Higher The Three-Domain System and Woese's RNA Analysis
Part of Classification — GCSE Biology
This higher tier covers Higher The Three-Domain System and Woese's RNA Analysis within Classification for GCSE Biology. Classification systems, taxonomy, and evolutionary relationships It is section 8 of 11 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Topic position
Section 8 of 11
Practice
25 questions
Recall
20 flashcards
Higher The Three-Domain System and Woese's RNA Analysis
Carl Woese's revision of the classification system in the 1970s–1990s represents one of the most significant changes in biology. Before Woese, the five-kingdom system divided life into: Prokaryotae (all prokaryotes), Protoctista, Fungi, Plantae, and Animalia. This system treated all prokaryotes as one group.
Woese's method: He sequenced the 16S ribosomal RNA (rRNA) gene from a wide variety of organisms. Ribosomal RNA was chosen because:
- All living organisms have ribosomes, so rRNA sequences can be compared across all life.
- rRNA changes very slowly through evolution (it is "highly conserved"), making it a reliable molecular clock for deep evolutionary comparisons.
- Small differences in rRNA sequence reflect millions of years of divergence.
The discovery: Woese found that organisms living in extreme environments (high salt, high temperature, no oxygen — e.g., methane-producing microbes in marshes) had rRNA sequences as different from true bacteria as either group is from eukaryotes. He named these organisms Archaea. This gave rise to the three-domain system: Archaea, Bacteria, and Eukaryota.
Why this matters: The three-domain system replaced the five-kingdom model because it is based on molecular evidence and more accurately reflects evolutionary relationships. It shows that the deepest division in life is not between single-celled and multi-celled organisms, but between prokaryotes with bacterial-type rRNA (Bacteria) and those with archaeal-type rRNA (Archaea), with all eukaryotes forming a third, more recently diverged domain.