scientists studied lima bean plants (Phaseolus lunatus) infested with either an armyworm, Spodoptera exigua, or a herbivorous mite, Tetranychus urticae. Both organisms feed on lima bean leaves, causing the leaves to release chemicals.

[Source: https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png]

The study was conducted to see which defence chemicals were produced by lima bean leaves when infested by armyworms or herbivorous mites. The scientists identified a mixture of compounds (C) released by the plant when attacked. Two of the chemicals in this mixture were identified (C1 and C2).

The scientists hypothesized that the defence chemicals in C act as signals to produce other chemicals (X, Y and Z) that are also involved in the defence of the plant.

The graphs show the amounts of chemicals X, Y and Z produced when the plants were infested by either one of the two herbivores or treated with the different chemicals C1 or C2.

[Source: R Ozawa and G Arimura, Involvement of Jasmonate- and Salicylate-Related Signaling Pathways for the
Production of Specific Herbivore-Induced Volatiles in Plants, Plant and Cell Physiology, 2000, 41, 4, 391–398,
by permission of Oxford University Press]

RNA was collected from leaves of the plants after each treatment (armyworm, herbivorous mite and the chemicals C1 and C2). DNA copies of the extracted RNA were made by a process called reverse transcription. Targeted genes in the DNA were then amplified.

The scientists then used the transcribed DNA of each treatment to study the activation of three genes of the plants’ defence system. The DNA was separated by gel electrophoresis. The activation was tested one hour after treatment and again after 24 hours.

Distinguish between the data for the leaves infested with the armyworm and the leaves infested with the herbivorous mite.

[3]

Compare and contrast the effects of treatment of the leaves using C1 and C2 with the effects of infestation.

[3]

Identify the process that was used to amplify the targeted genes.

[1]

Deduce, with a reason, which gene is first transcribed when exposed to C2.

[1]

Each gene is activated by one or more of the treatments. From the gel electrophoresis data, discuss the impact of the herbivorous mite infestation on gene activation compared to treatment with C1 and C2.

[3]

Using the gene activation data, deduce, giving two reasons, whether the armyworm or the herbivorous mite has infested lima bean plants over a longer period of time.

[2]

Markscheme

a. armyworm «infestation» produced more X than Y than Z/decreasing amounts AND herbivorous mite showed the opposite pattern/more Z than Y than X ✔

b. armyworm «infestation» produced more X than herbivorous mite ✔

c. armyworm «infestation» produced more Y than herbivorous mite / Y is the middle value for both ✔ Accept OWTTE.

d. armyworm «infestation» produced less Z than the herbivorous mite ✔

e. other valid distinction ✔

Clear distinction required not simple lists of values.

For mp b-d accept vice versa.

a. C1 caused the leaf to produce two of the same chemicals/Y and Z as the attack of herbivorous mites in a similar pattern «but in lower quantities» ✔ OWTTE

b. C1 produces the least «total» amount of chemicals of all the treatments ✔

c. C2 has very similar pattern to those caused by the armyworms «but in lower quantities» ✔ OWTTE

d. both herbivores caused a greater production of chemicals/all three chemicals compared to either C1 or C2 ✔

e. armyworms cause the greatest total amount of chemical production of any of the other treatments ✔

f. other valid comparison of chemical effect versus herbivore effect ✔

Clear comparison required between herbivore infestation and chemical treatment not simple lists of values.

PCR

Accept RT-PCR.

gene 1 is first transcribed «after C2 treatment» as it shows activation after one hour ✔

a. herbivorous mites induce activation of gene 2 first «at 1 hour» and then also gene 1 and gene 3 «at 24 hours»
OR
herbivorous mite «infestation» is the only treatment to affect all three genes/leads to greater gene expression overall ✔
b. gene 2 activation similar for mite and C1 «at both 1 and 24 hours» ✔

c. gene 3 activation similar for mite and C2 «both at 24 hours» ✔

d. gene 1 activation slower for mite compared to C2 but more intense (than C2 at 24 hours) ✔ Both parts OWTTE required for mpd.

e. gene 1 and gene 3 expressed in higher amounts «after 24 hours» in mite infestation compared to C2 ✔

a. the greater «gene expression» response of the lima bean plant to the mite infestation indicates a longer evolutionary relationship ✔ OWTTE.

b. herbivorous mites cause more genes to be expressed/higher intensity of gene activation ✔

c. herbivorous mites cause a more immediate/earlier response in gene activation ✔

Examiners report

It was common for students to earn three marks for this question. At the same time, students often outlined individual graphs separately rather than using comparative terms.

This question was less well answered. A number of students misinterpreted the question by comparing the results of treating with C1 and C2 only and not comparing these to the effects of infestation.

This question on DNA amplification was correctly answered by most students. A common incorrect answer was DNA profiling.

It was common for students to correctly identify Gene 1 as the first gene being expressed. It was also common for students to correctly justify their answer making reference to the data.

For this question, students were commonly able to earn marks as there were a number of things for students to notice in the data.

Students were able to identify the mite as the species that had infected the plant for a longer period of time. Fewer understood that the question was referencing evolutionary history. Some students were referencing the time period of data collection for the investigation.

Syllabus sections

Core » Topic 3: Genetics » 3.5 Genetic modification and biotechnology

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