At what stage in the evolution of the cell did the fundamental division into the primary kingdoms take place? What was the nature of the universal ancestor?
Long ago, however, there must have been still simpler forms of the cell. Consider the following argument. The translation mechanism is complex, comprising on the order of large molecular components. For such a mechanism to have evolved in a single step is clearly impossible. The primitive version of the mechanism must have been far simpler, smaller and less accurate. Otherwise the probability of error in making a protein strand would have been too great. Among the smaller and less specific proteins would have been the enzymes required to process genetic information.
If those enzymes were less precise than today's versions are, the cell's mutation rate must necessarily have been higher and the size of its genome correspondingly smaller. It evolved from an entity having simple properties, imprecise and general functions and a rather small complement of genes to an entity that functioned with many highly specific enzymes and a complex, precise genetic apparatus. To emphasize the primitive genetic and translational mechanisms of the earlier, simpler cells, I call them progenotes.
The discovery of the archaebacteria provides the perspective needed to approach the question of whether the universal ancestor was a prokaryote or a progenote.
Although the question is far from settled, the initial indications are that the universal ancestor was indeed a progenote. The time needed for the evolution of the first true bacteria or archaebacteria, then, had to be less than a billion years, and perhaps much less.
If archaebacteria should be found to differ from true bacteria in their mechanisms for controlling gene expression a possibility that has yet to be investigated , the implication would be that their common ancestor may have had only rudimentary mechanisms of genetic control.
The subunit structure of the RNA polymerase is quite constant among the true bacteria, whereas the archaebacterial polymerase structure is different. If that is so, it would appear that many of the modifications have evolved independently in each of the primary kingdoms.
Rather than searching for the hypothetical host, however, one should instead question whether there was such an entity. This is not a time to shape new discoveries in accordance with old prejudices. As I have indicated, the differences between the eukaryotic cell and the other major cell types at the molecular level are more extensive and pervasive than any of the differences visible with a microscope.
The eukaryotic nucleus appears to contain at least three kinds of genes: those of eubacterial origin presumed to have been appropriated from the genomes of the eukaryote's organelles , those of archaebacterial origin for example the gene for the ribosomal A protein , and those of an unidentified third origin exemplified by the cytoplasmic ribosomal RNA.
To what extent is the eukaryotic nucleus genetically a chimera: an entity composed of parts assembled from disparate sources? At what stage or stages of evolution did the presumed assembly take place? And what was the nature of the organisms that supplied the various genes and structures? Biologists have tended to look at the eukaryotic cell as having been formed by the association of fully evolved prokaryotic cells; their association is assumed to have created a "higher" type of cell, the eukaryotic.
The term prokaryote—"before the nucleus"—carries just this implication. The question that an profitably be asked now is whether the evolutionary agents that gave the eukaryotic cell its basic molecular character really were of this nature.
Rather than being an advanced, "higher" form, the eukaryotic cell may represent a throwback to the evolutionary dynamics of its long-gone ancestor, the progenote.
It is no longer necessary to rely solely on speculation to account for the origins of life. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. The urkaryote Logically the next question is: Where does the rest of the eukaryotic cell come from?
The progenote as ancestor The discovery of the archaebacteria provides the perspective needed to approach the question of whether the universal ancestor was a prokaryote or a progenote. Like all living cells, archaea rely on the replication of DNA to ensure that daughter cells are identical to the parent cell. The DNA structure of archaea is simpler than that of eukaryotes and similar to the bacterial gene structure.
The DNA is found in single circular plasmids that are initially coiled and that straighten out prior to cell division. While this process and the subsequent binary fission of the cells is like that of bacteria, the replication and translation of DNA sequences takes place as it does in eukaryotes.
Once the cell DNA is uncoiled, the RNA polymerase enzyme that is used to copy the genes is more similar to eukaryote RNA polymerase than it is to the corresponding bacterial enzyme.
Creation of the DNA copy also differs from the bacterial process. DNA replication and translation is one of the ways in which archaea are more like the cells of animals than those of bacteria. As with bacteria, flagella allow the archaea to move. Their structure and operating mechanism are similar in archaea and bacteria, but how they evolved and how they are built differ.
These differences again suggest that archaea and bacteria evolved separately, with a point of differentiation early on in evolutionary terms. Similarities among members of the two domains can be traced to later horizontal DNA exchange between cells. The flagellum in archaea is a long stalk with a base that can develop a rotary action in conjunction with the cell membrane. The rotary action results in a whiplike motion that can propel the cell forward. In archaea, the stalk is constructed by adding material at the base, while in bacteria, the hollow stalk is built up by moving material up the hollow center and depositing it at the top.
Flagella are useful in moving cells toward food and in spreading out after cell division. The main differentiating characteristic of archaea is their ability to survive in toxic environments and extreme habitats.
Depending on their surroundings, archaea are adapted with regard to their cell wall, cell membrane and metabolism. Other organic compounds such as alcohols, acetic acid, or formic acid are used as alternative electron acceptors by methanogens. These reactions are common in gut-dwelling archaea. Acetotrophic archaea also break down acetic acid into methane and carbon dioxide directly.
These acetotrophs are archaea in the order Methanosarcinales, and are a major part of the communities of microorganisms that produce biogas. Other archaea, called autotrophs, use CO 2 in the atmosphere as a source of carbon, in a process called carbon fixation. In addition, the Crenarchaeota use the reverse Krebs cycle while the Euryarchaeota use the reductive acetyl-CoA pathway. Carbon—fixation is powered by inorganic energy sources. Phototrophic archaea use sunlight as a source of energy; however, oxygen—generating photosynthesis does not occur in any archaea.
Instead, in archaea such as the Halobacteria, light-activated ion pumps generate ion gradients by pumping ions out of the cell across the plasma membrane. This process is a form of photophosphorylation. The ability of these light-driven pumps to move ions across membranes depends on light-driven changes in the structure of a retinol cofactor buried in the center of the protein.
Besides these, archaeal energy sources are extremely diverse, and range from the oxidation of ammonia by the Nitrosopumilales to the oxidation of hydrogen sulfide or elemental sulfur by species of Sulfolobus, using either oxygen or metal ions as electron acceptors.
Archaea usually have a single circular chromosome, the size of which may be as great as 5,, base pairs in Methanosarcina acetivorans, the largest known archaean genome. One-tenth of this size is the tiny , base-pair genome of Nanoarchaeum equitans, the smallest archaean genome known. It is estimated to contain only protein-encoding genes. Smaller independent pieces of DNA, called plasmids, are also found in archaea. Both bacteria and archaea have a cell wall that protects them.
In the case of bacteria, it is composed of peptidoglycan, whereas in the case of archaea, it is pseudopeptidoglycan, polysaccharides, glycoproteins, or pure protein. Bacterial and archaeal flagella also differ in their chemical structure. A scientist isolates a new species of prokaryote. He notes that the specimen is a bacillus with a lipid bilayer and cell wall that stains positive for peptidoglycan.
Its circular chromosome replicates from a single origin of replication. Is the specimen most likely an Archaea, a Gram-positive bacterium, or a Gram-negative bacterium? How do you know? The specimen is most likely a gram-positive bacterium. Since the cell wall contains peptidoglycan and the chromosome has one origin of replication, we can conclude that the specimen is in the Domain Bacteria.
Since the gram stain detects peptidoglycan, the prokaryote is a gram-positive bacterium. Skip to content Prokaryotes: Bacteria and Archaea. Learning Objectives By the end of this section, you will be able to do the following: Describe the basic structure of a typical prokaryote Describe important differences in structure between Archaea and Bacteria.
Common prokaryotic cell types. Prokaryotes fall into three basic categories based on their shape, visualized here using scanning electron microscopy: a cocci, or spherical a pair is shown ; b bacilli, or rod-shaped; and c spirilli, or spiral-shaped. David Cox; scale-bar data from Matt Russell.
The Prokaryotic Cell Recall that prokaryotes are unicellular organisms that lack membrane-bound organelles or other internal membrane-bound structures Figure. The features of a typical prokaryotic cell.
Flagella, capsules, and pili are not found in all prokaryotes. The three domains of living organisms. Bacteria and Archaea are both prokaryotes but differ enough to be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third domain of life.
Major groups of Archaea and Bacteria are shown. The Proteobacteria. Phylum Proteobacteria is one of up to 52 bacteria phyla. Proteobacteria is further subdivided into five classes, Alpha through Epsilon. Archaeal phyla. The Plasma Membrane of Prokaryotes The prokaryotic plasma membrane is a thin lipid bilayer 6 to 8 nanometers that completely surrounds the cell and separates the inside from the outside.
Bacterial and archaeal phospholipids. Archaeal phospholipids differ from those found in Bacteria and Eukarya in two ways. First, they have branched phytanyl sidechains instead of linear ones. Second, an ether bond instead of an ester bond connects the lipid to the glycerol.
The Cell Wall of Prokaryotes The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes. Visual Connection. Cell walls in Gram-positive and Gram-negative bacteria. Bacteria are divided into two major groups: Gram positive and Gram negative. Both groups have a cell wall composed of peptidoglycan: in Gram-positive bacteria, the wall is thick, whereas in Gram-negative bacteria, the wall is thin.
In Gram-negative bacteria, the cell wall is surrounded by an outer membrane that contains lipopolysaccharides and lipoproteins. Porins are proteins in this cell membrane that allow substances to pass through the outer membrane of Gram-negative bacteria.
In Gram-positive bacteria, lipoteichoic acid anchors the cell wall to the cell membrane.
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