Eukarya - One of the Three Domains of Life

     The Eukarya is the domain of eukaryotic cells. Eukaryotic cells have their DNA organized into chromosomes located in a nucleus. In fact, the term eukaryote means "true kernel" or "true nucleus". Eukaryotic cells also have other membrane-bound organelles such as mitochondria, plastids, and lysosomes. Neither a nucleus nor membrane-bound organelles are present in prokaryotic cells, the cell type found in the domains Archaea and Bacteria.

     Until recently, the eukaryotic organisms were divided into the four "Kingdoms" Protista, Fungi, Animalia, and Plantae based on features such as single-celled or multicellular and modes of nutrition. Recent investigations using DNA sequences from many different organisms have resulted in a rather different classification system for eukaryotes, as well as the Three Domain system presented here. Part of the new classification system (which is still being developed) is presented in this exercise.

     The Eukarya are highly diverse and include most of the familiar organisms such as plants, animals, and mushrooms. In addition, there are large numbers of single-celled organisms, both familiar (i.e. amoeba) and unfamiliar (i.e. Tricomonads). Most of the diversity of basic cell structure present in Eukarya is found in the single-celled organisms.

     Mitochondria and plastids, two organelles of eukaryotic organisms, were derived from prokaryotic cells by a process called endosymbiosis. Endosymbiosis is thought to have taken place when a primitive eukaryotic cell engulfed a bacterial cell. Instead of being digested, the bacterium became a permanent part of the eukaryotic cell. Gradually, the endosymbiotic bacterium was transformed into an organelle.

Points to remember:

• Eukarya are eukaryotic, which means they have a cell nucleus.
• Eukaryotic cells also contain membrane bound organelles.
• Eukaryotic plastids and mitochondria were derived from endosymbiosis of bacterial cells.
• Eukarya are more diverse than any other domain of life.

Representative organisms:

     Since most of the organisms familiar to the average student are eukaryotic, representative organisms are not presented here. However, more information on the different types of eukaryotic organisms is presented in the diversity exercise diagram.

Links:

     Additional information on Eukarya is available from the University of California at Berkeley Museum of Paleontology. This site includes general information on features of the eukaryotic cell and detailed information on the systematics and ecology of eukaryotes.