Viral replication – lytic and lysogenic cycles
BiologyMicrobiology & VirusesAges 15–16
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Microbiology & Viruses
A phage attaches to a bacterium and injects its DNA, the host cell synthesizes viral DNA and coat protein, assembles dozens of new virions, then bursts open to release them (lytic cycle). In the lysogenic cycle, viral DNA inserts into the bacterial chromosome as a prophage, replicating with the host cell through repeated divisions until induction (UV light) switches it to the lytic cycle. Students step through each stage and change the speed and the number of virions released. A third screen compares the structure of TMV, phage T4 and the influenza virus with a viroid, a prion chain reaction and a lichen.
Lesson: Overview of viruses: viral replication cycles (lytic and lysogenic)
What it shows
Viruses cannot reproduce on their own: they carry DNA or RNA inside a protein capsid, sometimes wrapped in an envelope, and use a host cell's enzymes and ribosomes. A bacteriophage shows attachment, penetration, biosynthesis, assembly and release. In the lytic cycle the host bursts; in the lysogenic cycle the viral DNA is copied as a prophage at every division until a trigger such as UV light switches it to the lytic cycle. Viroids are even simpler, a bare circle of RNA, and prions are misfolded proteins with no nucleic acid. Lichens, often taught alongside them, are fungus–alga partnerships.
How to use
Turn off auto-run and use Next stage so the class can describe each step before reading the caption. Repeat on the Lysogenic cycle tab, count the cells carrying a prophage and press Induce (UV). Change Virions released per cell to link burst size with the spread of infection. On Structures compared, pick each Specimen, follow the numbered labels, run the viroid and prion animations and compare all six in the table.
Parameters you can change
- Screen Lytic, Lysogenic (then induced to lytic), Structures compared: viruses, viroids, prions and lichens
- Speed 0.25–3 ×
- Virions released per cell 10–100
- Auto-run continuously (off: step through stages manually)
- Specimen on the Structures compared screen Tobacco mosaic virus (TMV), Bacteriophage T4, Influenza virus, Viroid (PSTVd), Prion, Lichen
Questions to explore
- Which stage determines which cells a virus can infect, and why?
- Why can bacteria carrying a prophage keep growing and dividing normally?
- How can a prion be infectious when it contains no DNA or RNA?
Common questions
What is the difference between the lytic and lysogenic cycles?
The lytic cycle makes new virions at once and bursts the host cell, whereas the lysogenic cycle inserts the viral DNA into the host chromosome as a prophage and keeps the cell alive. The prophage is copied at each division with no new virions forming, until a trigger such as UV light switches it to the lytic cycle.
Why don't antibiotics work against viruses?
Antibiotics do not work against viruses because they attack structures and processes found in bacteria, such as the cell wall and bacterial ribosomes, and viruses have neither. A virus has no metabolism of its own and reproduces with the host cell's enzymes and ribosomes, so viral infections are prevented with vaccines or treated with specific antiviral drugs.
Are viruses alive?
Viruses are usually not regarded as fully alive, because they are not made of cells, have no metabolism and cannot reproduce on their own. Outside a host a virion is an inactive particle of DNA or RNA in a protein capsid, but inside a host cell it directs the making of new copies, so viruses sit at the boundary of life.
