Transition from Five Kingdoms to Three Domains - Essay Example

Name Course Lecturer Date Transition from the five kingdoms to the three domains Introduction During the pioneering stages of classification, all organisms were classified under the broad categories of plants and animals. However, this approach became rather contentious and was beset by many problems. It excluded prokaryotes, photosynthetic protists and fungi which are also living organisms that exist on the face of the earth. Consequently, the five kingdom system was proposed by Robert H. Whittaker and soon implemented in 1969 (Gupta 112). Under this system, prokaryotes were placed in the kingdom Monera while the other four kingdoms comprised of eukaryotes; kingdoms Plantae, Fungi, Animalia and Protista. Animalia consists of eukaryotes that are multicellular and consist of complex systems in terms of structure, nutrition and development. Plantae on the other hand consist of cells that can make their own food through photosynthesis. Fungi are organisms that live on decomposing matter of other organisms and other organic molecules. Kingdom protista is composed of all other eukaryotes that cannot be defined as plants, animals or fungi. Most protists are unicellular although simple multicellular organisms believed to be descendants of unicellular protists are classified in this kingdom. Figure 1 Five kingdom system of classification (https://highered.mcgraw-hill.com/sites/dl/free/0072986751/391131/Chapter_27.pdf) Over the past 20 years, research has revealed vital information that has called into questioning of the five kingdom system. Scientists argue that molecular rRNA changes slowly through evolution hence changing evolutionary events. As a result, molecular data has led to reorganization of the five kingdoms into three domains: Bacteria, Archaea and Eukarya. Archaea and bacteria are basically unicellular and their nucleus is not membrane bound. They are distinguished on the basis of cell wall and lipid biochemistry. Gupta (129) asserts that archaea possess biochemical properties that enable them to live in hostile environments such as salty water bodies, anaerobic swamps and acidic environments. Cellular and molecular data also indicates that eukarya and archaea are related more closely than they are related to bacteria. It shows that eukarya and archaea do share a more recent ancestor than with bacteria. This is clearly reflected in the Figure 2 below. Figure 2 Three domain system of classification (https://highered.mcgraw-hill.com/sites/dl/free/0072986751/391131/Chapter_27.pdf) Figures 1 and 2 show a clear relationship between the five kingdom and the three domains; all eukaryotic kingdoms (animalia, protista, plantae and fungi) are included in the domain Eukarya while the kingdom monera was split into domains bacteria and archaea. Transition from the five kingdoms to the three domains According to Reynaud and Devos (3324) the rationale behind the transition from the five kingdom system to the three domains came from the increasing mass of evidence in particular rRNA phylogenies. The evidence suggested that archaebacteria should be accorded an equal taxonomic status as eukaryotes and bacteria. The tripartite scheme generally suggests that the new subdivisions occurred among prokaryotes, which are archaea and bacteria, while the eukaryotes remained the same. The members of bacteria and archaea are united in the prokaryotic realm by the common features such as general cell size, lack of organelles and nuclear membrane, presence of circular and large chromosomes that contain smaller circular DNA plasmids. Close similarities between gyrases and topoisomerases of bacteria and archaea depicts comparable chromosome structures in the two groups. Genes in many of the archaeal species appear to be arranged into bacteria-like operons. Furthermore, many gene clusters and archaeal operons are organized in a similar manner as those of bacteria. For example chloroplast and bacterial ribosomal operons are arranged in the order 16S-23S-5S which resembles that of archaea rRNA. In addition, FtsZ, a cell division protein found in bacteria has also been discovered in many archaeal species. Several archaea members possess type II restriction enzyme systems which also occur in bacteria. Bacterial and archaeal protein-coding genes lack spliceosomal introns which are typically found in eukaryotic cells. These similarities in archaea and bacteria probably explain why they were for, many years, classed under the same kingdom, kingdom monera Reynaud and Devos 3317). Figure 3 Structure of a bacterium (http://www.google.co.ke/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/Average_prokaryote_cell-_en.svg/320px-Average_prokaryote_cell-_en.svg.png&imgref) The figure shows basic features of a prokaryotic cell which is are also present in archaea. It is important to take note of some biochemical and cellular features of archaea that has earned them an evolutionary relationship with bacteria and eukaryotes. These features are either unique to archaea or unique combinations previously thought to be exclusive to bacteria or eukaryotes. Some unique archaeal characteristics include absence of fatty acid synthetase and acyl ester lipids,they contain isopranyl ether lipids, modified tRNA molecules, a range of antibiotic sensitivities and splitting of the RNA polymerase subunits. Lipids contained in archaea differ from those of eukaryotes and bacteria in four main ways. First, archaea contain ether linkages between the hydrocarbon chains and glycerol whereas eukaryotes and bacteria have ester linkages. Second, archaeal hydrocarbon chains contain isopranyl cahins that are highly methylated while the hydrocarbon chains in eukaryotes and bacteria are mainly straight chain fatty acyl chains. Third, the glycerol ethers in archaea contain 2,3-sn-glycerol different form the other two domains which contain 1,2-sn-glycerol. Fourth, some archaeal lipids are tetraethers which do not have comparable ester lipids. In addition, the biosynthetic enzymes for archaeal lipid are also quite different. Metabolic regimes in archaea also differ greatly from those of bacteria and eukaryotes. For instance, both pyrophosphate-dependent and ATP-dependent phosphofructokinases can occur in eukaryotes and bacteria while archaea can use either pyrophosphate-linked or ADP-dependent kinases. These differences provide reasons why there was necessity of recognizing archaea as a separate domain from bacteria. Figure 4 Basic structures of archaea and bacterial membranes. (http://www.google.co.ke/imgres?imgurl=http://www.nature.com/nrmicro/journal/v5/n4/images/nrmicro1619-f3.jpg&imgrefurl=http://www.nature.com/nrmicro/journal/v5/n4/fig_tab/nrmicro16). The diagram shows the main differences in the lipid and glycerol structures of archaea and bacteria. Bacteria contain the ester linkages and the 1,2-sn-glycerol while archaea contain the ether linkages and 2,3-sn-glycerol. Nester (587) asserts that tne of the most vital cellular processes in the regulatory steps of a cell is protein degradation. This process occurs in all kinds of cells but is significantly different depending on the type of cell. In eukaryotes the protein complex known as 26S proteasome is responsible for ATP-dependent proteolysis. This protein has a complex structure consisting of 20S core subunits and 34 polypeptides. On the other hand, the bacterial sequence consists of 20S proteasome β-type subunit. Although the bacteria and archaea domains show many similarities in their genomic organization, many genes of the archaea also show a great deal of similarities with eukaryotic homologs. This evidence was revealed by earlier studies with antibiotics. Bacteria exhibit sensitivity towards antibiotics, an inhibitor of 70S ribosomes. Eukaryotes and bacteria are both refractory towards streptomycin by they react towards certain anti-80S ribosome directed inhibitors for instance anisomycin. These domains also share sensitivity towards aphidicolin, a DNA polymerase inhibitor. There are other significant similarities between eukaryotes and archaea with regard to DNA transcription, translational and replication components. Eukaryotic and archaeal DNA polymerase are homologous and do not resemble any of the bacteria DNA polymerase with the exception of E. coli. RNAPs of archaea are evolutionarily closer to those of eukaryotes. Bacterial RNAPs are simpler in structure and are composed of four subunits while those of archaea and eukaryotes have a minimum of seven subunits. Eukaryotes and archaea share other trsnscription features that are apparently absent in bacteria such as certain transcription factors and TATA box-like binding sites. tRNA genes introns of the eukaryotes and archaea are similar in size and are located only at certain locations of the cell (Nester 616). Conclusion Although there is some opposition against the three domains systems of classifying organisms it largely accepted by many people and in actual fact it is the current paradigm in classification. This approach, which has subsequently replaced the five kingdom system, has enabled inclusion of majority of the organisms that were previously left out and incorrectly recognized. Although there is much similarity between archaea and bacteria, research has revealed a wide range of differences between the two domains in term of structural features as well as genomic composition thus qualifying them as two distinct domains. Eukaryotes on the other hand are quite distinct from bacteria and archaea although some few molecular properties are shared with the rest of the domains. Work cited; Gupta, Radhey. The natural evolutrionary relationships among prokaryotes. Critical review in microbiology 26.2 (2000): 111-131. Reynaud, Emmanuel and Devos, Damien. Transitional forms between the three domains of life and evolutionary implications. Prokaryots biological sciences 278. 1723 (2011): 3321-3328. Nester, Eugene. Microbilogy: a human perspective. Dubuque. Lowa: McGraw-Hill, 2001. Read More