Step by Step
L
Lytic cycle — active replication, cell destruction
In the lytic cycle, a phage injects its DNA, hijacks the host cell's machinery to make many copies of itself, and ultimately lyses (bursts) the cell to release new viral particles.
Example: a bacteriophage rapidly hijacking a bacterial cell's replication machinery, producing hundreds of new phage particles before bursting the cell open to release them.
L2
Lysogenic cycle — dormant integration as a prophage
In the lysogenic cycle, phage DNA instead integrates into the host bacterial chromosome as a prophage, replicating passively along with the host cell and potentially persisting silently for many generations.
Example: a prophage remaining integrated and dormant within a bacterium's chromosome across many generations of cell division, with no active viral replication occurring during this dormant period.
I
Induction — stress triggers the switch to lytic
Certain stressors, particularly UV exposure and the SOS DNA-damage response, can trigger a dormant prophage to excise itself from the host chromosome and switch into the active lytic cycle.
Example: UV radiation damaging a bacterium's DNA, triggering the SOS response, which in turn causes a previously dormant prophage to excise and enter the lytic cycle.
M
Medical relevance — toxin genes carried by prophages
Many important bacterial toxin genes are actually encoded within prophages, not the bacterium's own core genome — cholera toxin, diphtheria toxin, Shiga toxin, and Staph toxin superantigens are all examples. This means lysogenic conversion (acquiring a prophage) can directly make an otherwise harmless bacterium virulent.
Example: a bacterium that was previously non-toxigenic becoming capable of producing diphtheria toxin specifically because it acquired a prophage carrying that toxin gene through lysogenic conversion.
Applied Walkthrough
1
A strain of Corynebacterium that was previously non-toxigenic (unable to produce diphtheria toxin) is found to have suddenly gained this toxin-producing ability.
2
Ask: how could a bacterium acquire an entirely new toxin-producing capability like this? Through lysogenic conversion — acquiring a prophage that specifically carries the diphtheria toxin gene, integrating it into the bacterium's genome.
3
This illustrates the broader medical relevance of lysogeny: several of the most clinically important bacterial toxins (cholera, diphtheria, Shiga, Staph superantigens) are actually encoded by prophage genes, not the bacterium's own inherent genome.
4
If this same lysogenized bacterium were later exposed to a stressor like UV radiation, the SOS response could trigger the dormant prophage to excise and enter the lytic cycle, potentially releasing new phage particles capable of infecting and converting other bacteria as well.
Exam Application
Exams test whether you can distinguish the lytic cycle (active replication, cell destruction) from the lysogenic cycle (dormant integration as a prophage) and whether you know specific examples of toxin genes carried by prophages (cholera, diphtheria, Shiga, Staph superantigens) — a frequently tested connection between phage biology and bacterial virulence.
⚠ Common Trap
The most common trap is assuming bacterial toxin genes are always part of a bacterium's own core genome. Several of the most clinically significant toxins are specifically encoded by prophages acquired through lysogenic conversion — meaning the same bacterial species can be either harmless or highly virulent depending on whether it has acquired the relevant toxin-carrying prophage.
✓ Quick Self-Check
1. What happens during the lytic cycle?
The phage hijacks the host cell to make many copies of itself, then lyses the cell to release new viral particles.
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2. What happens during the lysogenic cycle?
Phage DNA integrates into the host chromosome as a prophage, replicating passively with the host and potentially persisting for generations.
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3. What can trigger a prophage to switch from lysogenic to lytic?
Stressors like UV exposure, triggering the SOS DNA-damage response.
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4. Name two toxins encoded by prophage genes rather than the bacterium's core genome.
Cholera toxin, diphtheria toxin, Shiga toxin, or Staph toxin superantigens (any two).
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5. What is lysogenic conversion, and why does it matter clinically?
A bacterium acquiring a prophage that carries a toxin gene, which can directly make an otherwise harmless bacterium virulent.
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