Features of Evaluation of the Cell - Coursework Example

EVOLUTION OF THE CELL As we all know cell is the structural and functional unit of each and every known living organism. In fact it is the smallest unit of an organism that is classified as living, and is sometimes called the building block of life (Alberts, 2002). There are different organisms with different types and number of cells. For instance, organisms such as bacteria are unicellular organisms. Multicellular organisms include human beings, animals, plants etc. basically all organisms can be classified into two groups eukaryotic and prokaryotic. Prokaryotic cells or organisms containing this type of cells are usually independent as every thing required for the survival, movement, and other aspect is present in just that single cell. On the other hand, eukaryotic cells are usually found in multicellular organisms and each set of cells is designed to perform specific function. When it comes to the evolution of cells, it is important to look at the origin of cells which has to do with the origin of life, and is one of the most significant steps in the theory of evolution. The birth of the cell can be considered as the transformation from pre-biotic chemistry to biological life. Later the single celled prokaryotes gave rise to the multicellular eukaryotic organisms. According to the endosymbiotic theory it is certain that DNA-bearing organelles such as the mitochondria and the chloroplasts are what remain of ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, where the rest of the cell seems to be derived from an ancestral archaean prokaryote cell (Figure1). There are also other theories that help us understand the evolution of cell. For instance, sex, as the stereotyped sequence of steps of meiosis and syngamy that persists in almost all extant eukaryotes, may have played a role in the transition from prokaryotes to eukaryotes. According to origin of sex as vaccination theory the eukaryote genome accreted from prokaryan parasite genomes in numerous rounds of lateral gene transfer. Sex-as-syngamy basically came up when infected hosts began transaction nuclearised genomes containing coevolved, vertically transmitted symbionts that conveyed protection against horizontal infection by more virulent symbionts (Sterrer, 2002). In fact the evolution of modern cells is perhaps the most challenging and important problem for the biologists (Brenner, 1998). Scientific curiosity in cellular evolution accelerated when the universal phylogenetic tree was determined (Zillig, et al., 1989). However only after the emergence of microbial genomics that biologists could really do much about the problem of cellular evolution. Initially the scientific world attempted to solve the issue using the Darwinian mode, and the main focus till now has been almost entirely on modelling the evolution of the eukaryotic cell (Figure 2). This is mainly because of the appeal of the endosymbiosis concept that has given rise to the chloroplast and mitochondrion. The scientific world has long played with an endosymbiotic origin for the eukaryotic nucleus, and even for the entire eukaryotic cell (Hartman and Fedorov, 2001). Endosymbiotic origin explanations are based on three characteristics: they (1) invoke cells that are essentially fully evolved; (2) develop the vital eukaryotic cell well after its archaeal and bacterial counterparts (or the prokaryotic part); and (3) center attention on eukaryotic cellular evolution, which implies that the evolutions of the “prokaryotic” cell types, the archaeal and bacterial, are of a different character—simpler, and, it would seem, less interesting. It can be said without any doubt that the universal phylogenetic tree brought classical evolution to end. Darwin said that “The time will come … when we shall have very fairly true genealogical trees of each great kingdom of nature” (Burkhardt and Smith, 1990). Only a century later the universal phylogenetic tree based on molecular (rRNA) sequence comparisons did exactly that and also went beyond to the final step to amalgamate all of the “great kingdoms” into one single “empire” (Woese, 1987). The main question posed by the universal tree is the nature of the entity represented by its root, the source of all existing life. There is enough support to hint that the basic organization of the cell is still not completed its evolution at the stage represented by the root of the universal tree. The basis for the evidence comes from the three main cellular information-processing systems. Translation was vastly developed by that stage: rRNAs, tRNAs, and the (large) elongation factors. Additionally almost all of the tRNA charging systems was in modern form (Woese et al., 2000). However, while it is evident that the majority of ribosomal proteins are universal in distribution, a few of them are not. A comparatively small cadre is specific to the bacteria, a fairly larger set general and restricted to the archaea and eukaryotes, and a few others are exceptionally eukaryotic. Most of the universal translational proteins and also those in transcription demonstrate the canonical pattern, i.e., the bacterial and archaeal versions of the protein are extremely different from one another that their difference is distinguished as one of “genre”. Except for the aminoacyl-tRNA synthetases the corresponding eukaryotic versions are practically the entire archaeal genre (Woese et al., 2000). However, still now why the canonical pattern exists is a major question that remains unanswered (Woese, 1987). In general, it can be said that transcription seems to have been rather less developed at the root of the universal tree. The two biggest subunits of the DNA-dependent RNA polymerase, β and β′ in bacterial nomenclature, are universal in distribution. But the remaining bacterial subunit (α) is only to some extent so. Bacterial α exists in two copies in the bacterial polymerase. Its archaeal or eukaryotic counterpart includes two individual proteins, both present in single copy in the enzyme and a part of each showing homology to portions of bacterial α and vice versa (Langer, et. al., 1995). It is assumed that a structural difference of this scale have to represent in any case some functional difference also. Studies have found that the archaeal transcription apparatus also contains other subunits, none of which are found in bacteria but all of which occur in eukaryotes (Langer, et. al., 1995). Additionally it is found that bacterial transcription initiation does not look like its archaeal or eukaryotic counterpart. Even though a universal transcription function seems to have existed by the end of the universal ancestor period, that mechanism seems undeveloped. Scientists have found that a modern type of genome reproduction mechanism did not exist at the root of the universal tree. Practically no homology exists between the bacterial genome replication mechanism and that are fundamentally common to the archaea and the eukaryotes. Therefore it can be said that modern genome replication mechanisms seem to have evolved twice (Olsen and Woese, 1996). These fundamental dissimilarities in the genetic machinery comprise a prima facie case to the effect that the era of cellular evolution sustained well into the evolutionary period encompassed by the universal phylogenetic tree. Additionally it was also seen that the order of maturation of the information processing systems was first translation, then transcription, and finally modern genome structure and replication (Olsen and Woese, 1996). Prokaryotic and Eukaryotic Cells It is a well known fact that in biology, when we use the word root, it refers to the nucleus of a cell. Additionally "pro" means "before," and "eu" means "true," or "good." Therefore, "prokaryotic" basically means "before a nucleus," and "eukaryotic" means "possessing a true nucleus." Therefore, the basic words "prokaryotic" and "eukaryotic" generally hint about one of the differences between these two cell types i.e. prokaryotic cells have no nuclei, while eukaryotic cells do have true nuclei. Before we look into the major differences it is important to note that despite their apparent differences, these two cell types have a lot in common. Both prokaryotic and eukaryotic cell perform most of the same kinds of functions, and in the same ways. Both are enclosed by plasma membranes, filled with cytoplasm, and loaded with small structures called ribosomes. Additionally both have DNA which carries the archived instructions for all the functions that need to be carried out by the cell. Physiologically they are very similar in a lot of ways. For example, the DNA in the two cell types is exactly the same kind of DNA, and the genetic code for a prokaryotic cell is precisely the same genetic code used in eukaryotic cells (Hurlbert, 1999). Beyond these similarities, there are also major differences. The following are a few differences when we take a closer look: a) The typical size of a prokaryotic cell is 0.2-2.0 mm in diameter, whereas a Eukaryotic Cell is 10-100 mm in diameter. b) A typical prokaryotic flagella has Consist of two protein building blocks whereas, Eukaryotic Cell has complex multiple microtubules. c) The first and the foremost difference is that the eukaryotic cells have a true nucleus which is bound by a double membrane. Prokaryotic cells have no nucleus. The function of the nucleus is to sequester the DNA-related functions of the large eukaryotic cell into a smaller chamber, for the purpose of better efficiency. Since this function is not necessary for the prokaryotic cell, they do not have nucleus. This is because of its much smaller size and all materials within the cell are comparatively close together. It should not be forgotten that prokaryotic cells do have DNA and DNA functions. d) If we look at the shape of both, it can be said that eukaryotic DNA is linear whereas prokaryotic DNA is circular or in other words they have no ends. e) If we take a closer look at the eukaryotic DNA it can be seen that it is having proteins called "histones" and is organized into chromosomes. On the other hand prokaryotic DNA is "naked," or has no histones linked with it, and is not formed into chromosomes. In fact the term "chromosome" does not strictly apply to anything in a prokaryotic cell. Therefore, a eukaryotic cell includes a number of chromosomes whereas a prokaryotic cell contains only one circular DNA molecule together with diverse collection of much smaller circlets of DNA called "plasmids." In other words it can be said that the smaller, simpler prokaryotic cell requires far fewer genes to operate and undergo a simple steps of duplication than the eukaryotic cell that undergo complex processes. f) Though both cell types have several ribosomes, the ribosomes of the eukaryotic cells are larger in size and are more complex when compared to the prokaryotic cell. In general ribosomes are structures in the cell made out of a special class of RNA molecules (ribosomal RNA, or rRNA) and a particular set of different proteins. If we take a closer look at the eukaryotic ribosome, it is composed of five kinds of rRNA and about eighty kinds of proteins where as a prokaryotic ribosomes are composed of only three kinds of rRNA and about fifty kinds of protein. g) The cytoplasm or the inner fluid of eukaryotic cells is composed of a large, complex collection of organelles and many of them enclosed in their own membranes. Examples are mitocondria, endoplasmic reticulum, etc. The prokaryotic cell contains no membrane-bound organelles which are independent of the plasma membrane. This is a very significant difference, and the source of the vast majority of the greater complexity of the eukaryotic cell. Cell organelles In a typical eukaryotic cell, mitochondrion is present. It is also called the power house of the cell as it is the source of energy for the cell. Additionally in plant cells, additional family of organelles called plastids are also present, and an example is the renowned chloroplast. In fact the mitochondria and chloroplasts more or less have a similar evolutionary origin. Both mitochondria and chloroplasts are clearly the descendants of independent prokaryotic cells, which have taken up permanent residence within other cells through a distinguished and common phenomenon called endosymbiosis. A special structure not found in a prokaryotic cell is called a mesosome. Not all prokaryotic cells have these. The mesosome is an elaboration of the plasma membrane which is a sort of decoration of ruffled membrane intruding into the cell. Structurally the mitochondrion is a double-membrane organelle, with a smooth outer membrane and an inner membrane which protrudes into the interior of the mitochondrion in folds called cristae. This membrane is very similar in appearance to the prokaryotic plasma membrane with its mesosomes. However, the similarities are a lot more significant than its manifestation. Both the mesosomes and the cristae are used for the same function. Particularly the aerobic part of aerobic cellular respiration is carried out by these organelles. Cellular respiration is an important process by which a cell converts the raw, potential energy of food into biologically useful energy, and there are two general types, anaerobic and aerobic. In realistic terms, the big distinction between anaerobic and aerobic respiration is that aerobic cellular respiration has a much higher energy yield than anaerobic respiration. Aerobic respiration is undoubtedly the evolutionary offspring of anaerobic respiration. In fact, aerobic respiration really is anaerobic respiration with additional chemical sequences added on to the end of the process to allow utilization of oxygen. Thus explaining its evolutionary pattern. So it is reasonable of scientists to think that a mitochondrion evolved from a once-independent aerobic prokaryotic cell which entered into an endosymbiotic relationship with a larger, anaerobic cell (College of DuPage, 2004). In conclusion, it can be said that prokaryotic and eukaryotic cells are more alike than different and all the present day scientific evidence strongly points out that all cells are related through evolution. (Source: www.uic.edu) (Source: http://www.fluwikie.com) References Alberts, B. (2002) Chapter 21: Cell Movements and the Shaping of the Vertebrate Body, In: Molecular Biology of the Cell, fourth edition, published by Garland Science. Brenner S. Science. 1998;282:1411–1412. Burkhardt, F.; Smith, S. (1990) The Correspondence of Charles Darwin. Vol. 6. Cambridge, U.K.: Cambridge Univ. Press, pp. 1856–1857. College of DuPage, (2004) Prokaryotic and Eukaryotic Cells, Retrieved on 6 November 2007 from http://www.cod.edu/people/faculty/fancher/ProkEuk.htm Hartman H, and Fedorov A. (2001) Proc Natl Acad Sci USA.;99:1420–1425. Hurlbert, R E. (1999) Chapter II: Eukaryotic vs. Prokaryotic cells, In: Microbiology 101/102 internet text, Retrieved on 6 November 2007 from http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html Langer D, Hain J, Thuriaux P, Zillig W. (1995) Proc Natl Acad Sci USA;92:pp. 5768–5772. Olsen G J, Woese C R. (1996) Trends Genet.;12:pp. 377–379. Sterrer W. (2002). On the origin of sex as vaccination. Journal of Theoretical Biology 216: 387-396. Woese C R. (1987) Microbiol Rev.;51: pp. 221–271. Woese C R, Olsen G J, Ibba M, Soll D. (2000) Microbiol Mol Biol Rev.;64:pp. 202–236. Zillig W, Klenk H P, Palm P, Leffers H, Puhler G, Grupp F and Garrett R. (1989) Endocytol Cell Res.;6:1–25. Read More