I. INTRODUCTION

C. CLASSIFICATION: THE THREE DOMAIN SYSTEM

The overall purpose of this Learning Object is:
1) to compare the three domains of cellular organisms found in nature; and
2) to learn which of these are prokaryotic and which is eukaryotic.

LEARNING OBJECTIVES FOR THIS SECTION

The Three Domain System, proposed by Woese and others, is an evolutionary model of classification based on differences in the sequences of nucleotides in the cell's ribosomal RNAs (rRNA), as well as the cell's membrane lipid structure and its sensitivity to antibiotics.

Comparing rRNA structure is especially useful. Because rRNA molecules throughout nature carry out the same function, their structure changes very little over time. Therefore similarities and dissimilarities in rRNA nucleotide sequence are a good indication of how related or unrelated different cells and organisms are.

This system proposes that a common ancestor cell gave rise to three different cell types, each representing a domain. The three domains are the Archaea (archaebacteria), the Bacteria (eubacteria), and the Eukarya (eukaryotes). The Eukarya are then divided into 4 kingdoms: Protists, Fungi, Anamalia, and Plantae. A description of the three domains follows:

1. The Archaea (archaebacteria)

The Archaea possess the following characteristics:

• Archaea are prokaryotic cells.
• Unlike the Bacteria and the Eukarya, the Archaea have membranes composed of branched hydrocarbon chains attached to glycerol by ether linkages (see Fig. 1).
• The cell walls of Archaea contain no peptidoglycan.
• Archaea are not sensitive to some antibiotics that affect the Bacteria, but are sensitive to some antibiotics that affect the Eukarya.
• Archaea contain rRNA that is unique to the Archaea as indicated by the presence molecular regions distinctly different from the rRNA of Bacteria and Eukarya.

Archaea often live in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles. One reason for this is that the ether-containing linkages in the Archaea membranes is more stabile than the ester-containing linkages in the Bacteria and Eukarya and are better able to withstand higher temperatures and stronger acid concentrations.

2. The Bacteria (eubacteria)

The Bacteria possess the following characteristics:

• Bacteria are prokaryotic cells.
• Like the Eukarya, they have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages (see Fig. 1).
• The cell walls of Bacteria, unlike the Archaea and the Eukarya, contain peptidoglycan.
• Bacteria are sensitive to traditional antibacterial antibiotics but are resistant to most antibiotics that affect Eukarya.
• Bacteria contain rRNA that is unique to the Bacteria as indicated by the presence molecular regions distinctly different from the rRNA of Archaea and Eukarya.

Bacteria include mycoplasmas, cyanobacteria, Gram-positive bacteria, and Gram-negative bacteria.

3. The Eukarya (eukaryotes)

The Eukarya (also spelled Eucarya) possess the following characteristics:

• Eukarya have eukaryotic cells.
• Like the Bacteria, they have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages (see Fig. 1).
• Not all Eukarya possess cells with a cell wall, but for those Eukarya having a cell wall, that wall contains no peptidoglycan.
• Eukarya are resistant to traditional antibacterial antibiotics but are sensitive to most antibiotics that affect eukaryotic cells.
• Eukarya contain rRNA that is unique to the Eukarya as indicated by the presence molecular regions distinctly different from the rRNA of Archaea and Bacteria.

The Eukarya are subdivided into the following kingdoms:

a. Protista Kingdom
Protista are simple, predominately unicellular eukaryotic organisms. Examples includes slime molds, euglenoids, algae, and protozoans.

b. Fungi Kingdom
Fungi are unicellular or multicellular organisms with eukaryotic cell types. The cells have cell walls but are not organized into tissues. They do not carry out photosynthesis and obtain nutrients through absorption. Examples include sac fungi, club fungi, yeasts, and molds.

c. Plantae Kingdom
Plants are multicellular organisms composed of eukaryotic cells. The cells are organized into tissues and have cell walls. They obtain nutrients by photosynthesis and absorption. Examples include mosses, ferns, conifers, and flowering plants.

d. Animalia Kingdom
Animals are multicellular organisms composed of eukaryotic cells. The cells are organized into tissues and lack cell walls. They do not carry out photosynthesis and obtain nutrients primarily by ingestion. Examples include sponges, worms, insects, and vertebrates.

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Updated: Feb. 21, 2007
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