Three Domain System

Three Domain System The Three Domain System, developed by Carl Woese, is a system for classifying biological organisms. Over the years, scientists have developed several systems for the classification of organisms. From the late 1960s, organisms had been classified according to a Five Kingdom system. This classification system model was based on principles developed by Swedish scientist Carolus Linnaeus, whose hierarchical system groups organisms based on common physical characteristics. The Three Domain System As scientists learn more about organisms, classification systems change. Genetic sequencing has given researchers a whole new way of analyzing relationships between organisms. The current system, the Three Domain System, groups organisms primarily based on differences in ribosomal RNA (rRNA) structure. Ribosomal RNA is a molecular building block for ribosomes. Under this system, organisms are classified into three domains and six kingdoms. The domains are Archaea, Bacteria, and Eukarya. The kingdoms are Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia. Archaea Domain This domain contains single-celled organisms known as archaea. Archaea have genes that are similar to both bacteria and eukaryotes. Because they are very similar to bacteria in appearance, they were originally mistaken for bacteria. Like bacteria, Archaea are prokaryotic organisms and do not have a membrane-bound nucleus. They also lack internal cell organelles and many are about the same size as and similar in shape to bacteria. Archaea reproduce by binary fission, have one circular chromosome, and use flagella to move around in their environment as do bacteria. Archaea differ from bacteria in cell wall composition and differ from both bacteria and eukaryotes in membrane composition and rRNA type. These differences are substantial enough to warrant that archaea have a separate domain. Archaea are extreme organisms that live under some of the most extreme environmental conditions. This includes within hydrothermal vents, acidic springs, and under Arctic ice. Archaea are divided into three main phyla: Crenarchaeota, Euryarchaeota, and Korarchaeota. Crenarchaeota include many organisms that are hyperthermophiles and thermoacidophiles. These archaea thrive in environments with great temperature extremes (hyperthermophiles) and in extremely hot and acidic environments (thermoacidophiles).Archaea known as methanogens are of the Euryarchaeota phylum. They produce methane as a byproduct of metabolism and require an oxygen-free environment.Little is known about Korarchaeota archaea as few species have been found living in places such as hot springs, hydrothermal vents, and obsidian pools. Bacteria Domain Bacteria are classified under the Bacteria Domain. These organisms are generally feared because some are pathogenic and capable of causing disease. However, bacteria are essential to life as some are part of the human microbiota. These bacteria preform vital functions, such as enabling us to properly digest and absorb nutrients from the foods we eat. Bacteria that live on the skin prevent pathogenic microbes from colonizing the area and also aid in the activation of the immune system. Bacteria are also important for the recycling of nutrients in the global ecosystem as they are primary decomposers. Bacteria have a unique cell wall composition and rRNA type. They are grouped into five main categories: Proteobacteria: This phylum contains the largest group of bacteria and includes E.coli, Salmonella, Heliobacter pylori, and Vibrio. bacteria.Cyanobacteria: These bacteria are capable of photosynthesis. They are also known as blues include Borrelia burgdorferi (cause Lyme disease) and Treponema pallidum (cause syphilis). Eukarya Domain The Eukarya domain includes eukaryotes or organisms that have a membranes include algae, amoeba, fungi, molds, yeast, ferns, mosses, flowering plants, sponges, insects, and mammals. Comparison of Classification Systems Five Kingdom System: MoneraProtistaFungiPlantaeAnimalia Archaea Domain Bacteria Domain Eukarya Domain Archaebacteria Kingdom Eubacteria Kingdom Protista Kingdom Fungi Kingdom Plantae Kingdom Animalia Kingdom Three Domain System As we have seen, systems for classifying organisms change with new discoveries made over time. The earliest systems recognized only two kingdoms (plant and animal). The current Three Domain System is the best organizational system we have now, but as new information is gained, a different system for classifying organisms may later be developed.