18.1.3—Archaea and Bacteria as prokaryotes

Archaea and Bacteria are both domains of unicellular prokaryotes, so both lack a nucleus. They are nevertheless separate domains because molecular and structural evidence shows important differences.

Shared boundary — both have prokaryotic cells and no nucleus; a bacterial-looking shape or a similar size does not identify the domain.
Membrane evidence — archaeal membrane lipids are distinctive, whereas bacterial membrane lipids have a different structure.
Cell-wall evidence — bacterial cell walls contain peptidoglycan; archaeal cell walls do not contain peptidoglycan.
Genetic evidence — both have 70S ribosomes, but archaeal ribosomal RNA and ribosomal structure show greater similarity to Eukarya than to Bacteria.

Use the evidence in this order:

1. Confirm the organism is prokaryotic; this places it in the Bacteria-or-Archaea branch, not automatically in Bacteria.
2. Compare membrane-lipid, cell-wall and ribosomal-RNA evidence.
3. Assign the domain from the combined evidence rather than from habitat, shape or the fact that the cell lacks a nucleus.
4. Treat extreme-environment survival as a possible observation, not as the definition of Archaea; not all Archaea are extremophiles.

“Both lack a nucleus” does not mean Archaea and Bacteria are the same domain. This card stops at their comparison; Eukarya nested taxonomy, kingdoms and viruses belong to later cards.