AQA A-Level Biology Paper 1, June 2025: Question 7

📐 Step-by-Step Calculation

1. Mitotic divisions: 1 diploid A₁ epithelial cell undergoes 6 mitotic cycles:
   2⁶ = 64 primary spermatocytes (PS cells).
2. Meiotic division: Each PS cell undergoes 1 complete meiotic division to produce 4 sperm cells:
   64 × 4 = 256 sperm cells produced per original A₁ cell (2⁸ = 256).
3. Total A₁ cells required:
   Total needed = (1.5 × 10⁷) / 256 = 58,593.75 cells.
4. Convert to standard form:
   5.86 × 10⁴ (or 5.859375 × 10⁴ / 5.9 × 10⁴).

✅ Mark Scheme Breakdown

• 2 marks: Correct answer in standard form:
  5.859375 × 10⁴ or correctly rounded (e.g. 5.86 × 10⁴ , 5.9 × 10⁴ , 6 × 10⁴ ).
• 1 mark (partial credit):
  • Showing 2⁶ or 64 cells produced from mitosis.
  • Showing 2⁸ or 256 sperm per A₁ cell.
  • Calculating (1.5 × 10⁷) / 64 = 2.34 × 10⁵ (forgetting meiosis produces 4 gametes).
  • Correct final figure not in standard form (e.g. 58,594).

❌ Common Errors

• Forgetting Meiosis yields 4 cells: Only dividing by 64 instead of 256. Meiosis results in 4 haploid gametes per primary spermatocyte.
• Missing Standard Form: Leaving the answer as 58,594 forfeits the final mark.
• Incorrect Powers: Multiplying by 6 rather than 2⁶ (mitosis involves exponential doubling, not simple multiplication).

🧠 Exam Technique

Always write out every stage of working clearly. If your final arithmetic has an error or you forget to put it into standard form, showing 2⁶ = 64 or 256 guarantees you bank the method mark.

💡 DNA Mass Progression

• Start of Interphase: Given in question as 8 arbitrary units (diploid, un-replicated chromosomes).
• End of Interphase: DNA replicates during S phase, doubling DNA mass: 8 × 2 = 16 arbitrary units.
• End of Meiosis I: Homologous chromosomes separate into 2 daughter cells. The DNA per cell halves: 16 / 2 = 8 arbitrary units.
• End of Meiosis II: Sister chromatids separate into 2 daughter cells each. The DNA per cell halves again: 8 / 2 = 4 arbitrary units.

✅ Mark Scheme Criteria

• Mark 1: Axis presentation:
  • x-axis labelled with all 4 phases.
  • y-axis labelled "DNA mass per cell / arbitrary units" (or "au") with a linear scale covering at least 50% of the grid.
  • Bars must not touch (categorical/discrete data).
• Mark 2: Start of interphase plotted at 8 AND end of interphase plotted at 16.
• Mark 3: End of 1st division plotted at 8 AND end of 2nd division plotted at 4.

🧠 Bar Chart Rules

The independent variable is categorical (phases). In AQA mark schemes, bars for discrete categories must be separate and not touching. Ensure your scale on the y-axis is uniform, goes up to at least 16, and uses at least half the provided grid height.

❌ Common Misconceptions

• Drawing touching bars (histogram style): Immediate loss of Mark 1.
• Omitting units on y-axis: Writing just "Mass of DNA" without "arbitrary units" or "au" loses Mark 1.
• Confusing chromosome number with DNA mass: Chromosome number halves in Meiosis I (2n to n), but DNA mass doubles in S-phase (8 to 16) before halving twice (16 to 8 to 4).

💡 Chromosome Status at End of Meiosis I

• Figure 9 shows 6 chromosomes arranged as 3 homologous pairs of distinct sizes (1 large pair, 1 medium pair, 1 small pair).
• During Meiosis I, homologous pairs separate (reduction division: 2n → n).
• Therefore, each daughter cell receives one chromosome of each size (total of 3 chromosomes).
• Sister chromatids do not separate until Meiosis II; thus, every chromosome must still be drawn as 2 chromatids attached at a centromere (X-shaped).

✅ Exactly What to Draw in Figure 10

❌ Common Errors

• Drawing single chromatids (lines) rather than replicated chromosomes with 2 chromatids.
• Drawing 6 chromosomes (failing to realise reduction division has halved the chromosome number).
• Drawing all 3 chromosomes identical in length/size rather than 3 distinctly different sizes matching Figure 9.

✅ Mark Scheme

Name of mutation (1 mark):

• Non-disjunction (also accept aneuploidy or trisomy ).
• Reject: Polyploidy (this means whole extra sets of chromosomes, not just one extra).

Explanation (max 2 marks):

• Point 1: Homologous chromosomes / pairs do not separate (in anaphase I)
  OR Sister chromatids do not separate (in anaphase II).
• Point 2: Both chromosomes / both sister chromatids move to the same pole / same end of the cell
  OR Chromosomes fail to attach to spindle fibres.

🧠 Exam Technique: Precise Terminology

• Pairing the structure to the phase: If discussing Meiosis I, state homologous chromosomes fail to separate. If discussing Meiosis II, state chromatids fail to separate. Blurring these together can cause loss of marks.
• Direction of movement: Always state that both items move to one/the same pole, which results in one gamete having n+1 and the other n-1.
• Avoid "Polyploidy": Aneuploidy refers to an abnormal number of individual chromosomes (e.g. 2n+1). Polyploidy refers to 3n, 4n, etc.