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Lytic and lysogenic cycles

Cells (Unity and diversity) · Viruses · note 2 of 3

Spec A2.3.3, A2.3.4
HL only (what this means)HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean
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Lytic and lysogenic cyclesSpec A2.3.3, A2.3.4

In short

In the lytic cycle, bacteriophage lambda attaches to an E. coli cell, injects its DNA and uses the host's energy supply, nucleotides, amino acids and ribosomes to make new viruses, which are released when the cell bursts. In the lysogenic cycle, the viral DNA integrates into the host chromosome as a prophage and is copied each time the bacterium divides.

Viruses rely on a host cell for energy supply, nutrition, protein synthesis and other life functions. Bacteriophage lambda infects E. coli and can follow either of two cycles.

Lytic cycle

  1. Attachment: the tip of the phage tail binds to a specific receptor protein on the outer surface of E. coli.
  2. Injection: the phage injects its DNA into the cell; the empty capsid stays outside.
  3. Replication and synthesis: the linear viral DNA forms a circle. Viral genes are transcribed by the host's RNA polymerase and translated by host ribosomes, using the host's ATP, nucleotides and amino acids. The viral DNA is replicated many times.
  4. Assembly: capsid proteins assemble into heads and tails, and copies of the viral DNA are packed into the heads.
  5. Lysis and release: viral proteins make holes in the plasma membrane and a viral enzyme (endolysin) breaks down the cell wall, so the cell bursts, and around 100 new phages are released to infect other cells.

Lysogenic cycle

  1. Attachment and injection happen as in the lytic cycle.
  2. The lambda DNA integrates into the host chromosome at a specific site, catalysed by a viral enzyme, integrase. The integrated viral DNA is called a prophage.
  3. A viral repressor protein switches off the genes needed for the lytic cycle, so no new viruses are made and the host survives.
  4. Each time the bacterium divides by binary fission, the prophage is replicated with the host chromosome and passed to both daughter cells.
  5. If the host is stressed, for example when its DNA is damaged by ultraviolet light, the prophage is cut out of the chromosome and the lytic cycle begins. This is called induction.
Lytic and lysogenic cycles compared
FeatureLytic cycleLysogenic cycle
Viral DNAStays separate from the host chromosomeIntegrated into the host chromosome as a prophage
New virus particlesMade quicklyNot made while the prophage is dormant
Effect on the hostKilled by lysisSurvives and divides normally
How the virus spreadsReleased phages infect new cellsCopied to daughter cells when the host divides
Lambda phage attaches to E. coli and injects its DNA. Lytic cycle: viral DNA forms a circle, is replicated while capsid proteins are made using host ribosomes, new phages are assembled and the cell lyses, releasing about 100 phages. Lysogenic cycle: viral DNA integrates into the bacterial chromosome as a prophage, which is copied into both daughter cells at binary fission; induction (for example by UV damage) cuts the prophage out and starts the lytic cycle. (opens full size in a new tab)
Lambda phage can kill its host at once (lytic) or lie dormant as a prophage (lysogenic) until induction.
Exam tip:

In a lytic cycle answer, name the host resources used: ATP, nucleotides, amino acids, ribosomes and enzymes. These earn the marks for 'viruses rely on a host cell'.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What do all viruses have in common?

All viruses have a small, fixed size, nucleic acid (either DNA or RNA) as their genetic material, a capsid made of protein, no cytoplasm, and few or no enzymes. Beyond these shared features they are highly diverse in shape, in the type of nucleic acid they carry and in whether they have an envelope.

What is the difference between the lytic and lysogenic cycle?

In the lytic cycle a virus uses the host cell to make many new viruses quickly and then bursts the cell. In the lysogenic cycle the viral DNA integrates into the host chromosome as a prophage, so the host survives and copies the viral DNA each time it divides, until induction triggers the lytic cycle.

Why does the flu vaccine change every year?

The flu vaccine changes every year because influenza viruses evolve rapidly. Their RNA genome mutates often, gradually changing the surface antigens haemagglutinin and neuraminidase (antigenic drift), so antibodies made against earlier strains no longer bind well. The vaccine is reformulated each year to match the strains expected to circulate.

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