Kingdoms and Domains

What are the five kingdoms and what are their key features?

Prokaryotae — prokaryotic cells, no membrane-bound nucleus, no membrane-bound organelles, 70S ribosomes, usually unicellular, cell wall of peptidoglycan (in bacteria). Includes all bacteria. Protoctista — eukaryotic cells, mostly unicellular (though some are multicellular), do not fit neatly into other kingdoms, can be autotrophic or heterotrophic. Includes algae, amoeba and Plasmodium. Fungi — eukaryotic, mostly multicellular (except yeasts), cell walls made of chitin, heterotrophic by extracellular digestion (saprotrophic), reproduce by spores.

Plantae — eukaryotic, multicellular, cell walls of cellulose, autotrophic by photosynthesis, contain chloroplasts.

Animalia — eukaryotic, multicellular, no cell wall, heterotrophic (ingest food), can move.

What is the three-domain system and how does it differ from the five-kingdom system?

The three-domain system was proposed by Carl Woese based on molecular evidence from ribosomal RNA (rRNA) sequences. It adds a level above kingdoms. The three domains are: Bacteria (true bacteria — prokaryotes with bacterial rRNA), Archaea (extremophile prokaryotes with distinct rRNA sequences and different membrane lipids and cell wall chemistry from bacteria — they are actually more closely related to eukaryotes than to bacteria), and Eukarya (all eukaryotic organisms — includes the kingdoms Protoctista, Fungi, Plantae and Animalia).

Why did the discovery of Archaea change the classification system?

Before molecular analysis, Archaea were grouped with bacteria as they are also prokaryotic. However, analysis of rRNA sequences and other molecular evidence showed that Archaea are as different from bacteria as bacteria are from eukaryotes. Archaea have unique membrane lipids, different RNA polymerase structures, and genes that are more similar to eukaryotic genes in some respects. This evidence led Woese to propose the three-domain system, splitting the old Kingdom Prokaryotae into two separate domains. This is an example of how new evidence can lead to reclassification.

What evidence do scientists use to classify organisms?

Morphological and anatomical features — physical appearance, body plan, internal structure. Behavioural characteristics.

Biochemical evidence — comparing base sequences of DNA or mRNA, or amino acid sequences of proteins (e.g. cytochrome c, haemoglobin). The more similar the sequences, the more closely related the organisms.

Immunological evidence — antibodies raised against proteins from one species are tested against proteins from other species; closely related species show greater cross-reactivity.

rRNA sequencing — used by Woese to establish the three-domain system.

Fossil evidence — can help establish evolutionary relationships over time.