The Origin Of The Eukaryotic Cell Was A Unique Event
There is little doubt that all known eukaryotic cells share a common ancestor that arose only once in four billion years of evolution. Common traits range from the conserved position of many introns , to the structure of nuclear pore complexes , to complex traits such as syngamy and two-step meiosis . It is implausible that all of these shared properties arose by lateral gene transfer or convergent evolution . Common ancestry is much the most parsimonious explanation.
However, a single ancestor is perfectly consistent with multiple origins if all ‘protoeukaryotic’ lines arising later were driven to extinction by fully-fledged eukaryotes already occupying every niche, and if all earlier protoeukaryotes were displaced by modern eukaryotes . This cannot be addressed phylogenetically, as any phylogenetic evidence for their existence is lost. Nor is the fossil record any help. It is hard to distinguish between eukaryotic and prokaryotic microfossils let alone prove the existence of extinct lines of protoeukaryotes. While asserting the unprovable existence of extinct lines of eukaryotes is unsatisfying, if not unscientific, extinction is commonplace, and the argument seems, on the face of it, irrefutable.
The First Inhabitants Of Earth
When and where did life begin? What were the conditions on Earth when life began? Based on the fossil record, LUCA, Last Universal Common Ancestor, was the predecessor to bacteria and archaea. While we don’t know what these organsims were like genetically, we do know that they had no true nucleus and were morphologically similar to bacteria and archaea. They were the first forms of life on Earth and they existed for billions of years before plants and animals appeared. The Earth and its moon are thought to be about 4.54 billion years old. This estimate is based on evidence from radiometric dating of meteorite material, together with other substrate material from Earth and the moon. Early Earth had a very different atmosphere than it does today and was subjected to strong radiation thus, the first organisms would have flourished in areas where they were more protected, such as in ocean depths or beneath the surface of the Earth. During this time period, strong volcanic activity was common on Earth, so it is likely that these first organisms were adapted to very high temperatures. Early Earth was also bombarded with mutagenic radiation from the sun. The first organisms needed to be able to withstand all these harsh conditions.
The Evolution of Bacteria and Archaea
Placing Key Events On The Geologic Time Scale
How do each of these events map onto geologic time? Most of them are not instantaneous events, and so they span multiple time periods as follows:
- Hadean eon : No life present on Earth
- Origin of life , 2.8-2.6 BYA
- First cyanobacteria, capable of producing oxygen through photosynthesis, ~2.5 BYA
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Eukaryotic Cell Vs Prokaryotic Cell
The difference between a eukaryotic cell and a prokaryotic cell is simple: eukaryotic cells have membrane-bound organelles. Within a prokaryotic cell the DNA simply floats around the cytoplasm. While prokaryotic cells do have one type of organelle , these organelles are not covered in a plasma membrane.
Prokaryotic Cell Vs Eukaryotic Cell
The difference between the prokaryotic cell and the eukaryotic cell is simple. Eukaryotic cells have a nucleus surrounded by a nuclear membrane and other membrane-bound organelles that perform specific functions in the cell. These membranes form the endomembrane system, which creates a series of specialized chambers within eukaryotic organisms that can complete a diverse range of tasks. By contrast, a prokaryotic cell only has a cellular membrane with no membranes extending on the inside of the cell.
All of the reactions within a prokaryote take place within the cytosol of the cell. While this makes the cells slightly less efficient, prokaryotic cells still have a remarkable reproductive capacity. A prokaryote reproduces through binary fission, a process that simply splits duplicated DNA into separate cells. Without any organelles or complex chromosomes to reproduce, most prokaryotic cells can divide every 24 hours, or even faster with an adequate supply of food.
While many prokaryotic cells have adapted to free-living within the environment, others have adapted to live within the gut of other organisms. These commensal organisms survive by breaking down molecules inside the gut and allow the organism they are living within the ability to digest a wider variety of foods. For example, the human gut contains 2-3 pounds of bacteria that have evolved to help us digest complex carbohydrates, proteins, and fats.
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The Tale Of Edouard Chatton
In their famed paper of 1962, The Concept of a Bacterium, Roger Stanier and C. B. van Niel emphasized that the nature and relationships of bacteria, debated since the earliest days of bacteriology, remained unresolved. The authors lamented , Any good biologist finds it intellectually distressing to devote his life to the study of a group that cannot be readily and satisfactorily defined in biological terms and the abiding intellectual scandal of bacteriology has been the absence of a clear concept of a bacterium. Certainly, they acknowledged, a few of their predecessors had also known that the cell structure of bacteria and blue-green algae was different from that of other organisms, and thus they introduced Edouard Chatton . But they explained that a satisfactory description of the difference could be articulated only after the revolutionary advances in knowledge of cellular organization which followed the development of new techniques after of the Second World War. It is now clear, they proclaimed , that among organisms there are two different organizational patterns of cells, which Chatton called, with singular prescience, the eukaryotic and procaryotic type. The distinctive property of bacteria and blue-green algae is the prokaryotic nature of their cells. It is on this basis that they can be clearly segregated from all other protists , which have eucaryotic cells.
Three Domains Of Life On Earth
DNA sequence comparisons and structural and biochemical comparisons consistently categorize all living organisms into 3 primary domains: Bacteria, Archaea, and Eukarya . Both Bacteria and Archaea are prokaryotes, single-celled microorganisms with no nuclei, and Eukarya includes us and all other animals, plants, fungi, and single-celled protists all organisms whose cells have nuclei to enclose their DNA apart from the rest of the cell. The fossil record indicates that the first living organisms were prokaryotes , and eukaryotes arose a billion years later.
Study Tip: It is suggested that you create a chart to compare and contrast the three domains of life as you read.
The information below was adapted from OpenStax Biology 22.2
Archaea and Bacteria share a number of features, but are also distinct domains of life:
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Genome Outposts Are Required For Major Expansion Of Oxidative Phosphorylation
Mitochondrial genes are undoubtedly necessary for oxidative phosphorylation in eukaryotic cells. A large body of data shows that the rate of respiration depends on the copy number of mitochondrial DNA , with active cells having more copies of the genome . Cells depleted in mtDNA have a low respiratory capacity , while mutations that cause mtDNA depletion are typically associated with mitochondrial diseases . The rate of transcription of mitochondrial genes, notably ND5, controls the overall rate of assembly of respiratory complexes . Presumably, the reverse is true for prokaryotes: in the absence of local genome outposts, giant bacteria could not respire across a significantly wider area of bioenergetic membrane, whether internal or external. All known examples of giant bacteria do indeed have multiple genomes, invariably placed close to the plasma membrane suggesting that local genome outposts are necessary for respiration.
Figure 4
Volume of cytoplasm controlled by a single genome. The long reach of the eukaryotic gene. Mean eukaryotic cell volume is 15,000 times greater than mean bacterial cell volume , and is controlled by a single nuclear genome. In the case of Thiomargarita the cell volume is larger than most eukaryotic cells but is mostly filled with inert vacuole. The band of active cytoplasm contains multiple nucleoids, each one governing a volume of cytoplasm equivalent to a single E. coli cell, hence volume per gene is prokaryotic.
Are Humans Considered Animals
Humans can move on their own and are placed in the animal kingdom. Further, humans belong to the animal phylum known as chordates because we have a backbone. The human animal has hair and milk glands, so we are placed in the class of mammals. Within the mammal class, humans are placed in the primate order.
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Are Bacteria Multicellular
Highlights. Many bacteria have a multicellular phase of their lifecycle, which fall into three broad categories based on shape and mechanism of formation. A number of pressures may have selected for multicellularity, including physicochemical stress, nutrient scarcity, predation, and environmental variability.
What Is A Prokaryotic Cell
Prokaryotic cells are single-celled microorganisms known to be the earliest on earth. Prokaryotes include Bacteria and Archaea. The photosynthetic prokaryotes include cyanobacteria that perform photosynthesis.
A prokaryotic cell consists of a single membrane and therefore, all the reactions occur within the cytoplasm. They can be free-living or parasites.
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Only Endosymbiosis Can Fashion Giant Nuclear Genomes
Endosymbiosis characteristically results in the reductive evolution of endosymbiont genomes . In the case of the bacteria that eventually became mitochondria, almost the entire genome was either lost or transferred to the nucleus, leaving only a tiny residual genome in most cases . But mitochondria are by no means unique in this regard. Chloroplasts, too, have lost almost all the genes required by free-living cyanobacteria . Other bacterial endosymbionts living inside eukaryotic cells, such as Buchnera Wolbachia , Rickettsia and Carsonella have retained only stumps of genomes in the case of Carsonella, smaller than many plant mitochondrial genomes . This process probably reflects competition between individual endosymbiotic cells for succession to the next generation. The fastest replicators, typically with the smallest genomes and lowest demand for de novo protein synthesis, prevail. The outcome is that unnecessary genes are jettisoned and the genome is gradually pared away. But this process–no more than standard practice for populations of endosymbionts–has the most profound repercussions for the host cell.
Microbes On The Human Body
A particularly fascinating example of our normal flora relates to our digestive systems. People who take high doses of antibiotics tend to lose many of their normal gut bacteria, allowing a naturally antibiotic-resistant species called Clostridium difficile to overgrow and cause severe gastric problems, especially chronic diarrhea . Obviously, trying to treat this problem with antibiotics only makes it worse. However, it has been successfully treated by giving the patients fecal transplants from healthy donors to reestablish the normal intestinal microbial community. Clinical trials are underway to ensure the safety and effectiveness of this technique.
Figure 2:
Scientists are also discovering that the absence of certain key microbes from our intestinal tract may set us up for a variety of problems. This seems to be particularly true regarding the appropriate functioning of the immune system. There are intriguing findings that suggest that the absence of these microbes is an important contributor to the development of allergies and some autoimmune disorders. Research is currently underway to test whether adding certain microbes to our internal ecosystem may help in the treatment of these problems, as well as in treating some forms of autism.
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Is A Bacteria A Eukaryote
All cells fall into one of these two broad categories. Only the single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotespro means before and kary means nucleus. Animals, plants, fungi, and protists are all eukaryoteseu means trueand are made up of eukaryotic cells.
What Are Good Bacteria Called
Probiotics are live bacteria and yeasts that are good for you, especially your digestive system. We usually think of these as germs that cause diseases. But your body is full of bacteria, both good and bad. Probiotics are often called good or helpful bacteria because they help keep your gut healthy.
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Characteristics And Examples Of Prokaryotes
Organisms are placed in either one of the three major groups based on their cell type and other features. These three groups are called domains. They are Archaea, Bacteria & Eukarya.
Prokaryotes are found in the domains Archaea and Bacteria.
BACTERIA AS PROKARYOTES
- Bacteria are single-celled microorganisms found everywhere on earth.
- They are diverse in their structure and shape. Many bacteria live in varied ecosystems on the earth and our bodies.
- The presence of a flagella, cell membrane, process of protein synthesis and the presence of peptidogycan in their cell wall are characteristics of bacterial cells.
- In the human gut, the number of bacteria outnumbers the number of human cells.
ARCHAEA AS PROKARYOTES
- The archaean cells are also prokaryotes, and these are the same shape and size as the bacteria, and these are also unicellular. The Archaea share traits with both the bacteria and the eukaryotes.
- The Archaea lack peptidoglycan in their cell walls, have a cell membrane like the bacteria.
- The Archaean cells have a unique flagella and carry out protein synthesis by a process similar to those of eukaryotes but not of bacteria.
- Some archaea survive in extreme weather, like in hot springs. These are also found in oceans, soils, inside organisms, and marshlands.
Definition Of Eukaryotes And Prokaryotes
Prokaryotes are organisms without a cell nucleus , or any other membrane-bound organelles. Most are unicellular, but some prokaryotes are multicellular.
Eukaryotes are organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. The most characteristic membrane bound structure is the nucleus. This feature gives them their name, which comes from the Greek , meaning good/true, and , meaning nut, referring to the nucleus. Animals, plants, fungi, and protists are eukaryotes.
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Rna In Prokaryotes Versus Eukaryotes
Comparison of the different domains of life shows rather distinct RNA complements. All living organisms possess rRNA, tRNA, mRNA and a variety of other noncoding RNAs. However, there are major differences in the types and frequency of many classes of RNA between eukaryotes and prokaryotes. Since the various types of RNA are dealt with individually elsewhere, we will summarize the differences between the different domains of life here .
Figure 19.03. RNA in the Bacterial Cell
Some of the major forms of RNA characteristic of bacteria are illustrated. The CRISPR system generates crispr RNA that recognizes intruding DNA or RNA from viruses and the CRISPR/Cas system destroys the incoming nucleic acid. Transcription generates mRNA that is translated into protein by the ribosome, which contains ribosomal RNA. Expression of the mRNA may be controlled by a riboswitch that changes conformation upon binding a signal molecule . More often, the binding of small RNA controls mRNA expression, usually by promoting RNA degradation.
Figure 19.04. RNA in the Eukaryotic Cell
Prokaryotes use CRISPR for antiviral defense whereas eukaryotes use RNA interference.
Many classes of RNA that act within the eukaryotic nucleus, especially snRNA and snoRNA, are missing in prokaryotes.
NAD Capping of RNA
Axel Magalon, … Anne Walburger, in, 2012
Differences Between Eukaryotes And Prokaryotes
While eukaryotes are complex, usually multicellular organisms that have eukaryotic cells, prokaryotes are usually single-celled organisms that have simpler prokaryotic cells. Eukaryotes include all complex lifeforms including protozoa, plants, fungi, and animals, while prokaryotes are microscopic, mostly single-celled lifeforms, either archaea or bacteria. Eukaryotes cells have DNA in a nucleus surrounded by a nuclear envelope, while the cells of prokaryotes do not have a distinct nucleus. Instead, they have a nucleoid, a nucleus-like region where DNA is concentrated. In addition, the DNA of eukaryotes is condensed into structures called chromosomes, which allow for a lot of genetic information to be packed into a small space and are neatly organized when it comes time for the cell to duplicate its DNA and divide. They transmit genes from parents to offspring and, control cellular processes such as metabolism, and allow cell differentiation to take place during development. Eukaryotic cells are usually much larger than prokaryotic cellsthey are usually a couple hundred times the size of prokaryotic cellswith a lower metabolic rate and a lower growth rate. An extreme example is found in plasmodial slime molds. These eukaryotes can be up to a meter in length and are a single cell with many nuclei inside.
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Prokaryotes Vs Eukaryotes: What Are The Key Differences
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Every living organism falls into one of two groups: eukaryotes or prokaryotes. Cellular structure determines which group an organism belongs to. In this article, we will explain in detail what prokaryotes and eukaryotes are and outline the differences between the two.
Bacterial And Archaeal Diversity
Bacteria and Archaea were on Earth long before multicellular life appeared. They are ubiquitous and are highly diverse in their metabolic activities. This diversity allows different species within these clades to inhabit every imaginable surface where there is sufficient moisture. For example, in the typical human body, bacterial cells outnumber human body cells by about ten to one. Indeed, bacteria and archaea comprise the majority of living things in all ecosystems. Bacterial and archaeal species have been identified that thrive in environments that are inhospitable for most other living things. Bacteria and archaea, along with microbial eukaryotes, are also critical for recycling the nutrients essential for creating new biomolecules. They also drive the evolution of new ecosystems, some of which are natural and others man-made.
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