AQA A Level Biology

Revision Notes

2.1.7 Viruses

Key Features of Viruses

  • Viruses are non-cellular infectious particles that straddle the boundary between ‘living’ and ‘non-living’
  • They are relatively simple in structure; much smaller than prokaryotic cells (with diameters between 20 and 300 nm)
  • Structurally they have:
    • A nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded)
    • A protein coat called a ‘capsid’
  • Some viruses have an outer layer called an envelope formed usually from the membrane-phospholipids of a cell they were made in
  • All viruses are parasitic in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles

Cell Components- Virus, downloadable AS & A Level Biology revision notes

Viruses are not cellular like prokaryotes and eukaryotes – this is just one example of a virus structure


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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