Explain how the lytic cycle of a bacteriophage results in the release of new phage particles from a host bacterial cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The bacteriophage injects its DNA into the bacterium while the capsid stays outside.
The host’s ribosomes and metabolic machinery are hijacked to transcribe and translate viral genes, producing phage proteins (including capsid proteins) and replicating phage DNA.
New phage particles are assembled by self‑assembly of capsid proteins around the replicated DNA inside the cell.
Phage‑encoded lysozyme (or a similar lytic enzyme) degrades the bacterial cell wall, causing lysis and the release of hundreds of new phage particles.
The host’s ribosomes and metabolic machinery are hijacked to transcribe and translate viral genes, producing phage proteins (including capsid proteins) and replicating phage DNA.
New phage particles are assembled by self‑assembly of capsid proteins around the replicated DNA inside the cell.
Phage‑encoded lysozyme (or a similar lytic enzyme) degrades the bacterial cell wall, causing lysis and the release of hundreds of new phage particles.
Examiner tips
- Use the word ‘injects’ for the first step, ‘hijack’ for utilisation of host machinery, ‘self‑assembly’ for capsid formation, and ‘lysis’ for release.
- Show the sequence of events in order; the examiner expects a clear progression from injection to release.
- Mention the enzyme (lysozyme) to demonstrate understanding of the lytic mechanism.
Common mistakes
- Confusing the capsid with the DNA during injection; the capsid remains outside. Failing to specify that the host’s ribosomes are used. Omitting the enzyme that causes lysis (e.g., lysozyme).
Mark scheme (4 marks)
- The bacteriophage injects its nucleic acid (DNA) into the host bacterium, whilst the protein capsid remains outside.
- The host cell's ribosomes and metabolic machinery are hijacked / used to transcribe and translate viral genes, producing phage proteins (including capsid proteins) and replicating phage DNA.
- New phage particles are assembled (self-assembly of capsid proteins around replicated phage DNA) inside the host cell.
- Phage-encoded lysozyme (or equivalent lytic enzyme) breaks down the bacterial cell wall, causing lysis and release of hundreds of new phage particles.
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 →
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