AQA AS Level Biology Paper 2, June 2023: Question 7

✅ Correct Answer Points

• Point 1: A DNA mass of 1 indicates early interphase (G1 phase) / no cells have a mass less than 1, showing mitosis is the only division occurring.
• Point 2: A DNA mass of 2 indicates mitosis (or G2 phase, or each chromosome consisting of 2 chromatids).
• Point 3: A DNA mass between 1 and 2 indicates DNA replication is actively taking place (S phase).

💡 Key Knowledge

• 1 unit represents the baseline haploid/diploid amount of DNA before replication ($G_1$).
• 2 units represents the doubled DNA content following replication ($G_2$ and mitosis).
• The intermediate bars represent cells in the S phase where DNA is progressively synthesized.

🧠 Exam Technique

When interpreting histograms showing DNA distribution, always systematically anchor your answers to the three key stages: start value ($G_1$), end value ($G_2$/mitosis), and the transitional middle bars ($S$ phase).

❌ Common Errors

Students often lose marks by simply stating "DNA increases" without linking specific mass values ($1$ or $2$) to exact cell cycle phases.

✅ Correct Answer Points

• Difference: Fewer cells with a DNA mass of 1, OR more cells with a DNA mass between 1 and 2, OR more cells with a DNA mass of 2.
• Justification: Cell division is faster / uncontrolled / there is more frequent mitosis and cell replication.

💡 Key Knowledge

Tumour cells bypass normal cell cycle regulatory checkpoints, shortening the time spent in resting/growth phases ($G_0$/$G_1$) and increasing the proportion of actively dividing cells.

🧠 Exam Technique

Ensure you provide both parts of the command: state how the graph would look different (feature from the histogram) and justify it biologically using cell cycle dynamics.

❌ Common Errors

Writing vague statements like "mass of DNA is higher overall." The mark scheme explicitly notes that this phrasing is insufficient.

✅ Correct Answer Points

• Prophase: Chromosomes condense / shorten / thicken / become visible.
• Metaphase: Chromosomes line up on the equator / centre of the cell (and are attached to spindle fibres).

💡 Key Knowledge

Condensation during prophase prevents tangled DNA strands during movement. Alignment at the equator during metaphase ensures equal chromatid separation during anaphase.

🧠 Exam Technique

Keep descriptions precise and phase-specific. Do not confuse prophase events (condensation) with metaphase events (equatorial alignment).

❌ Common Errors

Stating that "DNA replicates" in prophase. DNA replication occurs strictly during interphase ($S$ phase), long before prophase begins.

📐 Step-by-Step Calculation

1. Rearrange the formula:
   P = F × V  →  V = P / F
2. Substitute the values:
   V = (6 × 10⁻¹⁹ W) / (3 × 10⁻¹¹ N)
3. Perform the calculation (in m s⁻¹):
   V = 2 × 10⁻⁸ m s⁻¹
4. Convert units to nm s⁻¹:
   Since 1 m = 10⁹ nm :
   2 × 10⁻⁸ × 10⁹ = 20 nm s⁻¹

❌ Common Calculation Traps

• Unit Neglect: Forgetting to convert metres per second ( m s⁻¹ ) into nanometres per second ( nm s⁻¹ ). Stopping at 2 × 10⁻⁸ will only net you 1 mark for working.
• Standard Form Errors: Entering powers incorrectly into your calculator. Always double-check negative exponents.