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FORMS OF LIFE.   Term Paper ID:29425 Click to get this paper

Essay Subject: Discusses three stages of cellular evolution.... More...

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Paper Abstract: Discusses three stages of cellular evolution. First Common Ancestor (Universal Ancestor). Last Common Ancestor. Two basic forms of life: prokaryotes (single-celled); eukarotes (monocellular organisms) which have given rise to three kingdoms of multicellular organisms that exist. New types of cell divisions. DNA molecules. Paper Introduction: There are basically two forms of life on earth: prokaryotes, which are cells without a nucleus, and eukarotes, which are cells which contain a nucleus (Barbieri, 2001). Prokaryotes are found almost exclusively as single-celled organisms which can inhabit any ecological niche. They can exist with or without light, oxygen or organic molecules. They synthesize all their components from inorganic molecules, can rapidly adapt to environmental changes, and can exchange genes horizontally. Prokaryotes have a single circular DNA molecule, a single cell compartment and an exoskeleton. Eukaryotes are mostly dependent on oxygen, and exists as monocellular organisms such as protozoa and protista, and have given rise to the three kingdoms of multicellular organisms that exist: plants, fungi and animals (Barbieri, 2001). They have Text of the Paper: The entire text of the paper is shown below. However, the text is somewhat scrambled. We want to give you as much information as we possibly can about our papers and essays, but we cannot give them away for free. In the text below you will find that while disordered, many of the phrases are essentially intact. From this text you will be able to get a solid sense of the writing style, the concepts addressed, and the sources used in the research paper. References Barbieri, M. He believes that, with time, this UniversalAncestor evolved into a small number of increasingly complex cell types,and these gave rise to the three primary kingdoms that now exist. Phylogenetic structure of theprokaryote domain: the three primary kingdoms. (2 1). E. PNAS, 74, pp. First there was a transition from the First CommonAncestor (Universal Ancestor) to the Last Common Ancestor. Prokaryoteshave a single circular DNA molecule, a single cell compartment and anexoskeleton. These studies have shown that the three primary kingdoms areall the same age, and so none can be an ancestor of another. PNAS, 95, pp. These bacteria became chloroplasts and mitochondria(Barbieri, 2 1). A Last Common Ancestor musthave existed because all three kingdoms have the same genetic code, use ATPas an energy base, and approximately 5 percent of bacterial genes havehomologues in eukaryotes. Woese (1998) proposes that there was a Universal Ancestor which gaverise to all life now on earth. (1977). This Universal Ancestor, through mutation and lateral genetransfer, developed into a diverse community of cells which survive as abiological unit today. 6854-6859. The First Common Ancestor had genomes composed almost completely ofRNAs, and in the transition to the Last Common Ancestor, DNA wassubstituted for RNA in the genes, probably using an enzyme similar toinverse transcriptase. It has been proposed that the first forms of life had no nucleus, butgave rise to nucleated cells by engulfing bacteria in a symbioticrelationship. They canexist with or without light, oxygen or organic molecules. Next there wasa dichotomy giving rise to eubacteria, and finally a dichotomy thatresulted in archaebacteria and eukaryotes. They can adapt to environmental changes by changinganatomically, but at the price of extinction of many forms over time. R., & Fox, G. peQuod, Italy: 2 1. Woese, C. The organic codes: The birth of semanticbiology.. Traces of this substitution can still be found: many modernenzymes produce DNA but are still able to function as inversetranscriptase. This means there was also a First Common Ancestor. 5 88-5 9 . It has been proposed that symbiosis of eukaryotes andbacteria required characteristics which bacteria do not possess, and sothere must have been a common ancestor for the two which was not abacteria. The reason may have been that DNA is a more stablemolecule and therefore more suited to heredity, or to separate genes frommessengers. Prokaryotes are found almost exclusively assingle-celled organisms which can inhabit any ecological niche. They synthesizeall their components from inorganic molecules, can rapidly adapt toenvironmental changes, and can exchange genes horizontally. The evidence for this is that the molecules are similar in allorganisms. To determine whichtree was the correct one, molecules of Tu and G elongation factors, andsome ATPase subunits existing before the dichotomies were compared, and theresults showed that there have been two different dichotomies - one thatgave rise to eubacteria and one when the archaebacteria and eukaryotesseparated. There are basically two forms of life on earth: prokaryotes, whichare cells without a nucleus, and eukarotes, which are cells which contain anucleus (Barbieri, 2 1). Woese'scomputations led to four possible phylogenetic trees: the three kingdomsequally old; archaebacteria closer to eubacteria; eubacteria closer toeukaryotes; and archaebacteria closer to eukaryotes. Eukaryotes have DNA molecules arranged in linear fibers, andtheir cytoplasm contains several compartments (mitochondria, chloroplasts,lysosomes, etc.). The latter process is capable of generating endless differentstructures. Eukaryotes are mostly dependent on oxygen, and exists as monocellularorganisms such as protozoa and protista, and have given rise to the threekingdoms of multicellular organisms that exist: plants, fungi and animals(Barbieri, 2 1). They have developed new types of cell division (mitosisand meiosis), new types of movement, meiotic sexuality and embryonicdevelopment. These two intracellular organelles contain fragments ofancient circular DNA, and have 7 s ribosomes, which confirms theirbacterial origins. The common ancestor of these three kingdoms is referred to asthe Last Common Ancestor because it was not necessarily the first lifeform. The evidence presented above shows that all living organisms can betraced to a Last Common Ancestor. He believes that the Universal Ancestorarose when the earth was still very hot, and changed over time as the earthcooled. (1998). Woese, C. The commonality of RNAs over time, thepresence of bacterial DNA in mitochondria and chloroplasts, the sharing ofthe genetic code, the homology between bacterial and eukaryotic genes, andthe use of a common energy source, ATP, all point to the three primarykingdoms sharing a common ancestor. Woese has calculated the distance between species from ribosomalRNAs, which have not changed significantly throughout evolution (Woese,1977). Theaverage life of a eukaryote type is four million years, whereas somebacterial prokaryotes have been estimated to be three billion years old. The universal ancestor. These combined findings suggest that there have been three stages ofcellular evolution. If this paper is not what you are looking for, you can search again: Search for: or Click here to request an essay written just for you.