18.1.2—Three domains

The three domains are Bacteria, Archaea and Eukarya. Domain is the highest taxonomic rank, and grouping organisms into domains helps represent deep evolutionary divisions.

Bacteria and Archaea contain prokaryotic cells: they lack a nucleus. Eukarya contains eukaryotic cells: genetic material is enclosed in a nucleus and the cell has compartmentalised structures, including membrane-bound organelles.

Cell type is an important first boundary, but it is not enough to classify all prokaryotes. Molecular evidence, especially comparisons of RNA genes, supports separating the two prokaryotic domains and recognising their deeper relationships.

Use the classification in this order:

1. Ask whether the cell is prokaryotic or eukaryotic.
2. If prokaryotic, keep Bacteria and Archaea as separate possible domains rather than treating “prokaryote” as a domain.
3. Use molecular evidence alongside cell organisation before assigning the domain.
4. State the result as Bacteria, Archaea or Eukarya; detailed Archaea–Bacteria differences and lower Eukaryotic kingdoms are separate questions.

A simple cell shape cannot by itself identify the domain. “Prokaryote” describes cell organisation, whereas Bacteria and Archaea are separate domains; Eukarya is the eukaryotic domain. Viruses are outside this cellular three-domain classification.