Taxonomy

• Each group within a phylogenetic biological classification is called a taxon (plural taxa).
• Taxonomy is the study of taxa (groups) and their position in a hierarchal order, where they are known as taxonomic ranks.
• A domain is the highest taxonomic rank, and can be further subdivided into : Bacteria, Archaea and Eukarya.
• Bacteria are a group of single-celled prokaryotes with the following features: lack of membrane-bound organelles, unicellular, 70s ribosomes, murein cell walls and a single DNA loop without histones.
• Archaea are a group of single-celled prokaryotes, however they differ from bacteria in numerous ways. For example, their genes and protein synthesis are more similar to eukaryotes than prokaryotes.
• Archaea membranes contain fatty acid chains connected to glycerol by ether linkages (instead of ester bonds).
• The cell walls of archaea contain no murein, and they possess a more complex form of RNA polymerase.
• Eukarya are a group of organisms that are made up of one or more eukaryotic cells. One of the features that they possess are membrane-bound organelles, and their membranes contain fatty acid chains connected to glycerol via ester linkages.
• Eukarya don't all contain cell walls, but when they do they contain no murein.
• Eukarya contain 80s ribosomes rather than 70s (bacteria). 80s ribosomes are larger than 70s ribosomes.
• The Eukarya domain is divided into four kingdoms: Protocista, Fungi, Plantae and Animalia. Each kingdom consists of phyla, and diversity within phyla allows them to be further divided into classes.
• Classes can be further divided into orders of organisms, and then further into families, then genera, then species.
• Organisms with different domains have obvious differences in characteristics, whilst different species have less obvious differences (they are more similar).