Classification of the Three Domains (OCR A Level Biology)

• Taxonomy is the practice of biological classification
• It involves placing organisms into a series of categories or taxa
• There are different ranks within the hierarchical classification system used in biology
• The highest rank is the domain
  • There are three domains of life

• Cell type has a major role in the classification of organisms into the three domains but do not confuse cell types and domains
• The two cell types are prokaryotic cells and eukaryotic cells
  • 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

The three domains of life

• Although it was well known that most forms of life could be divided into prokaryotes and eukaryotes, advances in the studies of molecular biology, biochemistry and cell structure in the latter part of the 20th century showed that the prokaryotes are not one uniform group
• Based on molecular analyses of RNA genes in particular, and by looking at features such as ribosomal RNA (rRNA), aspects of protein synthesis and the structure of cell membranes and flagella, scientists realised that using cell type to classify organisms was insufficient and that prokaryotes could be divided into two separate groups (domains)
• The three domains are:
  • Bacteria (prokaryotes)
  • Archaea (prokaryotes)
  • Eukarya (eukaryotes)

The taxonomic classification system with the three domains

Archaea

• Organisms within this domain are sometimes referred to as the extremophile prokaryotes, as archaea were first discovered living in extreme environments (although 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 have 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 is 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 than the smallest known single-celled eukaryotes
• Bacterial cells divide by binary fission
• Example: Staphylococcus pneumoniae is a species of bacteria 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 that are only 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

Key differences between archaea & bacteria

• 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