Developments in biology: the three-domain system
Inheritance, variation and evolution • Classification of living organisms
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
From morphology to molecules
Earlier classification relies on observable features such as anatomy, life cycle and physiology. Similar appearance or lifestyle can cause unrelated organisms to be grouped together, so morphology alone gives an incomplete picture of evolutionary relationships. Molecular methods provide direct evidence of genetic relatedness, allowing classification to reflect evolutionary descent more accurately. The use of DNA and RNA sequence comparisons changes classification by revealing genetic distances and conserved sequences across all life forms .
Woese and 16S ribosomal RNA
A conserved region of ribosomal RNA (16S rRNA) provides a stable genetic marker for comparing very different organisms. Small changes in 16S rRNA accumulate slowly through evolution, so sequence comparison identifies deep evolutionary splits. Carl Woese’s analysis of 16S rRNA sequences identifies three major genetic lineages, leading to the proposal of the three-domain system. The method causes re-evaluation of previously accepted groups and reveals that some organisms that look like bacteria form a separate lineage, Archaea .
Definition of the three domains
The three-domain system divides cellular life into Bacteria, Archaea and Eukaryota. Bacteria consist of the familiar prokaryotic microbes with peptidoglycan cell walls and typical bacterial biochemistry. Archaea are prokaryotic in cell plan but differ in membrane lipids, cell wall chemistry and some molecular machinery; many inhabit extreme environments. Eukaryota contain all organisms with membrane-bound nuclei and organelles, including animals, plants, fungi and protists. The three-domain split reflects genetic divergence at the deepest branch points of the tree of life .
Key cellular and molecular differences
Eukaryotic cells have nuclei and membrane-bound organelles, while Bacteria and Archaea lack nuclei and typical eukaryotic organelles. Archaea differ from Bacteria in membrane lipid chemistry and in some aspects of transcription and translation machinery; these differences correspond to distinct genetic signatures. Many bacterial cell walls contain peptidoglycan, whereas archaeal cell walls lack peptidoglycan and often use different polymers. These molecular and biochemical differences align with sequence-based separations found in rRNA comparisons fileciteturn0file6turn0file19.
Limitations and continuing revision
Horizontal gene transfer (HGT) moves genes between unrelated lineages and complicates reconstruction of evolutionary history using single genes. Single-marker studies (for example, one rRNA gene) give strong evidence for deep splits but can miss more recent or reticulate relationships. Ongoing developments in whole-genome sequencing and bioinformatics refine domain boundaries and relationships, and classification remains subject to revision as new genomic data appear. Viruses remain excluded from the three-domain scheme because they lack cellular structure and do not fit the definition of living cells fileciteturn0file12turn0file19.
Key notes
Important points to keep in mind