Comprehensive Study Notes on Domain Archaea and Domain Bacteria

Evolutionary Relationships of Archaea, Bacteria, and Eukarya

  • Independent Evolution: Biologists currently believe that Domain Archaea and Domain Bacteria evolved independently from a common ancestor.
  • Phylogenetic Proximity:
    • Molecular evidence suggests that Archaea are more closely related to Eukarya than they are to Bacteria.
    • The Domain Eukarya is understood to have evolved from Archaea after the Archaea lineage split off from the Bacteria (represented in Figure 1.1).

General Characteristics and Classification of Domain Archaea

  • Taxonomic History:
    • In the traditional five-kingdom system, this domain was categorized under Kingdom Monera.
    • They were initially classified as a specialized group of bacteria and were historically called "archaebacteria."
  • Etymology: The name "Archaea" is derived from the Greek word archaios, which translates to "ancient."
  • Divergence: These organisms are prokaryotes that diverged from the bacterial lineage in very ancient times.
  • Physical Dimensions:
    • Individual archaeans typically range from 0.1 μm0.1\,\mu m to over 15 μm15\,\mu m in diameter.
    • Certain species form aggregates or filaments that can reach lengths up to 200 μm200\,\mu m.
  • Morphology: Archaea occur in various shapes, including:
    • Spherical
    • Rod-shape
    • Spiral
    • Lobed
    • Rectangular
  • Reproductive Mechanisms:
    • Archaea reproduce asexually.
    • Methods of reproduction include binary fission, multiple fission, fragmentation, or budding.
    • Notably, mitosis and meiosis do not occur within the domain Archaea.

Structural Uniqueness: Cell Membrane Composition

The uniqueness of Archaea is significantly defined by the chemical composition of their cell membranes, which differs from both Bacteria and Eukarya.

  • Archaeal Membrane Lipids:
    • Linkage Type: Contain lipids with an ether-linkage between the glycerol and the fatty acid chains.
    • Chain Structure: The fatty acid chains in Archaea are branched.
    • Functional Benefit: This specific chemical structure (ether-linkage and branching) makes their cell membranes more resistant to extreme environmental conditions.
  • Bacteria and Eukarya Membrane Lipids:
    • Linkage Type: Both domains possess membrane lipids where fatty acids are attached to glycerol by ester linkages.
    • Chain Structure: Their fatty acid chains are unbranched.
  • Visual Reference (Figure 1.2): Highlights the difference between the branched chain and ether linkage in Archaea versus the unbranched chain and ester linkage in Bacteria and Eukarya.

Structural Uniqueness: Cell Wall Composition

  • Archaea Cell Walls:
    • They lack both cellulose and peptidoglycan.
    • The wall contains distinct proteins and polysaccharides.
    • Certain archaea possess a specific substance known as pseudopeptidoglycan.
  • Bacterial Cell Walls:
    • Characterized by the presence of peptidoglycan, which is a polymer consisting of sugars and amino acids that provides structural support.
  • Eukarya Cell Walls:
    • Cell walls are not present in all Eukarya. When they do exist, they are composed of:
      • Cellulose (found in plants).
      • Chitin (found in fungi).

Genetic Relationship and Regulatory Features

  • Genetic Sequences: Archaea share several specific genetic sequences and regulatory features with eukaryotes.
  • Evolutionary Significance: These shared genetic markers highlight the closer evolutionary relationship between Domain Archaea and Domain Eukarya compared to their relationship with Domain Bacteria.