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
Diversity of Viruses Diagrams
The diversity of viral structures
Coronavirus structure
Classification of Viruses table
| Nucleic acid | Single or double stranded | Virus | Host Organism | Disease |
| DNA | Single | Canine parvovirus type 2 | Dog | Canine parvovirus |
| DNA | Double | Varicella zoster virus (VZV) | Human | Chickenpox & shingles |
| RNA | Single | Morbillivirus | Human | Measles |
| RNA | Double | Human immunodeficiency virus | Human | AIDS |
