A virus cannot divide. It has no cytoplasm to split and no machinery to copy itself with. What it can do is get its instructions inside a cell and let that cell build the next generation for it — either straight away, or years later.
📚 What you need to know
Viruses do not undergo cell division. They are parasitic and can only reproduce inside a host cell.
Every virus must attach, inject its nucleic acid, use the host’s protein synthesis machinery, assemble new particles and release them.
The lytic pathway ends with the host cell bursting. The enzyme lysozyme, coded for by the virus, causes this.
Fully formed, functional virus particles are called virions.
In the lysogenic pathway, viral nucleic acid combines with the host DNA and stays inactive.
A viral gene codes for a repressor protein that stops the viral genes being transcribed and translated. This dormant period is called latency, or a period of lysogeny.
The host cell keeps dividing, so every new cell carries the viral nucleic acid.
An environmental change, such as UV light or certain chemicals, can trigger the switch into the lytic pathway.
What every virus has to do
Whatever the pathway, the same five things must happen. Learn them as a checklist, because a “describe how a virus replicates” question is asking for exactly this.
The five requirements
attach → inject nucleic acid → use the host’s ribosomes → assemble new particles → release them
Notice what the virus contributes: instructions, and nothing else. The amino acids, the ribosomes, the enzymes and the ATP all belong to the host cell. That is what “parasitic” means here, and it is also why viruses are so hard to treat with drugs — almost everything you would want to attack belongs to the patient.
The lytic pathway
Lytic comes from lysis, meaning bursting. New virus particles are released by breaking the host cell open, which kills it.
The cycle closes because every released virion can start again on a fresh cell. One infected bacterium can produce hundreds of new particles in under an hour.
Attachment. The virus binds to the host cell membrane using its attachment proteins.
Entry. It injects its nucleic acid into the cytoplasm. In a bacteriophage the capsid stays outside.
Biosynthesis. The virus uses the host cell’s proteins and enzymes to make new viral nucleic acid and new viral proteins, including new capsids.
Assembly. The parts are put together and matured into complete, working virus particles called virions.
Release. The host cell lyses — bursts — releasing the virions to infect more cells. The enzyme responsible, lysozyme, is coded for by the virus’s own genetic material.
Lysozyme is a nice detail to include. It shows the bursting is not an accident of overcrowding — the virus carries a gene whose whole job is to break the cell open at the right moment. Mentioning that the enzyme is coded for by the viral genome usually earns the mark on its own.
The lysogenic pathway
Some viruses do not go straight to lysis. Instead they hide, and they can hide for a very long time.
Step 3 is the clever part. The virus is not doing any work here — the host cell is copying and spreading the viral genome for it, generation after generation.
New virus particles are not immediately released, and the infection does not immediately cause disease.
The viral nucleic acid combines with the host DNA, becoming part of the host’s genome.
A viral gene codes for a repressor protein, which prevents the viral nucleic acid from being transcribed and translated. Nothing viral gets built.
This dormant state is called latency, and the time it lasts is a period of lysogeny.
The host cell carries on as normal, including dividing, so every daughter cell inherits the viral nucleic acid. The number of infected cells grows steadily without a single virus particle being made.
The viral DNA stays inactive until a change in the cell’s environment — UV rays or certain chemicals — triggers it to enter the lytic pathway.
Lytic pathway
Lysogenic pathway
Viral nucleic acid
Stays separate and is used straight away
Combines with the host DNA
New virus particles
Made immediately
Not made during lysogeny
Fate of the host cell
Destroyed by lysis
Survives and keeps dividing
Symptoms of disease
Appear quickly
Do not appear until the lytic pathway starts
Key protein
Lysozyme, which bursts the cell
A repressor protein, which keeps the viral genes silent
How it spreads
Virions infect new cells
The host cell divides and passes the viral genome on
What ends it
Lysis of the cell
An environmental trigger switching it to the lytic pathway
🧠
Lytic bursts, lysogenic lurks
Lytic ends in lysis — the cell bursts. Lysogenic means the virus is generated later — it lurks in the host DNA until something wakes it up.
Why hiding is a good strategy. A virus in the lytic pathway kills its host and has to find a new one immediately. A virus in the lysogenic pathway gets copied for free every time the cell divides, causes no symptoms, and provokes no immune response. It only comes out of hiding when conditions suggest the host is in trouble — which is exactly when leaving is a good idea.
Worked examples
WE 1
Describe the lytic pathway
Describe the stages of the lytic pathway in a bacteriophage. (4 marks)
Stage 1: attachment and entry
The phage binds to the bacterial surface using its attachment proteins and injects its DNA into the cytoplasm.
Stage 2: biosynthesis
The host cell’s enzymes and ribosomes are used to make copies of the viral nucleic acid and new viral proteins, including capsids.
Stage 3: assembly
The parts are assembled and matured into complete virus particles called virions.
Stage 4: release
Lysozyme, coded for by the viral genome, bursts the host cell, releasing the virions to infect more cells.
Attach, inject, biosynthesis, assemble, lyseuse the word “virions” for the finished particles — it is the term the question expects
WE 2
Explain a latent infection
A person is infected with a virus but shows no symptoms for several years. Explain how this is possible. (3 marks)
Point 1: the pathway
The virus has entered the lysogenic pathway, so its nucleic acid has combined with the host DNA.
Point 2: why nothing happens
A viral gene codes for a repressor protein, which stops the viral nucleic acid being transcribed and translated, so no new virus particles are made.
Point 3: what ends it
This latency continues until an environmental trigger, such as UV light or certain chemicals, switches the virus into the lytic pathway and symptoms appear.
Integrated, repressed and dormant until something triggers lysis“repressor protein” and “latency” are the two terms being tested here
WE 3
Compare the two pathways
Outline two differences between the lytic and lysogenic pathways. (2 marks)
Difference 1: the fate of the cell
In the lytic pathway the host cell is destroyed by lysis, whereas in the lysogenic pathway it survives and continues to divide.
Difference 2: the viral nucleic acid
In the lytic pathway the viral nucleic acid is used immediately to build new particles; in the lysogenic pathway it combines with the host DNA and stays inactive.
Cell destroyed and virus built now, against cell kept and virus hiddeneach difference must mention both pathways to count as a comparison
💡 Exam tips
Say viruses do not undergo cell division — they replicate inside a host.
Name lysozyme for lysis and the repressor protein for latency. Both are single-word marks.
Use virions for fully assembled, functional virus particles.
Stress that the host supplies the ribosomes, enzymes and energy.
Name a trigger for the switch: UV rays or certain chemicals.
In the lysogenic pathway, the viral genome spreads because the host cell divides, not because virions are released.
⚠ Common mistakes
Saying the virus divides. It is assembled from parts made by the host cell.
Writing that the whole virus enters the cell. In a bacteriophage only the nucleic acid goes in.
Saying the lysogenic pathway never becomes lytic. A trigger can switch it at any time.
Claiming the host cell dies during lysogeny. It survives and keeps dividing.
Confusing lysozyme with lysogeny. One is the enzyme that bursts the cell; the other is the dormant period.
Forgetting that the virus uses the host’s ribosomes. It has none of its own.
Up next: Origin & Evolution of Viruses — where viruses might have come from, and why influenza needs a new vaccine every year.
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