Viruses (Cambridge International A-Level Biology)

Spec point — 18.1.6 · outline how viruses are classified, limited to the type of nucleic acid (RNA or DNA) and whether this is single stranded or double stranded

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

The diversity of viral structures

Coronavirus structure