1. What were the historical limitations of previous classification systems that led to and are solved by the three domain system that is in use today?
In the mid-1700s, Swedish botanist Carl Linnaeus was credited as one of the first scientists to develop taxonomy for living organisms. It is for this reason that he is known as the “father of taxonomy.” Linnaeus’ taxonomy grouped living things into two kingdoms: plants and animals (Tortora, 2013, p. 273).
By the 1900’s, scientists had discovered microorganisms that had characteristics that were dramatically different from those of plants and animals. Therefore, Linnaeus’ taxonomy needed to be enhanced to encompass microorganisms. In 1969, Robert H. Whittaker, working at Cornell University, proposed a new taxonomy system that consisted of five kingdoms called Whittaker’s five kingdom taxonomy. These kingdoms are Prokaryote, or monera, and eukaryotes comprised of the other four (Protista, plantae, fungi, and Animalia.) The Kingdom Prokaryote had been based on microscopic observations (Tortora, 2013, p. 273).
Scientists widely accepted Whitaker’s taxonomy until 1977, when Carl Woese, in collaboration with Ralph S. Wolfe at the University of Illinois, proposed a new six-kingdom taxonomy. This came about with the discovery of archaea, which are prokaryotes that live in oxygen-deprived environments (Wikipedia, 2014).
Before Woese’s six-Kingdom taxonomy, scientists grouped organisms into eukaryotes – animals, plants, fungi, and one-cell microorganisms and prokaryotes (Wikipedia, 2014).
Woese’s five-kingdom taxonomy in 1969 consisted of Eubacteria, Protista, Fungi, Plantae, and Animalia (Wikipedia, 2014).
By studying the ribosomal RNA sequence in prokaryotic cells, Woese determined that archaebacterial and eubacteria are two groups. Woese used three major criteria to define his six kingdoms: Cell type, level of organization, and nutrition (Wikipedia, 2014).
In the 1990’s, Woese studied rRna sequences in prokaryotic cells, providing that these organisms should be divided into two distinct groups. Today, organisms are grouped into three categories called domains that represented as bacteria, archaea, and eukaryotes (Tortora, 2014, p. 273).
The domains are placed above the phylum and kingdom levels. The term archaebacterial refers to the ancient origin of this group of bacteria that appear to have diverged from eubacteria. The evolutionary relationship among the three domains is domain bacteria (eubacteria), domain Eukarya (eukaryotes). Different classifications of organisms include: Bacteria; Eubacteria, Archaebateria; archaea, Eukarya; Protista, fungi, plantae, Animalia. The three domains are archaea, bacteria and Eukarya. Archaea lack peptidoglycan acid in the cell walls. Bacteria have a cell wall composed of peptidoglycan and muramic acid. Bacteria also have membrane lipids with ester-linked, straight-chained fatty acids that resemble eukaryotic membrane lipids. Most prokaryotes are bacteria. Bacteria also have plasmids, which are small, double-stranded DNA molecules that are extra chromosomal. Eukarytes are of the domain Eukarya and have a defined nucleus and membrane bound organelles and have linear chromosomes (Wikipedia, 2014).
Kingdom (biology). (2014, November 14). In Wikipedia, The Free Encyclopedia. Retrieved 22:35, November 14, 2014, from http://en.wikipedia.org/w/index.php?title=kingdom_(biology)&oldid=633796034
Tortora, G., Funke, B., Case, C. (2013). Microbiology: An Introduction. 11th Ed. Pearson. www.pearsonhighered.com
2. Why is the gene coding for ribosomal RNA (rRna) used for establishing phylogenetic relationships and how does this compare to previous methods for placing organisms in appropriate taxa?
There are numerous reasons for which genes encoding for rRNA are used for the purpose of establishment of phylogenetic relationship out of which, (a.) presence of these sequences in all creatures as; Bacteria, Archaea, and Eukaryota, (b.) facility to isolate them
