5.7B - Viral lifecycles

5.7B · The lytic viral pathway

Viruses do not contain all the cell machinery needed to reproduce independently, so they reproduce only inside living host cells. The GCSE lytic-and-lysogenic model is most clearly shown with a bacteriophage, a virus that infects a bacterial cell. This prevents an important overgeneralisation: not every human virus follows these exact two pathways.

The pathways share a starting point. The bacteriophage attaches to a compatible bacterial host cell, and its viral genetic material enters the cell. In the lytic pathway, events then proceed directly towards production and release:

1. Viral genes direct the host cell's enzymes to copy the viral genetic material.
2. The host cell's ribosomes use viral genetic instructions to make viral proteins.
3. The viral genetic material and proteins assemble into new virus particles.
4. The host cell lyses, meaning that it bursts open.
5. The new virus particles are released and can infect other host cells.

Genome integration is not a lytic step. Writing that viral DNA becomes part of host DNA describes the start of the lysogenic pathway instead. The defining lytic outcome is that new virus particles are assembled and the host cell is destroyed as they are released.

5.7B · The lysogenic viral pathway

The lysogenic pathway begins with the same attachment and entry, but the viral genetic material then follows a different route:

1. Viral DNA becomes integrated into the bacterial host's chromosome.
2. The host cell remains alive. Each time it copies its chromosome and divides, it also copies the viral DNA and passes it to daughter cells.
3. No new complete virus particles are assembled immediately, and the host cell does not lyse at this stage.
4. A trigger can activate the integrated viral DNA. It leaves the host chromosome and enters the lytic pathway, so viral components are made and assembled before the host cell eventually lyses and releases new viruses.