AQA AS Level Biology Paper 1, June 2024: Question 3

✅ Expected Answer / Mark Scheme

• (Virus) attachment protein binds to complementary cell receptors.
• Viral nucleic acid (DNA/RNA) enters the host cell.
• Viral nucleic acid is replicated, OR reverse transcriptase produces DNA from RNA.
• Viral proteins are synthesised (capsids/capsomeres).
• New viral components are assembled and released from the cell.

💡 Key Knowledge

Because viruses are non-living and lack cellular machinery, they must hijack a host cell to reproduce. Memorise the standard chronological sequence: Attachment → Entry → Replication → Protein synthesis → Assembly → Release.

🧠 Exam Technique

This is a classic chronological 4-mark AO1 recall question. Ensure your points flow in logical order. Examiners award marks sequentially for the journey from surface binding down to cell exit.

❌ Common Errors

Students often lose marks by using vague terminology like "the virus enters the cell whole" (for enveloped or complex viruses, usually only the nucleic acid or capsid enters) or confusing genetic information with specific nucleic acid terminology where strict mark schemes apply.

✅ Expected Answer / Mark Scheme

Virus replication has killed the bacteria / cells inside the clear zones.

💡 Key Knowledge

In microbiology, clear zones on a bacterial lawn represent areas where microbial cells have undergone lysis (bursting) due to viral infection and replication (bacteriophage activity forming plaques).

🧠 Exam Technique

Keep your answer direct. This is a 1-mark AO2 application question. Use precise biological verbs accepted by the mark scheme: killed , lysed , destroyed , or burst .

❌ Common Errors

Vague statements like "the viruses ate the bacteria" or saying the clear zones are just "empty space without nutrients" without explicitly linking it to viral killing/lysis will fail to gain the mark.

✅ Expected Answer / Mark Scheme

Clear zones increase in size, OR more clear zones form, OR clear zones merge/form one big clear zone.

💡 Key Knowledge

As time progresses, progeny viruses released from previously lysed bacterial cells will infect adjacent living bacterial cells, spreading outwards and enlarging the plaques.

🧠 Exam Technique

Be concise and descriptive about spatial changes. Any indication that the zones grow larger, multiply, or fuse together will secure the single mark.

❌ Common Errors

Refusing to mention the zones directly (e.g. talking about "cloudiness increasing") will miss the mark. The prompt asks specifically about the appearance resulting from ongoing viral action.

✅ Expected Answer / Mark Scheme

• Identity of stage: Cytokinesis (or Telophase) [1 mark]
• Explanation: Division of cytoplasm / pinching of cell-surface membrane / new cell-surface membrane forming / chromosomes are not visible [1 mark]

💡 Key Knowledge

Figure 4 clearly depicts the final stages of cell division where the single cell body constricts in the middle (cleavage furrow formation) to yield two separate daughter cells.

🧠 Exam Technique

Read the negative constraint carefully: "Do not refer to organelles." Mentioning mitochondria, nuclei, or the Golgi apparatus will automatically disqualify your explanation mark even if factually correct.

❌ Common Errors

Stating "mitosis" as the stage instead of the specific sub-stage/process (cytokinesis/telophase), or accidentally referencing nuclear envelopes or chromosomes condensing/forming nuclei.