biology 1.3

The Three Domains of Life

Introduction to the Three Domains

  • The Three Domains of Life comprise:
    • Bacteria
    • Archaea
    • Eukarya

Domain: Bacteria

  • Characteristics:
    • All are single-celled organisms.
    • Ubiquitous presence; found everywhere and extremely abundant.
    • Plays a crucial role in decomposing dead biological material.
    • A minuscule fraction of bacteria are pathogenic and can cause human diseases.

Domain: Archaea

  • Characteristics:
    • Composed of single-celled organisms.
    • Some reside in extreme environments, capable of tolerating conditions such as:
    • pH as low as 0
    • Temperatures reaching up to 121°C
    • Understanding of Archaea is limited compared to Bacteria.
    • Archaea consists of no recorded disease-causing organisms.
    • Includes significant groups such as:
    • Methanogens, which produce methane; contributing 25-30% of global warming.

Domain: Eukarya

  • Characteristics:
    • Members are referred to as eukaryotes.
    • Encompasses organisms such as animals, plants, fungi, and protists.
    • Includes all multicellular organisms and various single-celled organisms.

Common Characteristics of Bacteria and Archaea

  • Both domains consist of:
    • Single-celled organisms
    • Typically small in size
    • Surrounded by a rigid cell wall
    • Lack internal membranes, making them challenging to differentiate using a microscope.

Prokaryotic vs Eukaryotic Cells

Prokaryotic Cells:
  • DNA is located in a nucleoid region - no true nucleus present.
  • Generally range from 1-10 μm in size.
  • Typically surrounded by a cell wall.
  • Composed of a single circular chromosome.
  • Do not have histones associated with DNA.
  • Reproduce via binary fission, a simple asexual reproduction method.
Eukaryotic Cells:
  • DNA is encased within a membrane-bound nucleus.
  • Typically size ranges from 10-100 μm.
  • Some eukaryotic cells have cell walls; not universal.
  • Composed of multiple linear chromosomes.
  • DNA is associated with proteins known as histones.
  • Reproduce via more complex processes: mitosis or meiosis.

Key Differences Between Prokaryotes and Eukaryotes

  • DNA Organization:

    • Prokaryotes: single circular chromosome.
    • Eukaryotes: multiple linear chromosomes.

  • Histones Presence:

    • Prokaryotes: No histones present.
    • Eukaryotes: DNA is associated with histones.

  • Reproduction Method:

    • Prokaryotes: Reproduce by binary fission.
    • Eukaryotes: Reproduce by mitosis (asexual) or meiosis (sexual).

Visual Representations

  • Images and Collages:
    • Eukaryote collage adapted from SACE Stage 1 Biology Workbook.
    • Bacteria collage sourced from Wikimedia Commons.
    • Archaea collage also sourced from Wikimedia Commons.

Additional Information

  • Macrophages and Chromosomes:
    • Visuals include images of macrophages and chromosome structures to provide visual representation of concepts discussed.