Bacteria are all prokaryotes. However, new insight into molecular biology changed this view of life. A type of prokaryotic organism that had long been categorized as bacteria turned out to have DNA that is very different from bacterial DNA.
This difference led microbiologist Carl Woese of the University of Illinois to propose reorganizing the Tree of Life into three separate Domains: Eukarya, Eubacteria true bacteria , and Archaea. Archaea and bacteria also share certain genes, so they function similarly in some ways. But archaeans also share genes with eukaryotes, as well as having many genes that are completely unique. The ability of some archaea to live in environmental conditions similar to the early Earth gives an indication of the ancient heritage of the domain.
The early Earth was hot, with a lot of extremely active volcanoes and an atmosphere composed mostly of nitrogen, methane, ammonia, carbon dioxide, and water. There was little if any oxygen in the atmosphere. Archaea and some bacteria evolved in these conditions, and are able to live in similar harsh conditions today.
Many scientists now suspect that those two groups diverged from a common ancestor relatively soon after life began. So although archaea physically resemble bacteria, they are actually more closely related to us!
If not for the DNA evidence, this would be hard to believe. The archaea that live in extreme environments can cope with conditions that would quickly kill eukaryotic organisms. Alkaliphiles thrive at pH levels as high as that of oven cleaner. Halophiles, meanwhile, live in very salty environments. Some lack peptidoglycan, similar to eukaryotes and archaea. It has been surmised that these bacteria migh be an intermediate step between an ancestor that emerged from a bacterium domain Bacteria and an archael-eukaryotic ancestor prior to its split into the domains Archaea and Eukarya.
Archaea often live in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles. One reason for this is that the ether-containing linkages in the Archaea membranes is more stabile than the ester-containing linkages in the Bacteria and Eukarya and are better able to withstand higher temperatures and stronger acid concentrations. Bacteria also known as eubacteria or "true bacteria" are prokaryotic cells that are common in human daily life, encounter many more times than the archaebacteria.
Eubacteria can be found almost everywhere and kill thousands upon thousands of people each year, but also serve as antibiotics producers and food digesters in our stomachs. The Bacteria possess the following characteristics:.
Bacteria include mycoplasmas, cyanobacteria, Gram-positive bacteria, and Gram-negative bacteria. The Eukarya also spelled Eucarya possess the following characteristics:. The Eukarya are subdivided into the following four kingdoms:. It used to be thought that the changes that allow microorganisms to adapt to new environments or alter their virulence capabilities was a relatively slow process occurring within an organism primarily through mutations, chromosomal rearrangements, gene deletions and gene duplications.
Those changes would then be passed on to that microbe's progeny and natural selection would occur. This gene transfer from a parent organism to its offspring is called vertical gene transmission. It is now known that microbial genes are transferred not only vertically from a parent organism to its progeny, but also horizontally to relatives that are only distantly related, e. This latter process is known as horizontal gene transfer. Through mechanisms such as transformation, transduction, and conjugation, genetic elements such as plasmids, transposons, integrons, and even chromosomal DNA can readily be spread from one microorganism to another.
Microbes are known to live in remarkably diverse environments, many of which are extremely harsh. This amazing and rapid adaptability is a result of their ability to quickly modify their repertoire of protein functions by modifying, gaining, or losing their genes.
This gene expansion predominantly takes place by horizontal transfer. A biofilm is a microbial community held together in a gummy-textured matrix that consists primarily of polysaccharides secreted by the organisms, together with some proteins and nucleic acids. Biofilms grow attached to surfaces. Some of the best-studied biofilms are composed of prokaryotes, although fungal biofilms have also been described, as well as some composed of a mixture of fungi and bacteria.
Biofilm Development : Five stages of biofilm development are shown. During stage 1, initial attachment, bacteria adhere to a solid surface via weak van der Waals interactions. During stage 2, irreversible attachment, hairlike appendages called pili permanently anchor the bacteria to the surface.
During stage 3, maturation I, the biofilm grows through cell division and recruitment of other bacteria. An extracellular matrix composed primarily of polysaccharides holds the biofilm together. During stage 4, maturation II, the biofilm continues to grow and takes on a more complex shape.
During stage 5, dispersal, the biofilm matrix is partly broken down, allowing some bacteria to escape and colonize another surface. Micrographs of a Pseudomonas aeruginosa biofilm in each of the stages of development are shown. Biofilms are present almost everywhere: they can cause the clogging of pipes and readily colonize surfaces in industrial settings.
In recent, large-scale outbreaks of bacterial contamination of food, biofilms have played a major role. They also colonize household surfaces, such as kitchen counters, cutting boards, sinks, and toilets, as well as places on the human body, such as the surfaces of our teeth. Interactions among the organisms that populate a biofilm, together with their protective exopolysaccharidic EPS environment, make these communities more robust than free-living, or planktonic, prokaryotes.
The sticky substance that holds bacteria together also excludes most antibiotics and disinfectants, making biofilm bacteria hardier than their planktonic counterparts.
Overall, biofilms are very difficult to destroy because they are resistant to many common forms of sterilization. Privacy Policy. Skip to main content. Prokaryotes: Bacteria and Archaea. Search for:. Prokaryotic Diversity. Classification of Prokaryotes Prokaryotic organisms were the first living things on earth and still inhabit every environment, no matter how extreme.
Learning Objectives Discuss the origins of prokaryotic organisms in terms of the geologic timeline. Key Takeaways Key Points All living things can be classified into three main groups called domains; these include the Archaea, the Bacteria, and the Eukarya.
Prokaryotes arose during the Precambrian Period 3. Prokaryotic organisms can live in every type of environment on Earth, from very hot, to very cold, to super haline, to very acidic.
The domains Bacteria and Archaea are the ones containing prokaryotic organisms. The Archaea are prokaryotes that inhabit extreme environments, such as inside of volcanoes, while Bacteria are more common organisms, such as E.
These hallmark genes are shared by an extremely diverse group of viruses with different replication strategies, although none of the genes is strictly universal among viruses. The discovery of the hallmark genes reveals the evolutionary unity of the viral empire Koonin et al. Finally, viruses and related mobile genetic elements that lack capsids e.
These selfish genetic elements are major agents of gene transfer. The genomes of many eukaryotes, particularly animals and plants, consist in large part of inactivated remnants of such elements up to 80 percent of the genome in plants. Biologists sometimes debate whether viruses should be considered living organisms. However, the debates seem to be largely issues of semantics. Comparative genomics and metagenomics have transformed our understanding of the genetic universe.
New discoveries have revealed the previously unrealized prominence of the viral world. This second biological empire seems to be even more vast and diverse than the empire of cellular life-forms. A second key transformation in our understanding is that a complex network of treelike and netlike routes better explains evolution than does a single TOL. Even under this new network perspective, the three domains of cellular life — Bacteria, Archaea, and Eukarya — remain objectively distinct.
Although these domains are distinct, the eukaryotes are archaebacterial chimeras, which evolved as a result of, or at least under the strong influence of, an endosymbiotic event that gave rise to the mitochondria. Despite all the recent advances of evolutionary genomics, we still have to answer the most fundamental questions: How did cells evolve in the first place, what caused the fundamental differences between the two prokaryotic domains Archaea and Bacteria , and what triggered the emergence of the complex organization of the eukaryotic cell?
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