CIE A Level Biology (9700) 2019-2021

Revision Notes

18.2.4 Viruses


  • Viruses are microorganisms that can only be seen using an electron microscope
  • They have no cellular structure (and so are acellular and no metabolism
  • Viruses hijack the DNA replication machinery in host cells
  • The energy viruses need for replication is provided by respiration in the host cell
  • Viruses possess none of the characteristic features used for classifying organisms so they sit outside of the three-domain classification system
  • There is a wide-ranging debate as to whether viruses should be classified as ‘living’ or ‘non-living’ based on their inability to carry out the defining features of life outside of a host cell

Classifying viruses by their genetic material

  • Viruses are classified according to the type of nucleic acid (RNA or DNA) their genome is made from, and whether it is single-stranded or double-stranded
  • In cellular organisms like animals and plants, DNA is always double-stranded and RNA is usually always single-stranded
  • However, in viruses, DNA and RNA can be either single-stranded or double-stranded
  • As a result, there are four groups of viruses that exist:
    • DNA single-stranded viruses
    • DNA double-stranded viruses
    • RNA single-stranded viruses (this is the type of genome of SARS-CoV-2, the virus responsible for the COVID-19 pandemic)
    • RNA double-stranded viruses

Classification of viruses table

Classification of Viruses Table, downloadable AS & A Level Biology revision notes

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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