IB Biology Topic A3.2.9: Classification of All Organisms Using Evidence from Rrna Base Sequences Notes

rRNA Analysis Revealed Three Domains of Life

1. Early classification used five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.
2. This system ignored key molecular differences between organisms.
3. rRNA analysis later showed these groups did not reflect true evolutionary relationships.

Why rRNA Works as a Molecular Marker

1. rRNA is found in all living organisms, making it a universal molecular marker.
2. Its conserved regions barely change, so it can be compared across distantly related groups.
3. Its variable regions accumulate mutations steadily, showing 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 new rank above kingdoms called domains.

Domain Archaea

1. Prokaryotic, but distinct from bacteria.
2. Often live in extreme environments as thermophiles, halophiles, or methanogens.
3. Key features:
   1. Circular chromosomes.
   2. Cell wall without peptidoglycan.
   3. Glycerol-ether membrane lipids, which are unique to archaea.
   4. 70S ribosomes, with a small subunit more similar to eukaryotes.
   5. Histones present.
   6. Introns present in some genes.

Domain Eubacteria (Bacteria)

1. True bacteria: unicellular prokaryotes found in almost all environments.
2. Key features:
   1. Circular chromosomes.
   2. Cell walls with peptidoglycan.
   3. Glycerol-ester membrane lipids.
   4. 70S ribosomes, distinct from archaeal and eukaryotic ribosomes.
   5. No histones, and introns are extremely rare.

Domain Eukaryota

1. Includes animals, plants, fungi, and protists.
2. Complex cells with membrane-bound organelles.
3. Key features:
   1. Linear chromosomes inside a nucleus.
   2. 80S ribosomes in the cytoplasm, with 70S ribosomes in mitochondria and chloroplasts.
   3. Histones present.
   4. Introns common in genes.

Why the Three-Domain System Matters

1. It created a new level of classification above kingdoms.
2. It reflects true evolutionary ancestry.
3. It showed the "prokaryotes" were not a single unified group.
4. Archaea and Eukaryota share a more recent common ancestor than either does with Bacteria.