Others propose that the domains Archaea and Eukarya emerged from a common archaeal-eukaryotic ancestor that itself emerged from a member of the domain Bacteria. Some of the evidence behind this hypothesis is based on a "superphylum" of bacteria called PVC, members of which share some characteristics with both archaea and eukaryotes.
There is growing evidence that eukaryotes may have originated within a subset of archaea. In any event, it is accepted today that there are three distinct domains of organisms in nature: Bacteria, Archaea , and Eukarya.
A description of the three domains follows. There is a "superphylum" of bacteria called PVC, referring to the three members of that superphylum: the Planctomycetes, the Verrucomicrobia, and the Chlamydiae. Some of these bacteria show cell compartmentalization wherein membranes surround portions of the cell interior, such as groups of ribosomes or DNA, similar to eukaryotic cells. Some divide by budding or contain sterols in their membranes, again similar to eukaryotes.
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. Currently, all living organisms are classified into three domains:. Originally, the two prokaryotic domains were considered only as a single kingdom Monera , however biochemical differences were discovered between the two groups which warranted their reclassification into separate domains.
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