AQA AS Level Biology Paper 2, June 2025: Question 5

✅ Correct Answer

• Ribonucleic acid (accept RNA, rRNA, or ribosomal RNA)
• Protein (accept polypeptide or amino acids)

💡 Key Knowledge

Ribosomes are non-membrane-bound complexes consisting of ribosomal RNA (rRNA) and ribosomal proteins. In eukaryotes, rRNA is transcribed and assembled with proteins within the nucleolus before export to the cytoplasm.

❌ Common Errors & Pitfalls

• Specifying the wrong RNA: Writing mRNA or tRNA loses the mark immediately. Ribosomes are built from rRNA.
• Confusing with chromatin: Stating histones or DNA. Histones associate with eukaryotic nuclear DNA, not ribosomes.
• Partial answer: Giving only "protein" or only "RNA" scores 0 marks.

🧠 Exam Technique

When an exam paper numbers lines 1 and 2 for a single mark, read carefully: examiners expect both lines to be completed correctly to secure that single mark.

✅ Correct Answers (Choose 2 of each)

Similarities (any 2):

• Both involve DNA replication.
• Both involve division of cytoplasm (or cytokinesis / formation of cell-surface membrane / one cell divides into two).
• Both involve protein synthesis (or synthesis of ribosomes).

Differences in binary fission (any 2):

• No histones synthesised / no histones associated with DNA in binary fission (bacterial DNA is naked).
• No mitosis (or no prophase / metaphase / anaphase / telophase / spindle) in binary fission.
• No replication of membrane-bound organelles in binary fission (prokaryotes lack membrane-bound organelles).

🧠 Exam Technique: Using Table 2

Notice the directive: "Use the information in Table 2". Each row in the table gives you a clue:

• First phase (G₁): Ribosomes & proteins synthesised → applies to bacteria too (Similarity).
• Second phase (S): DNA replicated (Similarity) vs histones synthesised (Difference).
• Third phase (G₂): Membrane-bound organelles replicate → absent in bacteria (Difference).
• Mitosis: Stages listed → bacteria divide by binary fission, not mitosis (Difference).
• Cytokinesis: Cytoplasm divides & membrane forms → occurs in both (Similarity).

❌ Common Errors

• Writing general cell biology facts not in the table: E.g., "bacteria have peptidoglycan walls" or "eukaryotes have 80S ribosomes". These are correct biological facts, but the question explicitly demanded using the information in Table 2.
• Incomplete comparative statements: For differences, you must clearly state which process lacks the feature (e.g., "no histones in binary fission" rather than just writing "histones").
• Confusing mitosis with cell division: Binary fission is cell division, but it is not mitosis. Mitosis involves chromosome condensation, spindle apparatus, and chromatid separation.

✅ Correct Answer Breakdown

1. Point 1 (Suggest): Checks for mutations / changes in base sequence. [1 mark]
2. Point 2 (Explain - choose one route):
   • Prevents the formation of non-functioning proteins / enzymes (or altered tertiary structure). [1 mark]
   • OR Prevents uncontrolled cell division / development of tumours / cancer. [1 mark]

💡 Biological Context: The G₂/M Checkpoint

The third phase of interphase is the G₂ phase. Checking accuracy ensures errors in replication (such as mispaired bases, insertions, or deletions) are repaired before chromosomes condense and enter mitosis.

If an uncorrected mutation alters a triplet code, it may change the amino acid sequence (primary structure), altering hydrogen/ionic/disulfide bonds and leading to a non-functional tertiary structure (e.g., an altered enzyme active site).

🧠 "Suggest and Explain" Structure

Always split your response into two distinct halves:

• What is being checked? Base sequence mistakes / mutations.
• What is the consequence of avoiding that problem? Ensuring functional proteins are produced OR preventing unregulated proliferation (cancer).

❌ Common Errors

• Vague answers: Stating "to stop cells being damaged" or "so the cell works properly" without referring to mutations, proteins/enzymes, or cancer.
• Only identifying without explaining: Writing "to stop mutations" alone only scores the first mark; you must explain why avoiding mutations is biologically advantageous.