4.3.3 Classification of the Three Domains Flashcards
What is taxonomy?
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
What are 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)
What are 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
What are 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
What are 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
What are the differences between archaea and 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 compositio
What are membrane lipids like in archaea and bacteria?
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
What is the difference in ribosomal RNA between archaea and bacteria?
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
What is the difference in the composition of cell walls between bacteria and archaea?
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
