Why Both Prokaryotes And Eukaryotes Would Be Considered Successful Pdf
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Motivation: The folding of many proteins in vivo and in vitro is assisted by molecular chaperones.
Evolutionary Genomics pp Cite as. Organisms display astonishing levels of cell and molecular diversity, including genome size, shape, and architecture. In this chapter, we review how the genome can be viewed as both a structural and an informational unit of biological diversity and explicitly define our intended meaning of genetic information. A brief overview of the characteristic features of bacterial, archaeal, and eukaryotic cell types and viruses sets the stage for a review of the differences in organization, size, and packaging strategies of their genomes.
Energetics and genetics across the prokaryote-eukaryote divide
It differs markedly from the rich variety of cellular forms, sizes and structures found in eukaryotes. The prokaryotic cell is small, its volume and mass being approximately one thousand times less than those of an average eukaryotic cell Fig. However, it must not be considered merely a diminutive form of the eukaryotic cell but rather as strikingly different and remarkably adjusted to its peculiar and original way of life.
Small size has major consequences and it dictates some important biological properties. For instance, the surface to volume ratio is much higher about times in prokaryotic cells and this favors more rapid exchanges with the surrounding milieu. In the prokaryotic cell the cytoplasmic contents are more concentrated and in close and free contact with each other. Usually there are no internal compartments as we know them in the eukaryotic cell. High concentrations of ions and molecules combined to free movement and contact generally translate into surprisingly accelerated metabolism, rapid growth and multiplication.
A prokaryotic cell reproducing at the rate of two divisions per hour, which is by no means exceptional, will give a progeny exceeding one billion cells in less than two days of sustained exponential growth. Comparative size and shape of eukaryotic and prokaryotic cells. Also, if the hereditary patrimony of one of these cells is modified by a mutation or a genetic exchange and the new trait happens to be favorable, it will be propagated in a billion copies within a short period of time.
Once acquired, a new trait can be spread very rapidly within a bacterial community and, later, in other niches of the biosphere. It is generally contained by the outer membrane the cell wall which is rigid and resistant. As volume increases, so does the total internal pressure which eventually would physically destroy the cell lysis.
In part because of these physical constrains, during their evolution, the much larger eukaryotic cells have become compartmentalized. Giant prokaryotic cells are rare exceptions but they are now known to exist Prescott et al. Epulopiscium spp. It will be interesting to discover how this giant bacterium has solved the problems that are likely to be created by a large volume of cytosol to be contained by external envelopes only.
For example, one of the fundamental distinguishing characteristics of an eukaryotic cell is the organization of its DNA in separate chromosomes contained in a membrane-enclosed bag, the nucleus. This feature does not exist in prokaryotic cells Fig. However, several different prokaryotic cells contain intracellular membranes or sac-like membrane components that can be more or less numerous or complicated in arrangement.
Their functions vary from one kind of bacterium to another. For example, the cyanobacteria have thylakoids containing chlorophyll and capable of photosynthesis.
All nitrogen-fixers have lamellae. For a discussion of inclusions and intracellular membranes, the reader is referred to the many excellent papers on the subject, for example, Jensen In the present chapter, we will discuss, in very general terms, the main structure of the prokaryotic cell, some of the molecular but mostly the structural aspects that differentiate the prokaryotic cellular body from its eukaryotic counterpart.
We will hereafter refer to it as the large replicon. Also present are from one to seventeen Fox, much smaller replicons circular DNA molecules which the cell harbours on a temporary basis Fig. Their copies are able to move from one strain to another Fig.
These small replicons act as visiting genetic information molecules, easily exchangeable groups of genes, playing a major role in the solidarity of prokaryotes through a global communication System Sonea, Many of them also possess one or several flagellar structures that enable them to move. Bacterial flagella often move by rotation imparted from a basal body, a motor-like structure moved by protons.
Other protruding filamentous proteinic structures that are not essential in all conditions are fimbriae and pili. Fimbriae seem to be involved in the sticking adherence of the cells that produce them to inert surface or animal tissues e. Flagella, fimbriae and pili cannot be visualized under the ordinary light microscope without a special treatment and are not produced under all conditions. It is very significant that all prokaryotic strains studied until now possess surface receptors for temperate bacteriophages, and that about half of the strains also have receptors for self-replicating plasmids Fig.
The generalized presence of such receptors for exchangeable genetic information is additionnal proof of the importance of the genetic communication System in prokaryotes. Finally, many prokaryotic organisms secrete on their surface slimy materials often referred to as slime layers, capsules or, to use the more general term, glycocalyx.
We will return to these in later pages. As noted previously, in prokaryotes mechanical resistance is conferred by the outer layer of the envelope: the cell wall.
Basically, only three major specific shapes are adopted by prokaryotic cells and this fact too lends support to the concept of a unified prokaryotic world. These shapes have been recognized ever since the earliest times of examination with the ordinary light microscope Fig. The cross-links in P. It is present in eubacteria and its sugar derivative, N-acetylmuramic acid, has never been found in the cell membranes of eukaryotes.
A variety of different types of P. In Gram-negative bacteria, only between 10 to 15 per cent of the wall is P. They are important in the diffusion of nutrients and other substances. Other proteins present in the envelopes and whose presence and concentrations depend on the milieu and the growing conditions play a role in the entry of DNA during gene exchange processes.
Lipopolysaccharides play important roles in the physical interactions e. It is beyond the scope of this book to review the biochemistry of peptidoglycan formation but it is important to recall that the assembly of its basic constituent molecules synthesized in the cell and organized in a network outside the cytoplasm requires several different biochemical reactions related to transport through the cytoplasmic membrane and insertion into the existing web.
Some of the steps involved in the transport of these basic units and in the cross-linking between adjacent glycan chains can be inhibited by several antibiotics, of which penicillins beta-lactams and cephalosporins are of particular chemotherapeutic importance. Also, the P. These enzymes are useful, non-specific protective agents against bacterial infections and are also used extensively in the laboratory in experimental transformation and the production of cell-wall free bacteria protoplasts and spheroplasts.
Under natural conditions a bacterium that cannot keep its P. Antibiotics that interfere with the synthesis of the P. In addition to penicillins and cephalosporins, a group of substances called the glycopeptide antibiotics e. Acquisition of resistance to vancomycin results from the transfer of resistance genes able to move from one strain to another by common gene exchange processes transfection, mobilization and transduction so that the ability to produce protective protein and resist the lethal effects of glycopeptide antibiotics can easily disseminate from one strain to another Arthur et al.
The increased use of vancomycin in humans and of avoparcin as a growth promoter for farm animals is certainly a cause for justified concern since it could end up in an increased transfer of resistance genes to a large number of different pathogenic bacterial strains.
Archaea and chlamydia do not have it. However, their walls may contain a heteropolymer somewhat similar to peptidoglycan that lacks muramic acid and is called a pseudopeptidoglycan or, in the case of certain Archaea, be made of special proteins. Many Archaea are found in extreme conditions of temperature or salinity and it is not surprising that they have evolved surface layers different from those of the eubacteria. For example, bacterial lipopoly-saccharides LPS, also called endotoxins, are well known to activate macrophages in animals and to generate immunoregulatory substances e.
Their role in the triggering and the stimulation of the immune response receives considerable attention and is investigated in many laboratories.
They are now considered as modulins or modulators of many of the immune reactions and as mediators of homeostasis and, in large concentrations, of tissue pathology Henderson et al, This is also true of peptidoglycan which should not be viewed merely as a biologically inert corset that determines cell shape. Fragments of PG are potent biological effectors which modulate a remarkably diverse set of inflammatory and immune reactions in animals. A currently accepted model for the structure of the bacterial membrane is the fluid mosaic model of Singer and Nicholson.
Although it appears simple, the membrane fulfills several functions and can be considered a multipurpose organ. There are no mitochondria in a prokaryotic cell and the role of this eukaryotic energy generator respiration and electron transport is assumed by the membrane. It also acts as a boundary layer for the cytosol and as such it pumps metabolites in and catabolites out of the cell and probably plays an essential role in the equal and symmetric genome distribution before separation of the mother cell into two identical daughters.
The phospholipid leaflets are arranged in a bilayer structure into which various and numerous proteins are inserted. They play different roles, some of which are enumerated above electron transport, active transport of metabolites, catabolites, DNA, etc.
Bacterial membranes differ, on the one hand, from the eukaryotic cell membrane and they may also differ among strains both in the occurrence and nature of their basic constituents, the fatty acids. Another way in which bacterial membranes usually differ from eukaryotic ones is in lacking sterols such as cholesterol. However, many prokaryotic membranes contain sterol-like molecules that are synthesized from the same precursors as steroids.
They are called hopanoids and they probably stabilize the two fatty acid sheets. Hopanoids can be isolated from kerosene, an organic precursor of petroleum. It is present in large quantities in some sediments. In fact, it has been estimated that the total mass of hopanoids in all sediments may be around 10 tons, about as much as the total mass of organic carbon in all living organisms 10 12 tons Prescott et al, Since in all probability these hopanoids are of prokaryotic origin, this shows the enormous importance of prokaryotes in the formation of fossil fuel, in particular petroleum.
However, horizontal genetic exchanges are possible despite these differences. Some may appear randomly distributed, others equally spaced. Ribosomes in both prokaryotes and eukaryotes are an essential part of the translation in the synthesis of proteins machinery in cells. They act as the physical support on which the genetic information already transferred from the DNA to the messenger RNA is used to assemble amino acids in the correct order to make functional proteins.
In bacteria, ribosomes are smaller than in eukaryotic cells. They sediment less easily in the ultracentrifuge. They consist of about two-thirds ribonucleic acid and one-third protein and, following proper Chemical treatment, they dissociate into two subunits of different size and weight.
To fulfill their roles in protein biosynthesis, ribosomal RNA molecules must contain several functionally different regions. The nucleotide sequences in some of these regions are conserved and in others are highly varied. The smaller subunit of the prokaryotic ribosome contains a sequence 16S approximately nucleotides that is particularly suitable for genetic and phylogenetic comparison between different bacterial strains and also for studies of evolution.
During the very long geologic eons, when bacteria evolved into what taxonomists traditionnally call groups, families, genera or species, changes were imprinted in the sequence of ribosomal RNAs. These imprints or molecular signatures can be used to identify different bacteria and also to assess the most probable evolutionary distance between them Woese, Chloramphenicol, streptomycin, erythromycin and tetracyclines, for example, act as much more potent protein synthesis inhibitors in bacteria than in eukaryotic cells and are clinically useful antibacterial drugs although they are not entirely devoid of toxicity in eukaryotes.
Laboratory techniques can be used to stain the bodies of the bacterial cells with a cationic dye e. India ink.
It can be visualized as a defined area surrounding blue dots bacterial bodies. These transparent zones are usually called capsules and our knowledge of their chemistry is now extensive Bayer and Bayer, A large number of polysaccharides have been isolated from capsules and the slime material of bacteria.
Size, charge and composition of the capsular material are of primary importance in determining the roles and the usefulness of this structure for the bacteria.
Life in all its diversity is composed of only two types of cells: Eukaryotic and Prokaryotic. Prokaryotic cells are simple, one-celled organisms; such as bacteria. All other life is composed of Eukaryotic cells. Not surprisingly, Prokaryotic cells are far simpler in structure and they are also much smaller than Eukaryotic cells. A cell wall, which is a firm structure that encloses the cell membrane, can be seen in both prokaryotes and eukaryotes. Both cell types contain cytoplasm, in which ribosomes organelles that make Note: not all prokaryotic cells have flagella.
The cell of eukaryotic organisms animals, plants, fungi differs from that of prokaryotic organisms Archaea and Bacteria by the presence of several specialized organelles, such as: the nucleus containing the genetic information of the cell , the mitochondria site of cellular respiration , or the chloroplast site of photosynthesis in plants. The existence and organization of mitochondrial and chloroplast DNA, as well as their biochemistry and some structural traits, have led to their being considered as ancient bacteria integrated into a host cell by an endosymbiosis process. One possible hypothesis would be that current eukaryotes would descend from an archaeal ancestor who acquired a proteobacteria, the present mitochondria. Once this step was established, some cells would have incorporated cyanobacteria that are the origin of the chloroplast. At the same time, they have acquired the ability to carry out photosynthesis, and thus an autotrophic metabolism, a particularity of plants.
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The endosymbiotic theory deals with the origins of mitochondria and chloroplasts, two eukaryotic organelles that have bacteria characteristics. Mitochondria and chloroplasts are believed to have developed from symbiotic bacteria, specifically alpha-proteobacteria and cyanobacteria, respectively. The theory states that a prokaryotic cell was consumed or engulfed by a larger cell. By some unknown reason, the prokaryotic organelle was not consumed.
As of [update] the taxonomy was under revision  . Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.
It differs markedly from the rich variety of cellular forms, sizes and structures found in eukaryotes. The prokaryotic cell is small, its volume and mass being approximately one thousand times less than those of an average eukaryotic cell Fig. However, it must not be considered merely a diminutive form of the eukaryotic cell but rather as strikingly different and remarkably adjusted to its peculiar and original way of life. Small size has major consequences and it dictates some important biological properties.
All complex life on Earth is eukaryotic. All eukaryotic cells share a common ancestor that arose just once in four billion years of evolution. Prokaryotes show no tendency to evolve greater morphological complexity, despite their metabolic virtuosity.
The Prokaryotes pp Cite as. The aim of characterization in the present context is to obtain a complete collection of data describing the properties of a prokaryotic pure culture, i. When identification in this sense cannot be accomplished, the aim of identification must shift to characterization of a new species, i. It is clear that the amount of data required for the identification of an isolate with an established species is usually lower than the amount of data collected for characterization. And it is also clear that the final aim of characterization, as mentioned above, is never reached because continuing progress in scientific and technological methods allows the study of an ever-increasing number of characters or properties of a species.
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