A3.2.9 Classification of all organisms using evidence from rRNA base sequences (HL) Notes

rRNA Analysis Revealed Three Evolutionary Domains

1. Early classification used five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.
2. However, this system ignored key molecular differences between organisms.
3. rRNA analysis later revealed these groups didn't reflect true evolutionary relationships.

Why rRNA Was Chosen for Analysis

1. rRNA is found in all living organisms, making it a universal molecular marker.
2. It has conserved regions that barely change over time, allowing comparison across distantly related groups.
3. It also has variable regions that accumulate mutations steadily, providing evidence for evolutionary divergence.

Discovery of the Three-Domain System

1. In 1977, microbiologist Carl Woese and colleagues compared rRNA base sequences from different prokaryotes.
2. The results revealed two fundamentally distinct groups of prokaryotes:
   1. Eubacteria (true bacteria)
   2. Archaea (a separate lineage, more closely related to eukaryotes than to bacteria).
3. This led to a paradigm shift: a new taxonomic rank above kingdoms called domains.

1. Domain Archaea

1. Prokaryotic, but distinct from bacteria.
2. Often live in extreme environments (thermophiles, halophiles, methanogens).
3. Key features:
   1. Circular chromosomes.
   2. Cell wall without peptidoglycan (unlike bacteria).
   3. Glycerol-ether lipids in cell membranes (unique).
   4. 70S ribosomes, but with a small subunit more similar to eukaryotes.
   5. Histone-like proteins present.
   6. Introns present in some genes.

2. Domain Eubacteria (Bacteria)

1. True bacteria; unicellular prokaryotes.
2. Found in almost all environments.