Three Domain System of Classification

 

Chapter 1: Biodiversity and Classification

1.1 Three-Domain System of Classification

- Biodiversity refers to the variety of life at genetic, species, and ecosystem levels.

- Classification helps organize organisms and understand their evolutionary relationships (phylogeny).

- A phylogenetic tree is a diagram showing these relationships.

Five-Kingdom System (1969 - R.H. Whittaker)

- Proposed by Robert H. Whittaker, an ecologist.

- Divided all life into:

  1. Monera (Prokaryotes)

  2. Protista (Eukaryotes)

  3. Fungi (Eukaryotes)

  4. Plantae (Eukaryotes)

  5. Animalia (Eukaryotes)

Three-Domain System (1990 - Carl Woese)

- Proposed by Carl Woese, a microbiologist, based on molecular data, especially ribosomal RNA sequencing.

- Provided deeper evolutionary insight into prokaryotic organisms.

- Domains:

  1. Archaea (Prokaryotic, ancient lineage)

  2. Bacteria (Prokaryotic)

  3. Eukarya (All eukaryotic organisms)

Key Concept: Archaea and Bacteria evolved independently from a common ancestor.

Molecular evidence shows Archaea are closer to Eukarya than to Bacteria.

Domain Archaea

- Name derived from Greek archaios = "ancient".

- Size: 0.1 μm to over 15 μm; can form aggregates/filaments (up to 200 μm).

- Shapes: spherical, rod-shaped, spiral, lobed, rectangular.

- Reproduction: Asexual — binary fission, budding, fragmentation. No mitosis or meiosis.

 

Significance of Archaea

- Enzymes from archaea used in DNA replication at high temperatures (PCR).

- Methanogens: Involved in sewage treatment; produce biogas.

- Acidophilic archaea: Help extract metals (gold, copper, cobalt) from ores.

- Human gut: Methanogens contribute to intestinal gas production.

Major Groups of Archaea

1. Methanogens – Produce methane.

2. Halobacteria – Thrive in salty (saline) environments.

3. Thermococci – Live in hot environments.

4. Thaumarchaeota – Participate in nitrogen cycle.

Unique Characteristics of Archaea

Metabolism: Archaea show methanogenesis, sulfur reduction, and live in extreme environments. Bacteria perform photosynthesis, nitrogen fixation, and fermentation. Eukarya use respiration, fermentation, and photosynthesis (in plants).

Genetic Material: Archaea show similarities in gene sequences and gene regulation with Eukarya. Bacteria have genetic material distinct from both Archaea and Eukarya. Eukarya have complex chromosomes with histones.

Cell Wall: Archaea do not have cellulose or peptidoglycan. Some may have pseudopeptidoglycan. Bacteria have peptidoglycan, while Eukarya may have cellulose (plants) or chitin (fungi).

Cell Membrane: Archaea have ether linkages and branched fatty acids in their membrane, which makes them more resistant to heat and acid. Bacteria and Eukarya have ester linkages with unbranched fatty acids.