• Taxonomy is the practice of biological classification
• It involves placing organisms into a series of categories or 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
• 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)

The taxonomic classification system with the three domains

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

• Domains are the highest taxonomic rank that exist within the hierarchical classification system of organisms
• Initially, all organisms within the Archaea domain were classified as Bacteria
• Then several unique features possessed by Archaea were discovered that separated them from both Bacteria and Eukarya
• The main differences between Archaea and Bacteria are seen in:
  • Membrane lipids
  • Ribosomal RNA
  • Cell wall composition

Membrane lipids

• The membrane lipids found in the cells of Archaea organisms are completely?unique
• They are not found in any?bacterial?or?eukaryotic?cells
• The membrane lipids of Archaea consist of?branched?hydrocarbon chains bonded to glycerol by?ether?linkages
• The membrane lipids of Bacteria consist of?unbranched?hydrocarbon chains bonded to glycerol by?ester?linkages

Ribosomal RNA

• Both Archaea and Bacteria possess 70S ribosomes
• The 70S ribosomes in Archaea possess a smaller subunit that is?more similar to the subunit found in Eukaryotic?ribosomes than subunits in Bacterial ribosomes
  • The?base sequences of ribosomal RNA?in Archaea show more similarity to the rRNA of Eukarya than Bacteria
  • The?primary structure of ribosome proteins?in Archaea show more similarity to the ribosome proteins in Eukarya than Bacteria

Composition of cell walls

• Organisms from the?Bacteria?domain have cells that always possess cell walls?with peptidoglycan
• Organisms from the?Archaea?domain also have cells that always possess cell walls, however these?do not contain peptidoglycan

Characteristics & features of the three domains table