Prokaryotes, Archaea, Bacteria and Eukarya

Learning Outcomes

  • Explain the differences between the three domains of life: Bacteria, Archaea, and Eukarya.
  • Distinguish between prokaryotic and eukaryotic cells.
  • Identify the structure and function of key prokaryotic cell components.
  • Understand the classification of bacteria based on different criteria: shape, Gram stain, and metabolism.
  • Recognize the unique characteristics of Archaea.
  • Describe the characteristics of Fungi, yeast, and lichens and their importance in our life.
  • Recognize the peculiarities of viruses, their variety in structure and genome.

Three-Domain System

  • Life is classified into three domains:
    • Bacteria
    • Archaea
    • Eukarya
  • Eukaryotic cells evolved from endosymbiotic prokaryotic cells:
    • Mitochondria were derived from endosymbiotic bacteria.
    • Chloroplasts originated from photosynthetic bacteria.
  • Number of known living species (estimated):
    • Bacteria: 10,000, but in millions overall.
    • Archaea: 260, estimated 1 million.
    • Eukarya: between 400,000 to 1 million.

Similarities Between the Three Domains

  • Conduct glycolysis.
  • Use DNA as the genetic material that encodes proteins.
  • Produce proteins by transcription and translation using a similar genetic code.
  • Replicate DNA semi-conservatively.
  • Have plasma membranes and ribosomes.

Differences Between the Three Domains

Prokaryotic Cells:
  • Lack a nucleus and cytoskeleton.
  • Do not divide by mitosis.
  • Do not have membrane-enclosed organelles (e.g., mitochondria, Golgi apparatus).
Eukaryotic Cells:
  • Have a nucleus and cytoskeleton.
  • Divide by mitosis and meiosis.
  • Contain membrane-bound organelles.

Characteristics of Prokaryotes

  • Much simpler than eukaryotes, usually single-celled.
  • DNA is circular and free in the cytoplasm.
  • No membrane-bound organelles present.
Prokaryotic Cell Shapes:
  • Coccus (spherical)
  • Bacillus (rod-shaped)
  • Spirillum (flexible or rigid spirals):
    • Flexible = spirochete.
    • Rigid = spirillum.

Prokaryotic Reproduction

  • Binary fission is the main mode of reproduction, which can occur as fast as every 20 minutes under optimal conditions.
  • Some prokaryotes reproduce via budding or fragmentation.

Genetic Material Exchange in Prokaryotes (Eubacteria)

  • Gamete fusion does not occur; however, genetic material can be exchanged through:
    • Transformation: Uptake of DNA from the environment.
    • Transduction: Gene transfer via bacteriophages.
    • Conjugation: Transfer via direct contact (cytoplasmic bridge).

Domain Archaea

Kingdom Archaeabacteria:
  • Typically single-celled and distinct from bacteria.
  • Main types include:
    • Methanogens: Produce methane, live in anaerobic environments.
    • Extreme Halophiles: Thrive in high salt concentrations.
    • Extreme Thermophiles: Live in high-temperature environments.

Domain Eukarya

Kingdom Fungi:
  • Eukaryotic organisms with distinct characteristics:
    • Cell wall made of chitin.
    • Heterotrophic metabolism (cannot produce their own food).
    • Most are decomposers.
Importance of Fungi:
  • Play significant roles in ecosystems, food production, pharmaceuticals, and disease (e.g., penicillin).

Viruses

  • Defined as obligate parasites that require host cells for replication.
  • Variety in structure and classification, with genetic material being either DNA or RNA.
  • Viruses can affect humans, animals, and plants, causing various diseases.
Mode of Infection:
  • Viruses can enter host cells through various methods, such as phagocytosis and receptor-mediated endocytosis.
  • They may remain dormant (lysogenic) or actively replicate (lytic), affecting the host's cellular mechanisms.

Bacterial Classification

  • Classified by several methods:
    • Scientific naming (Genus species).
    • Staining techniques (e.g., Gram staining).
    • Morphology (shape and size).
  • Gram-positive bacteria retain crystal violet stain, while Gram-negative bacteria do not.

Summary

  • Life on Earth encompasses three domains: Bacteria, Archaea, and Eukarya.
  • Prokaryotes (Bacteria and Archaea) lack true nuclei, while Eukarya includes complex organisms.
  • Bacteria classify based on morphology, Gram stain, and metabolism.
  • Viruses are acellular entities that rely on host cells for reproduction and survival.