Domain is the most general taxonomic classification of organisms. Cell structure and genetics are two primary criteria used to classify organisms within three domains. Each domain has differences and shares similarities with the others.
All organisms were originally classified as either eukaryotes or prokaryotes. Eukarya are organisms with cells that contain a nucleus, which is a sac and its contents consisting of an organism's complete genetic material called deoxyribonucleic acid (DNA). Prokaryotes lack the nuclear membrane that encompasses the genetic material, so prokaryotic genetic material is lumped together as a mass called a nucleoid that does not have a membrane separating it from the rest of the cell structures.
Three Domains of Life
Domain is the most general classification of organisms. When it was discovered that the prokaryotes actually consisted of two very unique types of organisms, a new domain classification was created resulting in the three domains of life: Archaea, Bacteria, and Eukarya.
Archaea Often Live in Extreme Environments
Archaeans are microscopic, prokaryotic organisms that share many similarities with bacteria, such as their shape and appearance. Some are a rod-shaped bacillus, a spherical coccus, spiral or even triangular in shape! Some have one or more flagella, which are hair-like structures that can be used for movement.
It is the similar appearance of archaea to bacteria that led to the same classification of these organisms, but a more in-depth view of the cell walls led to the division of the prokaryotic classification. Archaean cell walls are composed of a phosphate layer on each side, followed by an L-glycerol layer connected to isoprene chains. The branching characteristic of the isoprene chains allows the chains to bond together in such a manner that can protect the cell in extreme environments like extreme heat or salinity.
Identification of Bacteria
The bacterial cell wall distinguishes bacteria from archaea. The cell membrane consists of a phosphate layer on each side, followed by a D-glycerol layer connected to unbranched fatty acids. This is in addition to a peptidoglycan layer.
Like archaeans, bacteria are single-celled organisms that can be rod-, spherical-, or spiral-shaped. Unlike archaea, they are less likely to exist in extreme conditions. Some of those that are thought to be bacteria that are adapted to extreme conditions are actually archaea. Halobacteria, an organism living in areas of high salinity, is one of these organisms and is also known as Haloarchaea, although the former, misleading name is still used.
Eukaryotic Cells Contain a Cell Nucleus
Eukarya are organisms whose cells contain a nucleus. The nucleus contains the organism's genetic material within a membrane called the nuclear envelope. Unlike the genetic material of archaea and bacteria, eukaryotic genetic material is organized rather than clumped together.
According to the University of California Museum of Paleontology article, "Eukaryota: More on Morphology," (accessed April 27, 2010) by Ben Waggoner of the Department of Biology at the University of Central Arkansas, and Brian R. Speer of the University of California in Berkeley, "Unlike bacteria and archaea, eukaryotes have their DNA in linear pieces that are bound up with special proteins, [called] histones, to make chromosomes, [which are] normally visible only in dividing cells."
"Although they show unbelievable diversity in form, they share fundamental characteristics of cellular organization, biochemistry, and molecular biology," say Waggoner and Speer in their "Introduction to the Eukaryota" (accessed April 27, 2010). In fact, Domain Eukarya includes animals, fungi, plants, and protists.
While one may think that archaea and bacteria are closely related, these two domains prove that appearances can be deceiving. Although archaea look like bacteria, they are more genetically similar to the more complex eukaryotes. Cellular structure and function demonstrates that these three domains are truly in classes of their own.
Diane Ursu - Diane Ursu joined Suite101 as a contributing writer in August 2009 and became a Feature Writer in January 2010. She is a freelance writer ...
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