AQA AS Level Biology Paper 2, June 2019: Question 6

9 marks · Medium difficulty · Short Answer

Describe and explain aseptic techniques, differences between mitosis and meiosis, chromatid numbers, and conclusions on chromosome movement from spore tube data in Neurospora.

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Question text

06 A scientist crossed a strain of the fungus Neurospora producing pink spores with a

strain of Neurospora producing white spores.

To cross these strains, he used aseptic techniques. He moved a small agar cube

containing one strain of the fungus onto a new agar plate. Then he placed a second

agar cube containing the other strain of fungus next to the first agar cube.

06.1 Describe and explain three ways in which the scientist would ensure he used aseptic

techniques to move each cube of agar onto a new agar plate.

[3 marks]

In the life cycle of Neurospora most stages are haploid. Fusion of two haploid strains

of this fungus produces diploid zygotes. Nuclear division in these zygotes occurs by

meiosis.

06.2 Give two differences between mitosis and meiosis.

[2 marks]

At the end of meiosis, this fungus produces cells called spores.

The spores are produced in narrow tubes that restrict their movement. As a result,

each tube contains a single line of spores. The spores are coloured either pink or

*18* white.

The spore colour gene is located on a pair of homologous chromosomes. Each

zygote produced in this cross has one chromosome with a pink allele (p) and one

chromosome with a white allele (w).

This is shown in Figure 7.

Figure 7

06.3 There are seven chromosomes in a spore nucleus.

Place a tick ( ) in the box next to the number that represents the number of

chromatids present in the zygote shown in Figure 7.

[1 mark]

28 20

The scientist recorded the arrangement of coloured spores inside many narrow tubes.

His results are shown in Table 2.

Table 2

06.4 Using all the information in this question, what can you conclude from the scientist’s

results about the movement of chromosomes in meiosis in this fungus?

[3 marks]

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

1. Wash hands to remove/kill microbes 3 max Each must include an

explanation

3. Accept use sterile/

Wear gloves/apron to prevent contamination;

antiseptic/bleach for

2. Burning Bunsen close by to create upward ‘disinfect’

current of air;

4. Accept autoclave/

3. Disinfect bench/work on disinfected cloth to kill

use alcohol/steam/boil

microbes/prevent contamination;

for flame

4. Flame instrument/equipment to sterilise/kill

06.1 microbes/prevent contamination; 4. Accept scalpel/

5. Lift lid slightly to prevent entry of microbes; inoculating loop/wire

loop/loop/forceps/

spreader for

instrument

1. 3. 4. & 5. Accept

microorganisms/

spores/bacteria/fungi/

contaminants for

‘microbes’

Differences must be

Mitosis given first 2 max

given as comparisons

1. One division, two divisions in meiosis;

2. (Daughter) cells genetically identical, daughter Ignore references to

cells genetically different in meiosis; asexual / sexual

3. Two cells produced, (usually) four cells produced reproduction, growth,

in meiosis; repair & replacement

4. Diploid to diploid/haploid to haploid, diploid to

06.2 haploid in meiosis;

2. Reference to

5. Separation of homologous chromosomes only in ‘genetically’ needed

meiosis; once

6. Crossing over only in meiosis;

7. Independent segregation only in meiosis; 4. Accept same

number chromosomes

in mitosis, but half the

number in meiosis

06.3 28; 1 Automarking

1. Separation of homologous chromosomes 3 max 1. Accept description

(occurred) of homologous

OR chromosome

(Independent) segregation (occurred); movement for

2. (Arrangement/separation/segregation of ‘separation’

chromosomes is) random/ (almost) equally

frequent (in tubes 1 and 2); 1. Accept alleles for

3. Crossing over occurred in tube 3/10 tubes; ‘chromosomes’

10 4. (Crossing over) is rare/infrequent/in only 10 1. Accept as a

tubes; labelled diagram of

06.4 chromosomes

3. Accept as labelled

diagram of crossing

over

3. and 4. “Crossing

over occurred in only

10 tubes” = 2 marks

TOTAL 9

How to answer it

Neurospora Genetics & Cell Division Study Guide

What this question tests

This question assesses your practical understanding of aseptic techniques, your ability to compare mitosis and meiosis, your knowledge of chromosome/chromatid numbers during cell division, and your analytical skills in interpreting genetic cross data and spore arrangements in fungi.

Question 0.6.1

Aseptic Techniques

Describe and explain three ways in which the scientist would ensure he used aseptic techniques to move each cube of agar onto a new agar plate. [3 marks]

✅ Acceptable Answers (Any 3)

Wash hands: To remove or kill microbes.
Wear gloves/apron: To prevent contamination of the culture.
Use a burning Bunsen burner nearby: To create an upward current of warm air, drawing spores/airborne contaminants away from the plates.
Disinfect bench: Using a disinfected cloth/antiseptic to kill microbes on working surfaces.
Flame instruments: Sterilise inoculating loops, forceps, or scalpels (e.g., passing through a Bunsen flame) to kill microbes.
Lift lid slightly: Only partially open agar plates to prevent entry of airborne microorganisms.

❌ Common Errors

Missing explanations: Simply stating "wear gloves" or "flame the loop" is insufficient. You must state the "why" (e.g., "...to kill/prevent microbes or contamination") to secure both description and explanation marks.
Using vague terms like "clean the bench" instead of "disinfect".

Examiner Tip: Every marking point requires a paired action and reason. Always ask yourself: "What am I doing, and what does it stop?"

Question 0.6.2

Mitosis vs. Meiosis

Give two differences between mitosis and meiosis. [2 marks]

✅ Correct Answers (Any 2 comparisons)

1 nuclear division in mitosis vs. 2 nuclear divisions in meiosis.
Daughter cells are genetically identical in mitosis vs. genetically different in meiosis.
2 cells produced in mitosis vs. (usually) 4 cells produced in meiosis.
Diploid to diploid in mitosis vs. diploid to haploid in meiosis.
No pairing/separation of homologous chromosomes in mitosis vs. occurs in meiosis.
No crossing over or independent segregation in mitosis vs. both occur in meiosis.

🧠 Exam Technique

Must be a comparison: If you write "Mitosis produces diploid cells", you might not get the mark unless you explicitly contrast it with meiosis (e.g., "...whereas meiosis produces haploid cells").
The word genetically only needs to appear once across your answer.

Question 0.6.3

Chromatids in the Zygote

There are seven chromosomes in a spore nucleus. Place a tick in the box next to the number that represents the number of chromatids present in the zygote shown in Figure 7. [1 mark]

✅ Correct Answer

28 (with a tick next to the final box)

📐 Step-by-Step Breakdown

A spore nucleus is haploid (n), so n = 7 chromosomes.
A zygote is formed by the fusion of two haploid nuclei, making it diploid (2n). Therefore, the zygote has 2 × 7 = 14 chromosomes.
Before nuclear division (in a zygote/cell ready to divide), each chromosome replicates to form two sister chromatids joined by a centromere.
Total chromatids = 14 chromosomes × 2 chromatids each = 28 chromatids.

Question 0.6.4

Interpreting Genetic Data

Using all the information in this question, what can you conclude from the scientist’s results about the movement of chromosomes in meiosis in this fungus? [3 marks]

✅ Mark Scheme Points (3 max)

1. Separation / Segregation: Homologous chromosomes separated / independent segregation occurred (deduced from tubes 1 and 2 where pink and white spores separated).
2. Random arrangement: Arrangement/separation/segregation of chromosomes is random, as tubes 1 and 2 occur with almost equal frequency (81 vs 78).
3. Crossing over frequency: Crossing over occurred (proven by tube 3, where alleles mixed: pink-white-white-pink or similar pattern).
4. Rare crossing over: Crossing over is rare or infrequent because it only appeared in 10 tubes out of nearly 170 total tubes.

💡 Key Knowledge

Table 2 shows tetrad analysis in fungi. Normal 2:2 segregation patterns (tubes 1 & 2) reflect standard independent assortment during meiosis I. Mixed banding patterns (tube 3) are direct evidence of chiasma formation and crossing over between non-sister chromatids during prophase I of meiosis.

Topics

Biology · Practical skills · Required Practicals · 3.4 Genetic information, variation and relationships between organisms · Experimental design · AS practicals (1–6)

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.