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A2.3.3 (HL)—Lytic cycle

In the lytic cycle, lambda phage DNA remains separate from bacterial DNA, directs production and assembly of new virions, and ends by lysing the host.

Syllabus
First assessment 2025
Objective
A2.3.3
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1A
Typical marks1–3

Common command terms

  • Identify
  • Compare
  • Contrast

Scoring notes

Common mistake
Reversing the cycles by placing viral DNA integration in the lytic cycle.

Recent exam appearances

November 2025Paper1B ["HL"] · TZ31(d)[ 3 ]A2.3.3 (HL)—Lytic cycle
May 2025Paper1A ["HL"] · TZ313[ 1 ]A2.3.3 (HL)—Lytic cycle
Practice this objective

Coverage 2025–2025 · Updated 15 Jul 2026

The Lytic Cycle Makes New Virions Quickly

HL only

In lambda's lytic cycle, the phage attaches to E. coli, injects its double-stranded DNA, redirects host resources, assembles new phages and lyses the bacterium.

The viral DNA remains separate from the bacterial chromosome. Viral genes use host energy, nutrients, ribosomes and enzymes to copy phage DNA and synthesize capsid and tail proteins; host DNA may be degraded.

Sequence: attachment -> DNA injection -> viral genome replication and protein synthesis -> assembly of heads, tails and genomes -> bacterial lysis and release.

One infected E. coli cell can assemble many lambda particles before the cell wall ruptures and releases them to infect neighbouring bacteria.

Lytic replication makes components before release; a phage does not grow larger and split like a bacterial cell.

Lytic cycle

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify / Compare / Contrast.

Command terms

Identify / Compare / Contrast

What earns marks

Build the answer around this relationship: Lambda phage attaches to E. coli and injects its DNA into the host.

Watch for

Reversing the cycles by placing viral DNA integration in the lytic cycle.

Representative question

Question 1

[Maximum number: 1]

What is a difference between the lytic and the lysogenic cycle of the bacteriophage lambda?

A

Only in the lytic cycle is lambda DNA inserted into the host DNA.

B

Only in the lysogenic cycle is the host metabolism used to replicate.

C

Only in the lytic cycle is the host metabolism used to replicate.

D

Only in the lysogenic cycle is lambda DNA inserted into the host DNA.

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 phage attaches to E. coli and injects its DNA into the host.
  • Lytic viral DNA remains separate from the bacterial chromosome.
  • Host machinery produces viral DNA and proteins that assemble into new phages.
  • The lytic cycle ends with bacterial cell lysis and virion release.