• Taxonomy is the practice of biological classification
• The phylogenetic classification system enables us to arrange species into groups based on their?evolutionary origins?and?relationships
• There is?no overlap between groups?and each group is called a taxon (plural taxa)
• By grouping organisms into taxa it can make them easier to understand and remember
• There are several?different ranks?or levels within the?hierarchical?classification system used in biology
  • Multiple smaller taxa can be put in the same larger taxa

   

• The highest rank is the?domain
• Cell type?has a major role in the classification of organisms into the three domains; but do not confuse cell types and domain
  • Prokaryotic?cells are easily distinguishable in that they lack a nucleus
  • Eukaryotic?cells have compartmentalised structures, with at least their genetic material segregated from the rest of the cell in a nucleus

   

• Based upon molecular analysis of RNA genes in particular, scientists have realised that using cell type to classify organisms is insufficient, and that?prokaryotes?could be divided into two separate groups (domains)
• The?three domains?are:
  • Archaea (prokaryotes)
  • Bacteria (prokaryotes)
  • Eukarya (eukaryotes)

   

Archaea

• Organisms within this domain are sometimes referred to as the extremophile?prokaryotes,?archaea were first discovered living in?extreme environments,?but not all archaea do
• Archael cells have?no nucleus?(and so are?prokaryotic)
• They were initially classified as bacteria until several unique properties were discovered that separated them from known bacteria, including:
  • Unique lipids being found in the membranes of their cells
  • No?peptidoglycan?in their cell walls
  • Ribosomal structure (particularly that of the small subunit) are more similar to the eukaryotic ribosome than that of the bacteria

   

• Archaea a similar size range as bacteria (and in many ways metabolism is similar between the two groups)
• DNA transcription is more similar to that of eukaryotes
• Example:?Halobacterium salinarum?are a species of the archaea domain that can be found in environments with high salt concentrations like the Dead Sea

Bacteria

• These are organisms that have?prokaryotic cells?which contain no nucleus
• They vary in size over a wide range: the smallest are bigger than the largest known-viruses and the largest are smaller that the smallest known single-celled eukaryotes
• Bacterial cells divide by binary fission
• Example:?Staphylococcus pneumoniae?is a bacteria species that causes pneumonia

Eukarya

• Organisms that have?eukaryotic cells?with nuclei and membrane-bound organelles are placed in this domain
• They vary massively in size from single-celled organisms several micrometres across to large multicellular organisms many-metres in size, such as blue whales
• Eukaryotic cells divide by mitosis
• Eukaryotes can reproduce sexually or asexually
• Example:?Canis lupus?also known as wolves

The three domains