Everything about Procaryote totally explained
The
prokaryotes (; singular
prokaryote /proʊˈkæriət/) are a group of
organisms that lack a
cell nucleus (= karyon), or any other
membrane-bound
organelles. They differ from the
eukaryotes, which have a cell nucleus. Most are
unicellular, but some prokaryotes are multicellular organisms. The word
prokaryotes comes from the
Old Greek pro- before +
karyon nut or
kernel, referring to the cell nucleus, +
suffix -otos,
pl. -otes; it's also spelled "procaryotes".
The prokaryotes are divided into two domains: the
bacteria and the
archaea. Archaea are a newly appointed
domain of life. These organisms were originally thought to live only in inhospitable conditions such as extremes of
temperature,
pH, and
radiation but have since been found in all types of
habitats.
Relationship to eukaryotes
A distinction between prokaryotes and
eukaryotes (meaning true kernel, also spelled "eucaryotes") is that eukaryotes do have "true" nuclei containing their
DNA, whereas the genetic material in prokaryotes isn't membrane-bound. Eukaryotic organisms may, as in the case of
amoebae, be unicellular or, as in the case of humans, be multicellular. The difference between the structure of prokaryotes and eukaryotes is so great that it's considered to be the most important distinction among groups of organisms. In 1977,
Carl Woese proposed dividing prokaryotes into the
Bacteria and
Archaea (originally Eubacteria and Archaebacteria) because of the major differences in the structure and genetics between the two groups of organisms. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the
three-domain system replacing the traditional
two-empire system. A criticism of this classification is that the word "prokaryote" itself is based on what these organisms are not (they are not eukaryotic), rather than what they're (either archaea or bacteria).
The cell structure of prokaryotes differs greatly from that of eukaryotes. The defining characteristic is the absence of a
nucleus. Instead, the
genomes of prokaryotes are held within an irregular
DNA/
protein complex in the
cytosol called the
nucleoid, which lacks a
nuclear envelope. Prokaryotes generally lack membrane-bound cell compartments: such as
mitochondria and
chloroplasts. Instead processes such as
oxidative phosphorylation and
photosynthesis take place across the prokaryotic
plasma membrane. However, prokaryotes do possess some internal structures, such as
vacuole and
cytoskeletons, and the bacterial order
Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures. Both eukaryotes and prokaryotes contain large
RNA/
protein structures called
ribosomes, which produce
protein. Prokaryotes are usually much smaller than eukaryotic cells..
It isn't surprising that many researchers have started calling prokaryotic communities multicellular (for example ). Differential cell expression, collective behavior, signaling,
programmed cell death, and (in some cases) discrete
biological dispersal events all seem to point in this direction. However, these colonies are seldom if ever founded by a single founder (in the way that animals and plants are founded by single cells), which presents a number of theoretical issues. Most explanations of
co-operation and the
evolution of multicellularity have focused on high relatedness between members of a group (or colony, or whole organism). If a copy of a gene is present in all members of a group, behaviors that promote cooperation between members may permit those members to have (on average) greater fitness than a similar group of selfish individuals (see
inclusive fitness and
Hamilton's rule). What to make of prokaryotic communities clearly founded by many (most likely unrelated) individuals, yet defined by (apparently) high levels of cooperation, communication, and coordinated behavior?
It is likely that these instances of prokaryotic sociality are the rule rather than the exception, a fact that has serious implications for the way we view prokaryotes in general and the way we deal with them in medicine. Bacterial biofilms may be 100x more resistant to antibiotics than free-living unicells and may be nearly impossible to remove from surfaces once they've colonized. Other aspects of bacterial cooperation—such as
bacterial conjugation and quorum-sensing mediated pathogenicity—present additional challenges to researchers and medical professionals seeking to treat the associated diseases.
Reproduction
Bacteria and archaea reproduce through
asexual reproduction, usually by
binary fission or
budding. Genetic exchange and recombination still occur, but this is a form of
horizontal gene transfer and isn't a replicative process, simply involving DNA being transferred between two cells, as in
bacterial conjugation.
Structure
Recent research indicates that all prokaryotes actually do have
cytoskeletons, albeit more primitive than those of eukaryotes. Besides homologues of actin and tubulin (
MreB and
FtsZ) the helically arranged building block of the
flagellum,
flagellin, is one of the most significant cytoskeletal proteins of bacteria as it provides structural backgrounds of
chemotaxis, the basic cell physiological response of bacteria. At least some prokaryotes also contain intracellular structures which can be seen as primitive organelles. Membranous organelles (a.k.a. intracellular membranes) are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, for example
photosynthesis or
chemolithotrophy. Additionally, some species also contain protein-enclosed
microcompartments mostly associated with special physiological properties (for example carboxysomes or gas vacuoles).
Morphology of prokaryotic cells
Prokaryotic cells have various shapes; the three basic shapes are:
Environment
Prokaryotes are found in nearly all environments on earth.
Archaea in particular seem to thrive in harsh conditions, such as high temperatures (thermophiles) or salinity (halophiles). Organisms such as these are referred to as
extremophiles. Many prokaryotes live in or on the bodies of other organisms, including humans.
Evolution of prokaryotes
It is generally accepted that the
first living cells were some form of prokaryote and may have developed out of
protobionts.
Fossilized prokaryotes approximately 3.5 billion years old have been discovered (less than 1 billion years after the formation of the earth's crust), and prokaryotes are perhaps the most successful and abundant organism even today. Eukaryotes only formed later, from symbiosis of multiple prokaryote ancestors; their first evidence in the fossil record appears approximately 1.7 billion years ago, although genetic evidence suggests they could have formed as early as 3 billion years ago.
While Earth is the only place in the universe where life is known to exist, some have suggested evidence of
life on Mars in the form of fossil or living prokaryotes; this is open to considerable debate and skepticism.
Prokaryotes diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct types of prokaryotes. For example, in addition to using
photosynthesis or organic compounds for energy like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as
hydrogen sulfide. This has enabled the bacteria to thrive and reproduce. Today,
archaebacteria can be found in the cold of
Antarctica and in the hot
Yellowstone springs.
Further Information
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