Key Concept:
Based on similarities in ribosomal RNA, living
organisms are classified into three domains: Bacteria, Archaea, and Eukarya.
In 1978, Carl Woese devised a system of classification based on the cellular organization of organisms. It groups all organisms in 3 domains as follows:
1. Bacteria - cell walls contain peptidoglycan. Includes all of the pathogenic prokaryotes as well as many of the nonpathogenic prokaryotes found in soil and water. The photoautotrophic prokaryotes are also in this domain.
2. Archaea - cell walls, if present, lack peptidoglycan. Includes prokaryotes that do not have peptidoglycan in their cell walls. They often live in extreme environments and carry out unusual metabolic processes. Archaea
include three major groups:
•The methanogens, strict anaerobes that produce methane from carbon dioxide and hydrogen.
•Extreme halophiles, which require high concentrations of salt for survival.
•Hyperthermophiles, which normally grow in extremely hot environments.
3. Eukarya, which includes:
•Protists (slime molds, protozoa, and algae)
•Fungi (unicellular yeasts, multicellular molds, and
mushrooms)
•Plants (includes mosses, ferns, conifers, and flowering
plants)
•Animals (includes sponges, worms, insects, and vertebrates)
Some Characteristics of the 3 domains:
Archaea (Sulfolobus)
Cell Type: Prokaryotic
Cell Wall: Varies in composition; contains no peptidoglycan
Membrane Lipids: Composed of branched carbon chains attached to glycerol by ether linkage
First Amino Acid in Protein Synthesis: Methionine
Antibiotic Sensitivity: No
rRNA Loop: Lacking
Common Arm of tRNA: Lacking
Bacteria (E. coli)
Cell Type: Prokaryotic
Cell Wall: Contains peptidoglycan
Membrane Lipids: Composed of straight carbon chains
attached to glycerol by ester linkage
First Amino Acid in Protein Synthesis: Formylmethionine
Antibiotic Sensitivity: Yes
rRNA Loop: Present
Common Arm of tRNA: Present
Eukarya (Amoeba)
Cell Type: Eukaryotic
Cell Wall: Varies in composition; contains carbohydrates
Membrane Lipids: Composed of straight carbon chains
attached to glycerol by ester linkage
First Amino Acid in Protein Synthesis: Methionine
Antibiotic Sensitivity: No
rRNA Loop: Lacking
Common Arm of tRNA: Present
Prokaryotic Cells and Eukaryotic Organelles compared:
Prokaryotic Cell
DNA: One circular; some two circular; some linear
Histones: In archaea
First Amino Acid In Protein Synthesis: Formylmethionine (bacteria), Methionine (archaea)
Ribosomes: 70S
Growth: Binary fission
Eukaryotic Cell
DNA: Linear
Histones: Yes
First Amino Acid In Protein Synthesis: Methionine
Ribosomes: 80S
Growth: Mitosis
Eukaryotic Organelles (Mitochondria and Chloroplasts)
DNA: Circular
Histones: No
First Amino Acid In Protein Synthesis: Formylmethionine
Ribosomes: 70S
Growth: Binary fission
The original nucleoplasmic cell was prokaryotic. However, infoldings in its plasma membrane may have surrounded the nuclear region to produce a true nucleus. Recently, French researchers provided support for this hypothesis with their observations of a true nucleus in Cemmala bacteria. Over time, the chromosome of the nucleoplasm may have acquired pieces such as transposons. In some cells, this large chromosome may have fragmented into smaller linear chromosomes. Perhaps cells with linear chromosomes had an advantage in cell division over those with a large, unwieldy circular chromosome. That nucleoplasmic cell provided the original host in which endosymbiotic bacteria developed into organelles. The cyanobacterium-like cell and the eukaryotic host require each other for survival. In sequencing the genome of a prokaryote called Thermotoga maritima, microbiologist Karen Nelson has discovered that this species has genes similar to members of both the Domain Bacteria and the Domain Archaea. Her findings suggest that Thennologa is one of the earliest cells. For this reason, Thermotoga is referred to as one of the "deeply branching genera," that is, it is near the origin or "root" of the evolutionary tree.
