AQA AS Level Biology Paper 2, June 2023: Question 5

5 marks · Medium difficulty · Short Answer

Identify chromosome arrangements after meiotic divisions in rice cells and explain similarities and differences in genetic variation causes between normal and mutant populations lacking crossing over.

Practise this question

Question

Figure 3 shows six diagrams labeled A to F of possible arrangements of chromosomes in cells. Diagrams A, B, and C show cells containing paired sister chromatids (X-shapes), with A having 12 pairs, B having 6 pairs, and C having 3 pairs. Diagrams D, E, and F show cells containing single uncopied chromosomes represented as vertical lines, with D showing 12 lines, E showing 6 lines, and F showing 3 lines. Below this, questions ask to identify diagrams representing rice cells with 12 homologous chromosome pairs after the first and second meiotic divisions, followed by a question discussing genetic variation in mutant rice lacking crossing over.
Question text

05.1 Figure 3 shows diagrams of six possible arrangements of chromosomes in cells.

Figure 3

Before meiosis, a cell of a rice plant has 12 pairs of homologous chromosomes

(24 chromosomes in total).

Give the letter of the diagram from Figure 3 that correctly shows the chromosome

content of rice cells after the first meiotic division and after the second meiotic

division.

[2 marks]

After first meiotic division

After second meiotic division

05.2 Scientists have produced a mutated rice variety in which there is no crossing over.

A population of the mutant rice variety produced by sexual reproduction shows

genetic variation. Populations of non-mutant rice varieties also show

genetic variation.

Suggest and explain the similarities and differences in the causes of genetic variation

within these rice populations.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme provides the answers for questions 05.1 and 05.2. For 05.1, after the first meiotic division is B and after the second is E, worth 2 marks. For 05.2, it awards up to 3 marks for similarities (independent segregation/assortment, random fertilisation, mutations) and differences (crossing over causing variation only in non-mutant populations), with a maximum of 2 marks for similarities.

Question Marking Guidance Mark Comments

After first meiotic division – B; 2

05.1 After second meiotic division – E; (2 x

AO2)

(Similarities) Max 2 for similarities

1. (Both populations) have (variation due to)

independent segregation/assortment (of

chromosomes/chromatids);

2. (Both populations) have (variation due to) 3 max

random fertilisation (of gametes);

05.2 (3 x

3. Both (populations) have (further) mutations; AO2)

(Difference)

4. Crossing over causes variation in non-mutant 4. Comparison can be

only; implied

How to answer it

Meiosis and Sources of Genetic Variation

What this question tests

This question assesses your understanding of cell division (meiosis I and meiosis II), chromosome behavior, ploidy levels, and the biological sources of genetic variation in sexually reproducing organisms (independent segregation, crossing over, random fertilization, and mutation).

Question 0.5.1

Chromosome Behavior During Meiosis

✅ Correct Answers

  • After first meiotic division: B
  • After second meiotic division: E
Total: 2 marks (2 × AO2)

💡 Key Knowledge

  • A starting diploid cell has 24 chromosomes (12 pairs).
  • Meiosis I: Homologous pairs separate. The cell becomes haploid, but chromosomes still consist of two sister chromatids. Diagram B shows 6 replicated (X-shaped) chromosomes.
  • Meiosis II: Sister chromatids separate. Diagram E shows 6 single-line unreplicated chromosomes.

🧠 Exam Technique

Count both the total number of structures and check whether they are single lines (unreplicated) or X-shapes (replicated chromatids). Remember that meiosis halves the chromosome number.

❌ Common Errors

  • Confusing meiosis I (separation of homologous pairs resulting in haploid cells with X-shapes) with mitosis or meiosis II.
  • Choosing diagrams with 12 structures instead of halving them to 6.
Question 0.5.2

Similarities and Differences in Genetic Variation

✅ Correct Answers (Max 3 marks)

Similarities (Max 2):

  • Both populations have variation due to independent segregation / assortment of chromosomes or chromatids.
  • Both populations have variation due to random fertilization of gametes.
  • Both populations have variation due to (further) mutations.

Difference (Max 1):

  • Crossing over causes genetic variation in non-mutant rice only (since the mutant has no crossing over).
Total: 3 marks max (3 × AO2)

💡 Key Knowledge

Meiosis generates genetic variation through two primary mechanisms: crossing over (prophase I) and independent segregation (anaphase I/II). If a mutation removes crossing over, independent segregation and other general sources of variation (random fertilization, gene mutations) remain fully operational.

🧠 Exam Technique

Always structure your answer clearly by explicitly labeling or separating similarities and differences. Use comparative language ("both populations..." versus "only the non-mutant...") to guarantee you hit marking points.

❌ Common Errors

Students often assume that lacking crossing over eliminates all genetic variation in the population, forgetting about independent segregation, random fertilization, and random mutations.

Topics

Biology · 3.4 Genetic information, variation and relationships between organisms

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.