A bacteriophage is a virus of bacteria
Bacteriophages attach to specific molecules on a bacterial surface. In many tailed phages, the protein head remains outside while the nucleic acid is injected through the tail. Host specificity therefore depends partly on whether the bacterial cell displays the receptor recognised by the phage.
Once the genome enters, infection can move toward rapid production of new phages or toward a dormant relationship with the host chromosome. These alternatives are the lytic and lysogenic cycles.
The lytic cycle: reproduce and rupture
During lytic infection, phage genes redirect bacterial metabolism. The viral genome is copied, capsid and tail proteins are synthesized, and components assemble into complete particles. Phage-coded enzymes eventually weaken the bacterial wall, water enters, and the cell lyses.
Lysis releases many virions at once and kills the host cell. A virulent phage follows this productive route rather than establishing a stable integrated state.
Tail fibres bind a bacterial receptor.
Phage nucleic acid enters the cell.
Genome and viral proteins are made.
New phage particles form.
The bacterium ruptures and releases phages.
The lysogenic cycle: integration before production
A temperate phage can integrate its DNA into the bacterial chromosome. The integrated viral DNA is called a prophage. It is copied whenever the bacterium replicates, so daughter cells inherit the phage genome without producing virions immediately.
Stress such as ultraviolet radiation or DNA damage can trigger induction. The prophage leaves the chromosome and enters the lytic programme. Lysogeny is therefore a reversible dormant strategy, not permanent inactivity.
Immediate synthesis, assembly and host lysis.
Integration as a prophage and replication with the host.
Stress shifts a prophage into the lytic route.
Why lysogeny can change a bacterium
A prophage may carry genes that alter bacterial phenotype, a process called lysogenic conversion. Some bacterial toxins are encoded by phage genes, so the bacterium becomes more pathogenic after acquiring the prophage.
Phages can also transfer bacterial DNA between cells. In generalized transduction, random bacterial fragments may be packaged during a lytic infection. In specialized transduction, genes close to a prophage insertion site may be removed with incorrectly excised phage DNA.
| Feature | Generalized | Specialized |
|---|---|---|
| Typical stage | Lytic packaging error | Faulty prophage excision |
| Bacterial genes moved | Potentially any fragment | Genes near the insertion site |
| Key idea | Random host DNA is packaged | Adjacent host DNA leaves with phage DNA |
Common examination traps
A prophage is viral DNA inside the bacterial chromosome, not a complete bacteriophage. Lysogenic infection does not immediately lyse the host, but the prophage can later be induced. Lysozyme-related phage enzymes are associated with breaking bacterial wall material during entry or release, not with genome replication.
When comparing cycles, follow three questions: Is viral DNA integrated? Are new virions being assembled now? Does the host survive this stage? Those answers usually identify the route.
The ideas to carry forward
- The lytic cycle produces virions and destroys the bacterium.
- The lysogenic cycle stores phage DNA as a prophage.
- Induction can move a prophage into the lytic cycle.
- Transduction transfers bacterial genes through phage particles.
Answer first. Then reveal the marking logic.
01Why can a bacterial population carry a prophage for many generations?2 marks · show the biological link
Answer: The prophage is copied as part of the host chromosome and passed to daughter cells during bacterial division.
02Which transduction route is limited to genes near the prophage site?2 marks · show the biological link
Answer: Specialized transduction, because it results from inaccurate excision of an integrated prophage.
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