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A2.3.4 (HL)—Lysogenic cycle

During lysogeny, lambda phage DNA integrates into the bacterial chromosome as a dormant prophage, replicates with its host, and may later enter lysis.

Syllabus
First assessment 2025
Objective
A2.3.4
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks3

Common command terms

  • Explain

Recent exam appearances

May 2025Paper2 ["HL"] · TZ14(c)[ 3 ]A2.3.4 (HL)—Lysogenic cycle
Practice this objective

Coverage 2025–2025 · Updated 15 Jul 2026

The Lysogenic Cycle Hides a Viral Genome

HL only

In lambda's lysogenic cycle, phage DNA integrates into the E. coli chromosome as a prophage instead of immediately producing new virions.

The prophage is copied whenever the bacterial chromosome replicates and is passed to daughter cells during binary fission. Stress such as DNA damage can induce excision and entry into the lytic cycle.

Sequence: attachment and injection -> integration as prophage -> copying with the host genome -> induction -> excision and lytic replication.

A lysogenic bacterium can divide repeatedly without bursting; after induction, the same viral DNA directs phage assembly and eventual lysis.

A prophage is not an inactive virus particle hidden inside the cell; it is viral DNA integrated into the host chromosome.

Lysogenic cycle

HL only

Assessment in practice

3 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain.

Command terms

Explain

What earns marks

Build the answer around this relationship: Lambda DNA integrates into the bacterial chromosome during lysogeny.

Representative question

Question 1

[Maximum number: 3]

Explain how the bacteriophage lambda reproduces in the lysogenic cycle.

Build A Virus Answer

HL only

A strong HL virus answer usually combines two moves: define the boundary, then explain the mechanism or consequence. Viruses have DNA or RNA plus a capsid but lack the machinery for independent metabolism. Their diversity is described by genome, capsid, and envelope. Lambda phage lets you contrast lytic takeover with lysogenic integration. Origin questions require competing hypotheses and polyphyly. Evolution questions require mutation, recombination, short cycles, large populations, and examples such as influenza or HIV.

  • Definition: non-cellular obligate parasite with genome plus capsid, lacking cytoplasm/ribosomes/metabolic enzymes.
  • Structure: compare genome, capsid shape, and envelope status.
  • Replication: lytic = takeover and lysis; lysogenic = prophage integration and induction.
  • Origins: likely polyphyletic, with virus-first, escaped-gene, and regressive hypotheses.
  • Evolution: rapid variation explains vaccine updates and treatment resistance.

Concept essentials

  • Lambda DNA integrates into the bacterial chromosome during lysogeny.
  • Integrated lambda DNA is called a prophage.
  • The prophage is copied with host DNA and inherited by daughter cells.
  • The lysogenic state delays virion assembly and host-cell lysis.
  • Induction can excise the prophage and initiate the lytic cycle.