Distinguish between the lytic and lysogenic cycles of a bacteriophage, with reference to the fate of the viral genome in each cycle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. In the lytic cycle the viral genome is replicated immediately and directs the production of new phage components, whereas in the lysogenic cycle the viral genome is integrated into the host chromosome as a prophage.
2. In the lytic cycle the host cell is lysed and destroyed at the end of the cycle, releasing new phage particles, whereas in the lysogenic cycle the host cell survives and continues to divide.
3. In the lysogenic cycle the prophage is replicated passively with the host DNA each time the cell divides, so daughter cells inherit the viral genome; in the lytic cycle no such vertical transmission occurs.
4. The lysogenic cycle can be induced or switched to the lytic cycle (e.g. by UV radiation), at which point the prophage excises from the host chromosome and enters the lytic pathway.
2. In the lytic cycle the host cell is lysed and destroyed at the end of the cycle, releasing new phage particles, whereas in the lysogenic cycle the host cell survives and continues to divide.
3. In the lysogenic cycle the prophage is replicated passively with the host DNA each time the cell divides, so daughter cells inherit the viral genome; in the lytic cycle no such vertical transmission occurs.
4. The lysogenic cycle can be induced or switched to the lytic cycle (e.g. by UV radiation), at which point the prophage excises from the host chromosome and enters the lytic pathway.
Examiner tips
- Use the word ‘distinguish’ to list contrasting features; keep each point short and to the point.
- Mention the fate of the genome, cell survival, and possibility of induction – these are the key markers.
- Use correct terminology: lytic, lysogenic, prophage, integration, excision, lysis.
- Keep answer within 4 marks – one sentence per point is sufficient.
Common mistakes
- Confusing the order of replication and assembly; the genome is replicated before new phage assembly in the lytic cycle.
- Forgetting that the lysogenic cycle involves integration and passive replication of the prophage.
- Mixing up lysis with cell survival – the lytic cycle lyses the cell, the lysogenic cycle does not.
Mark scheme (4 marks)
- In the lytic cycle the viral genome is replicated immediately and directs production of new phage components, whereas in the lysogenic cycle the viral genome is integrated into the host chromosome (as a prophage).
- In the lytic cycle the host cell is lysed / destroyed at the end of the cycle, releasing new phage particles, whereas in the lysogenic cycle the host cell survives and continues to divide.
- In the lysogenic cycle the prophage is replicated passively along with the host DNA each time the cell divides, so daughter cells inherit the viral genome, whereas in the lytic cycle no such vertical transmission occurs.
- The lysogenic cycle can be induced / switched to the lytic cycle (e.g. by environmental stressors such as UV radiation), at which point the prophage excises from the host chromosome and enters the lytic pathway.
Key terms in this question
lysogenic cycle · bacteriophage
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Distinguish" question →
- Decode the mark scheme abbreviations →
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