Higher The Three-Domain System and Woese's RNA Analysis

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.