Life's diversity is organized into three domains: Archaea, Bacteria, and Eukarya. This system, proposed by Carl Woese, revolutionized our understanding of life's relationships. It's based on genetic differences and cell structure.

Archaea and Bacteria are prokaryotes, lacking a nucleus. Eukarya includes all organisms with a nucleus. This classification helps us understand life's evolution and the unique characteristics of each group.

Domains

Archaea and Bacteria

• Archaea single-celled microorganisms that lack a nucleus (prokaryotes)
  • Thrive in extreme environments (hot springs, salt lakes, and hydrothermal vents)
  • Cell walls contain unique lipids and polysaccharides not found in other domains
• Bacteria single-celled microorganisms that lack a nucleus (prokaryotes)
  • Found in diverse environments (soil, water, and inside other organisms)
  • Cell walls contain peptidoglycan, a complex polymer of amino acids and sugars

Eukarya

• Eukarya organisms with cells containing a nucleus and membrane-bound organelles
  • Includes all multicellular organisms (plants, animals, and fungi) and some unicellular organisms (protists)
  • DNA is enclosed within the nuclear membrane
  • Organelles (mitochondria and chloroplasts) carry out specific functions within the cell
  • Cytoskeleton provides structure and enables cell movement

Cell Types

Prokaryotic Cells

• Prokaryotes single-celled organisms that lack a nucleus and membrane-bound organelles
  • Includes Archaea and Bacteria
  • DNA is located in the cytoplasm, not enclosed within a nuclear membrane
  • Ribosomes are smaller (70S) compared to eukaryotic ribosomes
  • Reproduce through binary fission, a form of asexual reproduction

Eukaryotic Cells

• Eukaryotes organisms with cells containing a nucleus and membrane-bound organelles
  • Includes plants, animals, fungi, and protists
  • DNA is enclosed within the nuclear membrane
  • Ribosomes are larger (80S) compared to prokaryotic ribosomes
  • Reproduce through mitosis (asexual) or meiosis (sexual reproduction)
  • Organelles (endoplasmic reticulum, Golgi apparatus, and lysosomes) perform specific functions within the cell

Classification Methodology

Carl Woese's Contributions

• Carl Woese American microbiologist who pioneered the three-domain system of classification
  • Proposed the domain Archaea as a separate group from Bacteria and Eukarya
  • Used molecular techniques to study evolutionary relationships among organisms
  • Challenged the traditional two-kingdom (plant and animal) classification system

16S rRNA Analysis

• 16S rRNA a component of the small subunit of prokaryotic ribosomes
  • Highly conserved across different species of prokaryotes
  • Sequence variations in 16S rRNA can be used to determine evolutionary relationships
  • Woese used 16S rRNA sequencing to establish the three-domain system

Horizontal Gene Transfer

• Horizontal gene transfer the exchange of genetic material between organisms other than by vertical transmission from parent to offspring
  • Occurs frequently among prokaryotes (Archaea and Bacteria)
  • Contributes to the rapid spread of antibiotic resistance in bacterial populations
  • Can complicate the classification of organisms based on genetic similarity