Explain how viruses that infect bacteria (bacteriophages) are used as vectors in genetic engineering, including the steps by which foreign DNA is introduced into a bacterial host cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The gene of interest is cut with restriction enzymes and ligated into a phage genome in vitro, creating a recombinant phage.
2. The recombinant genome is packaged into phage particles in vitro, producing infective phage vectors.
3. The phage infects the bacterial host, injecting its DNA (containing the foreign gene) into the cell.
4. The foreign DNA is either integrated into the bacterial chromosome or maintained as a prophage, allowing its expression and replication in the bacterium.
2. The recombinant genome is packaged into phage particles in vitro, producing infective phage vectors.
3. The phage infects the bacterial host, injecting its DNA (containing the foreign gene) into the cell.
4. The foreign DNA is either integrated into the bacterial chromosome or maintained as a prophage, allowing its expression and replication in the bacterium.
Examiner tips
- Use the exact sequence of steps: insertion, packaging, infection, integration/maintenance.
- Show the role of restriction enzymes and ligase in step 1.
- Mention that the phage injects DNA into the host in step 3.
- Explain that the foreign DNA can become a prophage or integrate into the genome in step 4.
Common mistakes
- Confusing plasmid vectors with phage vectors.
- Omitting the in vitro packaging step.
- Failing to state that the foreign DNA can integrate or be maintained as a prophage.
Mark scheme (4 marks)
- Foreign DNA / gene of interest is inserted into the bacteriophage genome (in vitro) using restriction enzymes and DNA ligase
- The recombinant phage is packaged into phage particles (in vitro packaging), producing infective phage vectors
- The recombinant phage infects the bacterial host cell by injecting its DNA (including the foreign gene) into the bacterium
- The foreign DNA integrates into / is maintained in the bacterial genome (or as a prophage), allowing expression/replication of the foreign gene in the bacterium
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Viruses (HL only) questions
- Explain how antigenic variation in influenza viruses makes it difficult to devel…
- Explain why viruses are not considered living organisms, with reference to their…
- Explain how RNA viruses such as HIV overcome the challenge of replicating their …
- Explain how the lytic cycle of a bacteriophage results in the release of new pha…
- Distinguish between the lytic and lysogenic cycles of a bacteriophage, with refe…
- Explain how a virus with a lipid envelope acquires that envelope and why its pre…