Lesson 2 of 6 / Viral and bacterial genetics
Lytic and lysogenic inheritance
How can phage DNA persist while the host keeps dividing?
I can explain lambda phage lytic and lysogenic cycles.
I can explain inheritance in phage lytic/lysogenic, influenza and HIV cycles.Syllabus
Syllabus 9477, Core 2(e). Explain inheritance in phage lytic/lysogenic, influenza and HIV cycles.
About 7 min
Make a guess. You are not marked.
A phage infects a bacterium, yet the bacterium keeps dividing normally for many generations. Is the phage DNA gone?
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It may be built into the host chromosome.
In lysogeny, phage DNA integrates as a prophage and is copied every time the bacterium divides.
The key idea
Lytic infection makes and releases new phages; lysogeny maintains a prophage copied with the host genome until induction.
Explore the mechanism
Produce particles now or inherit a prophage?
Attachment structures recognise a host. The genome enters; a complete phage does not divide as a cell.
Purple loop is bacterial chromosome; brown segment is integrated phage DNA. Small head-tail icons represent progeny, not exact counts. No bacterial nucleus is drawn.
Explanation
A phage attaches to suitable bacterial receptors and delivers its genome. In a lytic cycle, viral genes direct replication of phage nucleic acid and synthesis of phage proteins using host resources. Components assemble into virions, and cell lysis releases progeny.
Lambda phage can instead enter lysogeny. Its DNA integrates into the bacterial chromosome as a prophage, with lytic genes repressed. When the bacterium copies its chromosome and divides, the prophage is transmitted to descendants, allowing viral genetic inheritance without immediate virion production.
Under inducing conditions, prophage DNA can leave the host chromosome and enter the lytic programme. Integration is not the same as destruction of all bacterial DNA, and lysogeny is not a phage hiding as an intact miniature particle inside the nucleus; bacteria have no nucleus.
Compare outcomes by tracking both the viral genome and the host. A stable number of infected bacterial descendants can indicate prophage inheritance, whereas rising free-phage counts followed by lysis fits productive lytic infection. A model diagram is a sequence of events, not a fixed timing prediction for every phage.
Step by step
- 1
Follow the genome after entry
Replication for virions or integration as a prophage?
- 2
Track the host cell
Continued division differs from lysis.
- 3
Locate the switch
Induction permits a lysogen to produce phages.
Worked example
Work through the evidence
A bacterial clone transmits phage DNA for many generations without releasing virions. After stress it lyses and releases phage. Explain.
One way to explain it
The phage was inherited as a prophage during lysogeny. Induction activated a lytic cycle with viral replication, assembly and host lysis.
Why this answer works
- Viral genome persistence does not require constant particle production.
- The later release links the two alternative programmes.
Watch out for this
A student says: "Lysogeny means the phage genome is not replicated at all." What is wrong with this?
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It is copied with the host chromosome, even while lytic particle production is repressed.
Check your understanding
Which structure is a prophage?
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Phage DNA integrated into the bacterial chromosome
It can be inherited during bacterial division.
Exam-style questions
Check your understanding
Which statements are true of the lysogenic cycle of lambda phage? 1: Phage DNA integrates into the host chromosome. 2: The host cell bursts at once. 3: The prophage is copied when the host divides. 4: The prophage can later enter the lytic cycle.
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1, 3 and 4 only
In lysogeny the DNA integrates, is copied with the host chromosome and can later be induced. Immediate lysis belongs to the lytic cycle.
Quick recall
How can phage DNA persist while the host keeps dividing?