AQA AS Level Biology Paper 2, June 2024: Question 5

✅ Correct Answer / Marking Points

• Mark 1: Compare the DNA base sequence / nucleotide sequence (or mRNA sequence).
• Mark 2: The more closely related the species are, the more similar the base sequence / genes (or they share a more recent common ancestor).

💡 Key Knowledge

Over evolutionary time, random mutations accumulate in DNA. Closely related species shared a common ancestor recently, meaning there has been less time for mutations to alter base sequences; hence, their DNA sequences are more similar.

🧠 Exam Technique

Always state what is being compared. Simply writing "compare the genes" is too vague; specify base sequence or nucleotide sequence.

❌ Common Errors

Stating that they compare "amino acid sequences" — the question specifically asks about genes coding for rRNA. Ribosomal RNA is a non-coding functional RNA and is not translated into an amino acid sequence.

✅ Correct Answer

All cellular organisms have ribosomes / ribosomal RNA (rRNA).

💡 Key Knowledge

Ribosomes are essential for protein translation and are found in all three domains of cellular life (Bacteria, Archaea, and Eukarya). Because rRNA is universally present and highly conserved, it acts as a molecular "clock" that allows cross-domain comparisons.

❌ Common Errors

Writing "all organisms have DNA" or "all organisms need proteins". While true, this fails to explain why ribosomal RNA genes specifically can be used across every cellular organism.

✅ Correct Answer

• Reaching upper branches: To absorb/obtain more light / sunlight for photosynthesis [1 mark].
• Opening stomata only at night: Reduces transpiration / evaporation / water loss [1 mark].

🧠 Exam Technique

Always pair environmental adaptations with their physiological process:

• Light → Photosynthesis
• Night stomatal opening → Reduced transpiration / water loss (due to lower nocturnal temperatures and higher relative humidity).

❌ Common Errors

Writing merely "to get sunlight" without mentioning photosynthesis, or confusing water loss with heat loss.

✅ Correct Answer

• Point 1: To produce a single / few layer(s) of cells OR avoid overlapping cells [1 mark].
• Point 2: So that light can pass / penetrate through the tissue [1 mark].

💡 Key Knowledge

Optical (light) microscopes rely on transmitted light passing from a sub-stage lamp through the condenser, specimen, and objective lens. If tissue is too thick, light is absorbed or scattered, resulting in a dark, indistinguishable silhouette rather than individual cell structures.

❌ Common Errors

Giving vague responses such as "so you can see it clearly" or "to get higher magnification". You must explicitly reference light passing through and obtaining a single layer / avoiding overlapping.

✅ Correct Marking Points (Any 3)

• Use single, continuous, unbroken lines (no sketching; no crossing or hanging lines).
• Do not use shading or hatching.
• Draw only the outlines of tissues / do not draw individual cells (if making a low-power plan).
• Draw all structures to scale / proportional size.
• Include labels, annotations, or a title (with straight, uncrossed label lines).
• Include a magnification or scale bar.

🧠 Exam Technique

Carefully read the prompt condition: "Assume the student uses a sharp pencil." Do not write "use a sharp pencil" or "use a HB pencil", as the question has already eliminated this mark!

❌ Common Errors

• Suggesting artistic shading to "make it 3D" — strictly prohibited in scientific drawing.
• Sketchy, overlapping, or feathering pencil strokes.
• Arrowheads on label lines (label lines must touch the feature without an arrowhead).