Here is a term paper on the ‘Evolution of Cell’ for class 9, 10, 11 and 12. Find paragraphs, long and short term papers on the ‘Evolution of Cell’ especially written for school and college students.
Term Paper on Cell Biology
Term Paper Contents:
- Term Paper on the Introduction to Cell Biology
- Term Paper on the Spontaneous Generation of Life
- Term Paper on the Scale of Biological Time
- Term Paper on the Microfossils of Unicellular Organisms
- Term Paper on the Origin of Eukaryotic Cells
- Term Paper on the Organic Soup Theory
- Term Paper on the Pre-Biological Formation of Macromolecules
- Term Paper on the Transformation from Macromolecules to Cells
- Term Paper on the First Cell: An Extreme Heterotroph
- Term Paper on the Evolution of Energy-Generating Mechanisms
- Term Paper on the Origin of the Chloroplast and the Mitochondrion
Term Paper # 1. Introduction to Cell Biology:
Many of the cellular properties that seemed a few decades ago to be so mysterious as to be beyond comprehension are now clearly understood in straightforward chemical terms. A prime example is the structure, replication, and decoding of genes.
As knowledge of the chemical basis of cell function and structure has increased, so has our curiosity about the evolutionary route by which contemporary cells have come to be what they are. The path of cell evolution has been toward greater and greater efficiency and diversity of cellular operations, and for the most part this has been achieved by the evolution of continually greater complexity in cell function and structure. As a result, even the simplest contemporary cells we know about, the mycoplasmal bacteria, are still chemically very complex organisms.
Some reflections of the evolutionary path are observable in contemporary cells. For example, it is likely that the first eukaryotic cell evolved a long time ago from a progenitor cell that was probably not greatly different in structure from some contemporary prokaryotes. The progenitor cell, called a progenote, gave rise to both prokaryotes and eukaryotes.
The progenote has long since disappeared, and the many functional and structural differences that now separate contemporary prokaryotes and eukaryotes attest to a long evolutionary series of intermediate cell types, particularly in the eukaryotic line of descent; little trace of the intermediates has remained. The attempt to understand cell evolution ultimately leads us to the question of the origin of the first cell.
Although we know roughly when in the Earth’s history the first cell probably arose (more than 3.5 × 109 years ago), we do not know and probably never will be able to learn in specific terms how it arose. Still it is possible that we may gain a general understanding of the origin of the cell.
Not many years ago the question of how life originated seemed beyond serious scientific inquiry. The recent great increase in knowledge of the biochemistry and the molecular nature of cell function and structure now permits formulation of specific schemes of how the first cell might have come into existence.
To some extent it is now possible to devise hypotheses about the origin of the cell that can be tested in the laboratory. Particularly important in all of this thinking is the concept of an organic soup, a complex mixture of organic molecules that formed and accumulated during the first one billion years after the Earth was formed, and within which the cell subsequently originated. Laboratory experiments designed to test the organic soup concept. We will first consider briefly some early ideas about the origin of life.
Term Paper # 2. Spontaneous Generation of Life:
The ancient Greek scholars formulated the idea of spontaneous generation of life in which insects and other small animals were thought to arise spontaneously from mud or decaying organic matter. This idea persisted throughout the Middle Ages with descriptions of successful demonstrations of spontaneous generation of worms, flies, eels, frogs, and other organisms from mud and decaying materials. For example, mice were believed to arise spontaneously from cheese wrapped in rags and kept in a dark place.
Frogs were said to form from decaying vegetation in ponds. The appearance of maggots in rotten meat seemed a particularly clear case of spontaneous generation. The overthrow of the doctrine of spontaneous generation began in the late 1600s with the experiments of Francesco Redi, an Italian physician. In his most famous experiment he showed that maggots did not appear in rotting meat when the meat was protected from flies.
On meat exposed to the open air, maggots develop from eggs laid by flies. Redi’s experiments discredited the theory of the spontaneous generation of animals. However, microorganisms were discovered by Antonie van Leeuwenhoek, the father of microscopy, in 1677, about the same time as Redi’s experiments, and the controversy about spontaneous generation shifted from animals to microorganisms.
The debate reached a peak in the 1700s and was led by an Englishman, John Needham, and an Italian, Lazzaro Spallanzani. These two men did similar experiments but obtained different results. A solution of organic matter, for example mutton gravy, was boiled in a glass vessel, and the vessel was sealed. Needham sealed his vessels with corks and consistently observed the growth of microorganisms several days after boiling the solution. He concluded that the microorganisms had originated spontaneously out of the organic matter.
Spallanzani boiled the organic solutions longer and sealed them more carefully. In some experiments he sealed vessels containing organic solutions by melting the neck of the glass vessel and then boiled the contents. The pressure in the closed vessels was increased by the heating, and therefore the boiling point of the enclosed organic solution was raised. This is equivalent to the modern method of sterilization called autoclaving, in which solutions are heated to 120°C using steam to generate a pressure of 15 to 20 lbs per in2.
Spallanzani consistently observed that his organic solutions remained free of microorganisms. He concluded that the microorganisms that appeared in solutions heated and left open came from the air. However, others repeated Spallanzani’s experiments and observed the appearance of microorganisms.
We must conclude that these experimenters were less careful than Spallanzani and had not achieved sterilisation of their solutions. The outcome was, however, that Spallanzani’s experiments did not put an end to the controversy about spontaneous generation of microorganisms. We now know that simple boiling of a solution (Needham’s experiment) is insufficient to kill all forms of microorganisms.
Many kinds of bacteria and fungi develop into spores when conditions are unfavorable for growth. In the form of a spore the cell is dehydrated, metabolically inert, and enclosed by a thick, tough cell wall. When placed in an environment that is favorable for growth, spores germinate into a metabolically active form and resume growth and reproduction. Spores can survive for many years in a dried state.
They are not killed by brief treatment with boiling water but can be destroyed by prolonged boiling or by autoclaving. Bacterial and fungal spores are everywhere in our environment and are a common component of airborne dust. They are a major nuisance to those who work with animal and plant cell cultures, since a single contaminating spore germinates and grows rapidly in the nutrient media required for cell culture.
A further complication is that autoclaving breaks down some of the components in nutrient media required for growth of animal cells. To avoid the deleterious effects of heat, sterilisation is now often achieved by passing media through an extremely fine filter that removes any microorganisms or their spores. The argument about spontaneous generation was settled to the satisfaction of most scientists only by the experiments of Louis Pasteur in the 1860s.
One of his experiments consisted of sterilising nutrient broth by prolonged boiling in a flask with an S-shaped neck. The flask was open to the air, but airborne spores were trapped in the S-shaped neck, and the broth remained sterile. When the neck of the flask was subsequently broken off, allowing airborne dust to enter, microorganisms began to grow in the broth.
Pasteur also showed that microorganisms (presumably in spore form) could be collected by passing air through a filter made of cotton. The disproof of spontaneous generation leaves us with an enigma. If cells can arise only from preexisting cells, where did the first cell on Earth come from?
The experiments by Spallanzani and Pasteur showed that cells could not arise spontaneously from organic matter, at least under the conditions of their experiments. Modern experiments on this problem are designed to test the hypothesis that under certain conditions, particularly those that might have existed on Earth before the appearance of life, a simple primitive cell might have formed spontaneously.
Thus, biologists accept that cells do not now arise spontaneously from organic material, but most believe that given the right conditions and sufficient time (many millions of years), spontaneous generation of a cell did happen at least once, and all contemporary cells have descended from that first cell. In considering the current thinking and experimentation on the origin of life it is necessary to order real and postulated events onto a time scale beginning with the formation of the Earth.
Term Paper # 3. The Scale of Biological Time:
Until the late 1960s the oldest known fossils had been found in sedimentary rocks about 600 million years old. These represented highly evolved invertebrate animals such as trilobites, whose hard shells readily gave rise to fossils. The fossil record indicates that small invertebrates and plants were the only multicellular organisms in existence from about 600 to 500 million years ago.
This interval of 100 million years is called the Cambrian period. The Cambrian period was followed by a succession of periods that are defined by the fossils of progressively more highly evolved plants and animals, extending to the Quaternary period, in which we now live.
The period before the appearance of the first fossilised invertebrates, extending from the origin of the Earth 4.6 billion years ago up to the beginning of the Cambrian period 600 million years ago is called the Precambrian period. Six hundred million years is an underestimate of the time of appearance of the first multicellular organisms.
Multicellular organisms that were structurally simpler, lacking hard shells, and leaving no apparent fossil record undoubtedly preceded the appearance of such highly evolved forms as hard-shelled invertebrates. Recently, worm tracks have been discovered in rocks at least 100 million years older than fossils from hard-shelled animals, showing that soft bodied multicellular organisms were present at least 100 million years before the start of the Cambrian period, or 700 million years B.P. (before present).
We do not know when the first multicellular animals evolved, but it must have been longer than 700 million years ago. Also, however long ago the first multicellular animal evolved, it must have been preceded by an interval in which all organisms were unicellular prokaryotes and eukaryotes.
Furthermore, this interval was most likely preceded by a period in which only progenotes existed. All of this was largely supposition until the late 1960s, when the electron microscope was used in a successful search for microfossils of unicellular organisms in ancient rocks.
Term Paper # 4. Microfossils of Unicellular Organisms:
Many descriptions of microfossils in rocks formed far back in Precambrian times have been published in scientific journals in the last 20 years. The oldest microfossils discovered so far are filamentous and spherical structures that strongly resemble bacteria, including blue- green bacteria.
These microfossils occur abundantly in flint like rocks called chert in South Africa and in rocks of Western Australia, both of which are 3.5 × 109 years old. Cherts formed in Precambrian times also typically contain 0.5 to 1.0 percent organic material, including substances that could be breakdown products of chlorophyll and proteins.
On the basis of these studies it is now generally believed that life existed on Earth at least 3.5 × 109 years B.P., and that photosynthetic organism (blue green bacteria) had already evolved by that time. It is generally believed that the first cells were highly heterotrophic in their nutrition, obtaining energy and nutrients from an abundance of organic molecules in the environment. Evolution of photosynthetic prokaryotic organisms followed sometime later. Thus, we may conclude that the first cells, which were probably prokaryotic-like, arose sometime before 3.5 × 109 years ago, less than 109 years after formation of the Earth.
Term Paper # 5. Origin of Eukaryotic Cells:
The fossil record provides an estimate of when eukaryotes originated. Some of the microfossils found in bitter spring’s chert of Australia contain structures resembling cytoplasmic organelles and nuclei and are believed to represent unicellular eukaryotes. This chert is 900 million years old, and unicellular eukaryotes therefore seem to have been well established by that time.
Fossils of possibly nucleated cells have also been discovered in Beck Spring dolomite in California, which is about 1.3 × 109 years old. The oldest microfossils that might be remnants of eukaryotic cells have been found in rocks 1.4 to 1.6 × 109 years old from the Ural Mountains in the U.S.S.R. These microfossils are similar in form to existing kinds of unicellular green algae.
Shale from Montana about 1.4 × 109 years old has yielded microfossils of filamentous blue-green bacteria as well as much larger spheroidal, thick-walled structures that are believed to be cysts of eukaryotes. From these and similar discoveries it is generally believed that eukaryotes originated at least 1.4 × 109 years ago.
Thus, the origin of pre-prokaryotes (progenotes) preceded the origin of eukaryotes by about 2.1 × 109 years. Many differences separate contemporary prokaryotes and eukaryotes. They both probably evolved from progenotes, and it is virtually certain that not all the differences arose simultaneously.
Modern prokaryotes probably do not differ in structure nearly as much from the progenote ancestor as do modern eukaryotes. The origin of eukaryotes must have occurred with a single change, and this was followed by subsequent evolution of a succession of changes that now make eukaryotes very different from prokaryotes.
We have little idea of which difference represents the first step in divergence of eukaryotes from the progenote. It might have been the acquisition of any one of a number of properties-new kinds of genes, multiple chromosomes, a larger content of DNA, a nuclear envelope, histones, a new principle of gene regulation, and so forth.
The cell and molecular biology of only a small fraction of extant eukaryotes, but it is unlikely that any primitive eukaryotic cells have survived to give us insight into the origin of the eukaryotic cell line. Nevertheless, increased knowledge of the function and structure of contemporary eukaryotic cells may someday provide an indication of the origin and evolution of the eukaryotic cell.
Continued study of microfossils will no doubt result in more accurate estimates of the timing of such events as the origin of the first cells (progenotes), the evolution of the prokaryote and eukaryote lines, and evolution of the first simple multicellular organisms from unicellular eukaryotes.
Once the first cell had formed 3.5 × 109 or more years ago, we can understand, at least in principle, how the subsequent course of events was dictated by the evolution of an ever-increasing complexity in cell functions and structures. We know little about the particular steps, such as the evolution of regulatory genes, the evolution of photosynthesis, the evolution of the first eukaryotic cell, or the evolution of those genetic mechanisms that made possible the first multicellular organisms.
Future research in molecular biology, genetics, and cell biology may yet give us a better idea of these processes of cellular evolution. However, we are faced with a conceptually far more difficult problem than cellular evolution, and that is the matter of how the cell came into existence in the first place. It is impossible to formulate any reasonable scheme by which a cell might have formed directly from the inorganic materials present on the primitive Earth.
The jump from inorganic chemicals to organic molecules capable of self-replication is simply too enormous. A solution to this conceptual dilemma was first proposed by the Russian biochemist, Alexander I. Oparin, and the British biologist J.B.S. Haldane beginning in the 1920s, and is now generally called the organic soup concept.
Term Paper # 6. The Organic Soup Theory:
The great contribution of Oparin and Haldane to the subject of the origin of life was based on the idea that in the period before life arose the atmosphere of Earth contained hydrogen (H2), methane (CH4), ammonia (NH3), and water (H2O), but no free oxygen (O2). Thus, Oparin and Haldane proposed that the pre-life atmosphere of Earth was highly reducing in a chemical sense.
From a variety of evidence, geologists, cosmologists, and chemists now generally agree that primitive atmosphere was chemically reducing in nature. As an example of these lines of evidence, early Precambrian rocks contain ferrous iron, which is unstable in the presence of O2. Therefore, the early Precambrian rocks must have been laid down in the absence of atmospheric O2.
Oparin and Haldane both reasoned that a reducing atmosphere consisting of H2, CH4, NH3, and H2O would be favorable for the spontaneous formation of simple organic molecules and these might then polymerise spontaneously into macromolecules. These macromolecules might then accumulate in the oceans and lakes of the time, giving rise to organic soups.
It is doubtful that the accumulation would have been great because many kinds of organic molecules are unstable in aqueous solutions and are slowly and spontaneously hydrolysed. However, one may suppose that organic molecules may have become concentrated by adsorption to solid surfaces (a common phenomenon) or through the rapid evaporation of lakes.
It was Oparin’s idea that the first cell arose not from inorganic substances but from a mass of pre-biologically formed organic material. Oparin’s idea remained unknown in the West for a long time, probably because it was published in Russian. In 1938 Oparin’s book entitled The Origin of Life was published in English, and immediately his theory attracted wide attention.
Fifteen years later, in 1953, the Oparin-Haldane proposal about the spontaneous formation of organic molecules was tested directly. With the apparatus shown in Figure 1.5, a gaseous mixture of H2, CH4, NH3, and H2O was exposed to electric spark discharges. Water was first added to the flask. The air was pumped out with a vacuum pump and the apparatus was then filled with a mixture of hydrogen, methane, and ammonia.
The electric discharges, which were common in the primitive atmosphere (as lightning) provided, energy for the synthesis of molecules from the four starting components. The water in the flask was boiled to cause circulation through the apparatus and remove any reaction products from the spark zone. Reaction products collected in the condensing water in the condenser and accumulated in the water phase. The experiment was run for one week, and the water then analysed for any organic compounds that might have formed.
A complicated mixture of small amounts of hundreds or even thousands of compounds is theoretically possible and might have reasonably been expected. Instead a small number of compounds accounted for most of the reaction products.
Second, the molecules produced included several of major biological importance, particularly the amino acids glycine, alanine, aspartic acid, and glutamic acid. Fifteen percent of the carbon added (as CH4) to the apparatus was recovered in the compounds identified in Table 1.1. Additional carbon was converted into unidentified, tarlike, high-molecular-weight, organic polymers.
This experiment has been repeated with various modifications many times in other laboratories. As a result of these experiments almost all 20 amino acids, as well as purines, pyrimidine’s, ribose, nucleosides, and nucleotides have been produced a biologically under simulated early- Earth conditions.
In recent years radio-astronomy has provided evidence by microwave spectroscopy that abiological synthesis of large quantities of biologically important molecules occurs commonly in the universe outside the Earth. These molecules include H2, H2O, NH3, H2S, CO, and HCN and the organic molecules cyanoacetylene (C2HN), methanol (CH3OH), ethanol (CH3CH2OH), formaldehyde (CH2O), formic acid (HCOOH), formamide (HCONH2), acetonitrile (CH3CN), and acetaldehyde (CH3CHO).
This list is striking because it includes the very compounds that are the most important for the abiological synthesis of amino acids, purines, pyrimidines, and sugars. For example, formaldehyde, acetaldehyde, and hydrocyanic acid (HCN) react to form glutamic acid. Cyanoacetylene is a precursor of pyrimidines, and in particular can form a large amount of cytosine.
Aldehydes, HCN, and NH3 yield a variety of amino acids. Ribose, glucose, and other sugars are formed spontaneously in an alkaline solution of formaldehyde. Hydrocyanic acid is a precursor of glycine and purines.
A group of meteorites known as carbonaceous chondrites also contain organic molecules. The Murchison meteorite, which fell near Murchison, Australia in 1969, contains two percent carbon, much of which is present as a complex mixture of organic molecules.
Among these are glycine, alanine, valine, proline, glutamic acid, and aspartic acid. Similar findings have been made with the Murray meteorite, which fell in the U.S. in 1950. Pyrimidines and possibly purines are also present in the Murchison meteorite.
The carbonaceous chondrites condensed from the same gaseous mass (the solar nebula) from which the sun and planets condensed. It is likely, therefore, that the organic molecules found in the meteorites were formed in the gaseous nebula.
The Earth was also formed from dust and asteroids that condensed from the same gaseous mass, suggesting that organic molecules may have been present from the very beginning of the Earth’s history. Of course, the presence of organic molecules in the solar nebula raises the possibility that organic molecules are present in other gaseous masses throughout the universe.
In sum, the synthesis of organic molecules under simulated conditions of prolife Earth, the detection in other parts of the universe of substances that spontaneously react to form organic molecules, and the presence of organic molecules in meteorites all add plausibility to Oparin’s idea that the origin of the first cell was preceded by the formation and accumulation of large amounts of organic molecules of major biological importance.
Term Paper # 7. Pre-Biological Formation of Macromolecules:
With pre-biological synthesis of such monomers as amino acids, purines, pyrimidines, nucleotides, and sugars the origin of life no longer seems so inscrutable. However, to go from monomers to a cell, no matter how simple and primitive is still a big leap.
We now know, however, that under the appropriate conditions, a solution of amino acids can polymerise into large polypeptides, and nucleotides can polymerise into nucleic acid molecules.
Thus, it is generally supposed that accumulation of monomers was followed by the pre-biological formation of macromolecules and that the first cell arose by an aggregation of macromolecules. What is required for abiological formation of macromolecules? Remember that the polymerisation of amino acids into polypeptides and of nucleotides into polynucleotides occurs by dehydration-condensation of the monomers.
To form a peptide bond between two amino acids (condensation) a molecule of water must be removed (dehydration). Similarly, water is removed in the formation of the 3′, 5′-phosphodiester bond between two nucleotides. These polymerisations do not occur readily in aqueous solutions because the presence of many water molecules opposes dehydration, driving the reaction toward depolymerisation (hydration of the monomers) rather than polymerisation.
The problem has been solved in the cell by coupling the hydrolysis (bond breakage by addition of water) of ATP to the dehydration- condensation of amino acids or nucleotides into polymers. The dehydration is accomplished by forming an aminoacyl-tRNA at the expense of ATP in the case of peptide bond formation, and by splitting PPi from nucleoside triphosphate in the case of nucleic acid synthesis.
The overall process, however, is a coupling of water removal from monomers to water addition (hydration) to ATP (to form ADP + Pi + H+). It is highly unlikely that ATP was present in a sufficient amount in the pre-biological environment to drive polymerisation reactions.
However, polyphosphates (the simplest polyphosphate is pyrophosphate) can bring about polymerisation of amino acids by a dehydration reaction in the same manner as does ATP and polyphosphates could readily have formed under pre-life conditions. Polyphosphates can form when orthophosphate (PO43-) is warmed in the presence of urea and NH4+ (urea is a product of the organic soup experiment).
Polyphosphates have been used experimentally to promote the synthesis of AMP from adenine, ribose, and phosphate, and to drive the formation of polynucleotides. Other compounds might also have served as dehydration-condensation agents.
These compounds include carbodiimide, cyanate, cyanogen, and cyanovinyl phosphate, all of which might readily have formed under pre-life conditions. Monomers can also be caused to polymerise by adsorption on the surface of certain minerals such as clay and apatite compounds. Adsorption of amino acids to one common clay known as montmorillonite is followed by condensation of the amino acids into long polypeptide chains.
Another way to bring about polypeptide formation is to heat a mixture of dry amino acids to 130 to 180°C for a few hours. In the absence of water the necessary dehydration reaction is strongly favored. Large complex polypeptides formed by this method aggregate into microspheres when mixed with water. Protein-like molecules form when aminoacyl adenylates are mixed.
These aminoacyl adenylates are the activated form of amino acids used in cellular protein synthesis. Aminoacyl adenylates readily form abiologically when AMP and amino acids are mixed in the presence of dehydration-condensation agents. Not only do the abiologically formed protein-like polymers form stable microspheres but, remarkably, such polymers possess low levels of catalytic power, for example, in the decarboxylation of pyruvate to acetaldehyde and CO2.
Thus, at least some of the protein-like polymers qualify as primitive enzymes.
Term Paper # 8. Transformation from Macromolecules to Cells:
The strong probability that polypeptides and polynucleotides formed under pre-life conditions further reduces the difficulties in conceptualizing how life originated. Yet we are still faced with formidable problems in trying to understand the origin of the cell.
Abiologically formed macromolecules may aggregate into cell-like structures, such as the proteinoid microspheres in Figure 1.7, and these may grow and subdivide. What additional properties must an aggregate of macromolecules possess to be called a cell? The answer to this is not as easy as it might seem initially.
It seems essential that a genetic mechanism be present that allows the primitive cell to duplicate itself precisely. It is difficult to imagine how this could be achieved without a template-copying mechanism of the type present in nucleic acid replication. We may also add the requirement that the macromolecular components must have functions, however crude, that are useful for survival and reproduction. Enhancement by enzymatic catalysis of polymerisation of monomers into more macromolecules would be one such function.
These functions must be inherited through cell reproduction. The presence of a genetic inheritance carries with it the potential for chance improvement of macromolecular functions through mutation. Once genetically inherited functions have been acquired, the way is open for cellular evolution.
Suppose that the first cell had to contain nucleic acids because only nucleic acids are known to replicate them with a reasonable amount of precision and therefore constitute a mechanism of genetic inheritance.
The formation of nucleotides under simulated pre-life conditions has already been accomplished in the laboratory. In addition, these nucleotides can be caused to polymerise into short polynucleotides when a dehydration-condensation agent is added.
With an appropriate polymerase obtained from cells, a single-stranded DNA or RNA chain acts as a template for the synthesis of a complementary chain from nucleoside triphosphates. Essentially the same can be accomplished without a polymerising enzyme, that is, under pre-life conditions.
For example, oligomeric chains of thymidylate residues can be generated under pre-life conditions. In turn, oligomers composed of six thymidylate residues (hexamers) have been found to join together slowly to form longer polynucleotides of thymidylate residues in the presence of a dehydration-condensation agent.
If polyadenylate is added to the mixture, the rate of joining of the thymidylic acid hexamers is greatly speeded up; apparently the polyadenylic acid acts as a template that binds the hexamers of thymidylate residues by base-pairing and thereby aligns the hexamers so that end-to-end joining takes place much more rapidly. In bringing together hexamers of thymidylate residues, and by promoting their end-to-end ligation, polyadenylate is actually functioning as a catalyst.
Similarly, hexamers of polyadenylate residues may be coupled together into longer polynucleotides on a template of polyuridylate residues. Polycytidylate chains act as templates for polymerisation of GTP into chains up to 40 nucleotides long.
These experiments give evidence that polynucleotides might well be able to replicate themselves in the familiar template fashion without catalysis by protein enzymes. Therefore, it seems possible that cellular reproduction was from the beginning based on polynucleotide replication. We know of no way in which proteins can function as templates to guide their own reproduction.
Not long ago, nucleic acids were thought to lack the kind of catalytic capabilities necessary to give the first cell its first functions. Thus it seemed that proteins (primitive enzymes) must have been present early in primitive cells.
The supposition that proteins had to be present to provide enzymatic activity in primitive cells has changed dramatically as a result of recent studies on excision of introns from primary RNA transcripts in eukaryotes. The precursor ribosomal RNA transcript in Tetrahymena undergoes catalytic self-splicing in which the RNA intron acts as an enzyme to remove itself from the transcript by cutting and splicing together of RNA ends.
Moreover, the intron RNA can act as a ribonuclease to remove nucleotides from RNA, and astonishingly the intron RNA can also work as a polymerase, catalysing the formation of polycytidylate chains on a poly (G) template that contains. Segments of RNA such as the intron of precursor rRNA in Tetrahymena has been given the name ribozymes. These discoveries favor strongly the idea that the first cells used RNA molecules both to hold genetic information and as enzymes to catalyse RNA metabolism.
The subsequent switch from RNA to a DNA double helix as the genetic material is not a conceptually difficult problem. However, how self-catalysed RNA replication evolved into the current genetic mechanisms in which information in nucleotide sequences is translated into amino acid sequences via tRNA adaptors is much harder to envisage.
Term Paper # 9. The First Cell: An Extreme Heterotroph:
We cannot reasonably suppose that the first cell was capable of more than a few enzymatic activities. Hence, it must have depended on abiologically formed organic molecules in its environment for growth and reproduction. With the evolution of anabolic capabilities, stringent dependence on heterotrophic existence presumably decreased; indeed, depletion of essential components (e.g., amino acids and nucleotides) in the environment necessitated the evolution of anabolic activities.
Eventually, the exogenous supply of nutrients such as nucleotides and amino acids would begin to be depleted. Any primitive cell that acquired through chance the ability to catalyse the formation of one or another nucleotide or amino acid from some closely related molecule still available in the environment would have an advantage over its contemporaries. Hence early cellular evolution surely moved in the direction of autotrophy. It seems unnecessary to postulate that in its earliest form the cell possessed an enveloping membrane.
However, as soon as anabolic and energy-generating capabilities began to be acquired, a membrane became important for retention of valuable metabolic products. Hence, we may suppose that a plasma membrane of some sort appeared early in the evolution of metabolism.
Term Paper # 10. Evolution of Energy-Generating Mechanisms:
The acquisition of anabolic capabilities carries with it a requirement for energy to drive synthesis of molecules. The primitive environment may have contained some energy-rich substances (e.g., polyphosphates), but these would probably have been quickly used up. Therefore, the ability of the cell to generate its own energy supply probably occurred early in cell evolution.
For several reasons, it seems likely that the first energy generating mechanism was a primitive form of glycolysis. All contemporary cells possess glycolysis, which indicates that it evolved before prokaryotic and eukaryotic cells diverged from a common ancestor. Glycolysis is also mechanistically simpler than photosynthesis and presumably preceded photosynthesis in evolution. Since the early atmosphere of Earth lacked oxygen, respiration was impossible.
In fact, it is generally believed that the oxygen now present in the atmosphere is the result of photosynthesis. Hence, respiration probably evolved after photosynthesis. The order of evolution of energy-generating mechanisms is therefore believed to have proceeded from glycolysis to photosynthesis to respiration.
Term Paper # 11. Origin of the Chloroplast and the Mitochondrion:
The origin of these organelles represents one aspect of eukaryotic cell evolution about which we have gained some understanding. The mitochondrion is believed to have arisen through a symbiotic association between an early eukaryotic cell and a bacterium. According to this hypothesis a bacterium was engulfed by the cytoplasm of a eukaryotic cell, where it became a permanent resident, growing and reproducing with little if any detriment to its host.
Such arrangements are not uncommon among contemporary fungi and protozoa. Amoeba proteus, for example, harbors a bacterium that grows and multiplies in the cytoplasm of the amoeba. A single amoeba contains thousands of these bacteria. The bacterium has become adapted to the environment provided by the amoeba cytoplasm to the extent that it can no longer grow and divide outside the amoeba.
Whether the bacterium contributes anything to the well-being of the amoeba (a symbiotic relationship) or whether it is strictly a parasite is not known. The arrangement now present in A. proteus may resemble an early stage in mitochondrial evolution. The relationship between the early eukaryotic cell and its acquired bacterium evolved into a symbiotic one in which the bacterium became specialised for the respiratory synthesis of ATP, which it supplied to its host.
The eukaryotic host, in turn, evolved in the direction of providing more and more of the proteins necessary for the structure and function of the symbiont, and this was accompanied by a corresponding loss of genes (DNA) in the symbiont. Assuming that the host cell was originally aerobic, it lost its own enzymatic machinery for respiration since all respiration is carried out in mitochondria in contemporary cells.
The genetic and biochemical interrelationships between mitochondria and the rest of the cell are intricate and complex. Chloroplasts are also likely to have evolved from blue-green bacteria acquired by an early eukaryotic cell.
According to the hypothesis about the symbiotic origin of the mitochondrion and the chloroplast, the DNA in these organelles represents all that remains of the original chromosome of an aerobic bacterium or blue-green bacterium. The strength of the symbiosis hypothesis lies in basic similarities between the organelles and free-living bacteria.
First, the DNA of mitochondria and chloroplasts does not have histones associated with it, and neither does the DNA of bacteria or blue-green bacteria. The ribosomes of mitochondria and chloroplasts correspond in size to prokaryotic ribosomes rather than eukaryotic ribosomes.
The rRNAs of organellar ribosomes, which are coded for by organellar DNA, are about the same size as prokaryotic rRNAs and hence are smaller than eukaryotic rRNAs. Moreover, organellar rRNAs have nucleotide sequences that are present in prokaryotic rRNAs but not in eukaryotic rRNAs.
Finally, the drug chloramphenicol inhibits protein synthesis in bacteria and in mitochondria, but has no effect on protein synthesis in the cytoplasm of eukaryotic cells. Thus, mitochondrial and bacterial protein synthesis shares a basic property that is not present in eukaryotic protein synthesis. There is a variety of other, corroborating observations involving similarities in other organellar and bacterial properties, but the above are the main ones.