A level Biology B · Edexcel 9BI0 · Topic 2C
Viruses
Key points
- Viruses are acellular: a nucleic acid core (DNA or RNA) inside a protein capsid, sometimes surrounded by a lipid envelope taken from the host cell.
- They have no metabolism and can only reproduce inside a host cell, so they are usually not classed as living.
- Surface proteins attach to specific receptors on host cells, which decides which cells they infect.
- Lytic cycle: the virus injects its genome, the host makes new viral particles and the cell bursts (lyses) to release them.
- Lysogenic cycle: viral DNA joins the host chromosome and is copied with it, staying dormant until triggered.
- Retroviruses such as HIV use reverse transcriptase to make DNA from their RNA.
- Antibiotics do not work against viruses.
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Viruses are tiny particles that can only reproduce inside a living host cell. They cause diseases from the common cold to AIDS and Ebola, and their life cycles explain why they are so hard to treat.
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