The big idea: A virus cannot reproduce on its own — it has no ribosomes, no enzymes for energy and no way to copy itself.
So a virulent virus does something ruthless: it takes over a living host cell, forces that cell to mass-produce copies of the virus, then bursts the cell open to release them.
This take-over-and-burst route is called the lytic cycle (from lysis = bursting/splitting a cell). The classic example is a bacteriophage — a virus that infects bacteria.
A bacteriophage. The tail fibres anchor to a SPECIFIC receptor (step 1, attachment); the genome packed in the head is injected into the host (step 2, entry).
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Two words to keep straight: Virulent = a virus that goes straight into the lytic cycle and kills the host quickly.
Lysis = the host cell bursting and dying, releasing the new viruses.
The cycle in five steps (we unpack each next)
- Attachment — the virus locks onto a specific spot on the host
- Entry — the viral genome gets inside the cell
- Replication / synthesis — the host is hijacked to copy the virus's parts
- Assembly — the parts snap together into new viruses
- Lysis / release — the cell bursts, freeing many new viruses
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Each step has a clear cause and effect. Follow the logic and you never have to memorise the order — it falls out naturally.
| 1. Attachment | Tail fibres (or surface proteins) bind a specific receptor molecule on the host's outer surface. | The shape of the virus protein must match the host receptor — like a key in a lock. This match is what decides which cells a virus can infect. |
| 2. Entry | The viral genome (DNA or RNA) is injected into the cell; the empty protein coat is left outside. | Only the genetic instructions need to get in — they carry everything needed to commandeer the cell. |
| 3. Replication / synthesis | The host's own enzymes, ribosomes, nucleotides and ATP are redirected to copy the viral genome and translate it into capsid proteins. | The virus brings instructions, not factories. It hijacks the cell's existing machinery instead of building its own. |
| 4. Assembly | Fresh genome copies and capsid proteins self-assemble into many complete new virus particles (virions). | The capsid proteins fit together automatically around a genome copy — no extra machinery required. |
| 5. Lysis / release | The cell bursts (lyses) and dies, releasing a flood of new viruses that infect neighbouring cells. | Often a viral enzyme weakens the cell wall/membrane so it splits open. One infected cell can release hundreds of viruses. |
Why attachment is so specific — and why it matters: A virus can only attach if its surface proteins fit a receptor that the host cell actually has.
No matching receptor → no attachment → no infection. This is exactly why a phage that destroys one species of bacterium may be completely harmless to a different species, and why the cold virus infects your airway cells but not, say, your muscle cells.
The set of cells a virus can infect is its host range, and it is set right here, at step 1.
- Virulent virus
- A virus that immediately runs the lytic cycle, killing the host cell to reproduce.
- Receptor
- A specific molecule on the host's surface that the virus's proteins bind to during attachment.
- Host range
- The set of cell types or species a particular virus is able to infect — determined by attachment specificity.
- Virion
- A single complete virus particle (genome packed inside its protein capsid), ready to infect a new cell.
- Lysis
- The bursting and death of the host cell that releases the new viruses.
Net effect — the line to remember: The lytic cycle gives rapid amplification of the virus (one cell → many new viruses) and death of the host cell. That double outcome — lots of copies, dead cell — is the whole point of the cycle.
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How this is tested: A common HL task is to outline or describe the lytic cycle in order, or to explain why a virus only infects certain hosts.
Marks are awarded per correct step in the right sequence, so always answer attachment → entry → replication → assembly → lysis, and name the host machinery that gets hijacked.
IB-style question — outline the lytic cycle of a bacteriophage
A bacteriophage infects a bacterium and follows the lytic cycle. Outline the events of this cycle, in sequence, from the virus reaching the cell to the release of new viruses. [5]
How to score all five marks
- Attachment. The phage binds to a specific receptor on the bacterium's surface using its tail fibres. (The match between phage protein and receptor decides whether infection is even possible.)
- Entry. The phage injects its genome (DNA) into the cell; the empty protein coat stays outside.
- Replication / synthesis. The viral genome hijacks the host's machinery — its enzymes, ribosomes, nucleotides and ATP — to make many copies of the genome and many capsid proteins.
- Assembly. The new genomes and capsid proteins self-assemble into many complete new virus particles (virions).
- Lysis / release. The host cell bursts (lyses) and dies, releasing the new viruses to infect further cells.
Final answer
Attachment to a specific host receptor → injection of the viral genome → hijacking host enzymes/ribosomes/ATP to copy genome + make capsid proteins → self-assembly of new virions → lysis of the host cell, releasing many new viruses.
Check your answer: Five marks need five distinct, ordered points: attach (specific) → enter (genome injected) → replicate (host machinery hijacked) → assemble → lyse (cell dies, viruses released). Skipping the order — or writing 'the virus reproduces' as one lump — loses marks.