A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a cell nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' or 'kernel'. In the earlier two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. However, in the three-domain system, based upon molecular phylogenetics, prokaryotes are divided into two domains: Bacteria and Archaea. A third domain, Eukaryote, consists of organisms with nuclei.
Prokaryotes evolution before eukaryotes, and lack nuclei, mitochondria, and most of the other distinct organelles that characterize the eukaryotic cell. Some unicellular prokaryotes, such as cyanobacteria, form colonies held together by , and large colonies can create multilayered . Prokaryotes are asexual, reproducing via binary fission. Horizontal gene transfer is common as well.
Molecular phylogenetics has provided insight into the interrelationships of the three domains of life. The division between prokaryotes and eukaryotes reflects two very different levels of cellular organization; only eukaryotic cells have an nuclear membrane that contains its DNA, and other membrane-bound organelles including mitochondria. More recently, the primary division has been seen as that between Archaea and Bacteria, since eukaryotes may be part of the archaean clade and have multiple homologies with other Archaea.
Prokaryotic cells are generally smaller and similar than eukaryotic cells. Prokaryotic cells do not enclose their genetic material within a nucleus.
Structure
Prokaryotes have simple cell skeletons. These are highly diverse, and contain homologues of the eukaryote proteins actin and tubulin. The cytoskeleton provides the capability for movement within the cell.
Most prokaryotes are between 1 and 10 μm, but they vary in size from 0.2 μm in Thermodiscus spp. and Mycoplasma genitalium to 750 μm in Thiomargarita namibiensis.
Bacterial cells have various shapes, including spherical or ovoid cocci, e.g., Streptococcus; cylindrical bacilli, e.g., Lactobacillus; spiral bacteria, e.g., Helicobacter; or comma-shaped, e.g., Vibrio. Archaea are mainly simple ovoids, but Haloquadratum is flat and square.
| + Parts of the prokaryote cell (2025). 9780073383071, McGraw-Hill Education. ISBN 9780073383071 | |
| Flagellum (not always present) | Long, whip-like protrusion that moves the cell. |
| Cell membrane | Surrounds the cell's cytoplasm, regulates flow of substances in and out. |
| Cell wall (except in Mollicutes, Thermoplasma) | Outer covering that protects the cell and gives it shape. |
| Cytoplasm | A watery gel that contains enzymes, salts, and organic molecules. |
| Ribosome | Structure that produces proteins as specified by DNA. |
| Nucleoid | Region that contains the prokaryote's single DNA molecule. |
| Glycocalyx (only in some groups) | Glycoprotein covering outside the cell membrane. |
Reproduction and DNA transfer
In bacteria, gene transfer occurs by three processes. These are virus-mediated transduction; conjugation; and natural transformation.
Transduction of bacterial genes by bacteriophage viruses appears to reflect occasional errors during intracellular assembly of virus particles, rather than an adaptation of the host bacteria. There are at least three ways that it can occur, all involving the incorporation of some bacterial DNA in the virus, and from there to another bacterium.
Conjugation involves , allowing plasmid DNA to be transferred from one bacterium to another. Infrequently, a plasmid may integrate into the host bacterial chromosome, and subsequently transfer part of the host bacterial DNA to another bacterium.
Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the water around them. This is a bacterial adaptation for DNA transfer, because it depends on the interaction of numerous bacterial gene products.
The bacterium must first enter the physiological state called competence; in Bacillus subtilis, the process involves 40 genes. The amount of DNA transferred during transformation can be as much as a third of the whole chromosome. Transformation is common, occurring in at least 67 species of bacteria.
Among archaea, Haloferax volcanii forms cytoplasmic bridges between cells that transfer DNA between cells, while Sulfolobus solfataricus transfers DNA between cells by direct contact. Exposure of S. solfataricus to agents that damage DNA induces cellular aggregation, perhaps enhancing homologous recombination to increase DNA repair.
Colonies and biofilms
Microcolonies may join above the substratum to form a continuous layer. This structure functions as a simple circulatory system by moving water through the biofilm, helping to provide cells with oxygen which is often in short supply. The result approaches a multicellular organisation. Differential cell expression, collective behavior, signaling (quorum sensing), programmed cell death, and discrete biological dispersal events all seem to point in this direction.
Environment
The first organisms
The oldest prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after the formation of the Earth's crust. Eukaryotes only appear in the fossil record later. The oldest fossil eukaryotes are about 1.7 billion years old.Carl Woese, J Peter Gogarten, " When did eukaryotic cells (cells with nuclei and other internal organelles) first evolve? What do we know about how they evolved from earlier life-forms?" Scientific American, October 21, 1999.
Evolution
Taxonomy and phylogeny
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. Archaea were originally thought to be extremophiles, living only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of . The resulting arrangement of Eukaryota (also called "Eucarya"), Bacteria, and Archaea is called the three-domain system, replacing the traditional two-empire system.
Knowledge of prokaryote taxonomy is rapidly changing in the 21st century with the sequencing of large numbers of genomes, many of these without the isolation of cultures of the organisms involved. As of 2021, consensus had not been reached among taxonomists to rely exclusively on genomes as opposed to existing practices, describing species from cultures.
According to the 2016 phylogenetic analysis of Laura Hug and colleagues, using genomic data on over 1,000 organisms, the relationships among prokaryotes are as shown in the tree diagram. Bacteria dominate the diversity of organisms, shown at left, top, and right in the diagram; the archaea are shown bottom centre, and the eukaryotes in the small green area at bottom right. As represented by red dots on the diagram, there are multiple major lineages where no representative has been isolated: such lineages are common in both bacteria (such as Omnitrophica and Wirthbacteria) and archaea (such as Parvarchaeota and Lokiarchaeota). At the lower levels (species to class) and up to the level of phylum, the data provide strong support for the groupings, but the deepest (oldest) branches of the phylogeny are more uncertain.
The large diversity of bacterial lineages shown in purple on the right of the diagram. These represent the so-called "candidate phyla radiation of bacteria", namely those with a combination of small genomes and reduced metabolic capabilities: none of them have been found to be able to carry out the whole of the citric acid cycle by which many cells release usable energy, and few can synthesise and , building blocks of and . This may represent an ancient condition, or a loss of capabilities of symbiotic organisms.
As distinct from eukaryotes
Both eukaryotes and prokaryotes contain which produce proteins as specified by the cell's DNA. Prokaryote ribosomes are smaller than those in eukaryote cytoplasm, but similar to those inside mitochondria and , one of several lines of evidence that those organelles derive from bacteria incorporated by symbiogenesis.
The genome in a prokaryote is held within a DNA/protein complex in the cytosol called the nucleoid, which lacks a nuclear envelope. The complex contains a single circular chromosome, a cyclic, double-stranded molecule of stable chromosomal DNA, in contrast to the multiple linear, compact, highly organized found in eukaryotic cells. In addition, many important genes of prokaryotes are stored in separate circular DNA structures called plasmids. Like eukaryotes, prokaryotes may partially duplicate genetic material, and can have a haploid chromosomal composition that is merodiploid.
| + Prokaryotes vs Eukaryotes |
Prokaryotes lack mitochondrion and chloroplasts. Instead, processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic cell membrane. Prokaryotes possess some internal structures, such as prokaryotic cytoskeletons. It was previously suggested that the bacterial phylum Planctomycetota has a membrane around the nucleoid and contains other membrane-bound cellular structures. Further investigation revealed that Planctomycetota cells are not compartmentalized or nucleated and, like other bacterial membrane systems, are interconnected.
Prokaryotic cells are usually much smaller than eukaryotic cells. This causes prokaryotes to have a larger surface-area-to-volume ratio, giving them a higher metabolic rate, a higher growth rate, and as a consequence, a shorter generation time than eukaryotes.
Eukaryotes as Archaea
| + Eukaryotes as Archaea |
Another view is that the most important difference between biota may be the division between Bacteria and the rest (Archaea and Eukaryota). DNA replication differs fundamentally between the Bacteria and Archaea (including that in eukaryotic nuclei), and it may not be homologous between these two groups.
Further, ATP synthase, though homologous in all organisms, differs greatly between bacteria (including eukaryotic such as mitochondria and chloroplasts) and the archaea/eukaryote nucleus group. The last common ancestor of all life (called LUCA) should have possessed an early version of this protein complex. As ATP synthase is obligate membrane bound, this supports the assumption that LUCA was a cellular organism. The RNA world hypothesis might clarify this scenario, as LUCA might have lacked DNA, but had an RNA genome built by ribosomes as suggested by Woese.
A RNP world has been proposed based on the idea that may have been built together with primordial nucleic acids at the same time, which supports the concept of a ribocyte as LUCA. The feature of DNA as the material base of the genome might have then been adopted separately in bacteria and in archaea (and later eukaryote nuclei), presumably with the help of some viruses (possibly retroviruses as they could reverse transcribe RNA to DNA).
See also
External links
- Prokaryote versus eukaryote, BioMineWiki
- The Taxonomic Outline of Bacteria and Archaea
- The Prokaryote-Eukaryote Dichotomy: Meanings and Mythology
- Quiz on prokaryote anatomy
- TOLWEB page on Eukaryote-Prokaryote phylogeny
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