Home > Prokaryote
Prokaryotes are mostly unicellular organisms without a nucleus, in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between prokaryote and eukaryote cell structure is the most important in the living world. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. However, Woese has proposed dividing them into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) on the supposition that these have separate origins. This controversial arrangement is called the three-domain system.Prokaryotes do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of the human body.
The name prokaryote comes from the Greek pros meaning before and karyon meaning nut, referring to the nucleus. Prokaryotes also lack cytoskeletons and membrane-bound cell compartments such as vacuoles, endoplasmic reticulumThe endoplasmic reticulum or ER (endoplasmic means "within the cytoplasm", reticulum means "little net") is an organelle found in all eukaryotic cells. The ER modifies proteins, makes macromolecules, and transfers substances throughout the cell. Prokaryot, mitochondriaIn cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. Mitochondria are sometimes described as " cellular power plants" because their primary purpose is to manufacture adenosine triphosphate (ATP), which is used as a sourc and chloroplastChloroplasts are organelles found in plant cells and eukaryotic algae which conduct photosynthesis. Chloroplasts are similar to mitochondria but are found only in plants. Both organelles are surrounded by a double membrane with an intermembrane space; bots. In eukaryotes, the latter perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur within the cell membrane, and endosymbiontAn endosymbiont (also known as intracellular symbiont) is any organism that lives within cells of another organism, i. forming an endosymbiosis . For instance, some nitrogen fixing bacteria (e. in Rhizobium Sinorhizobium and Bradyrhizobium live in plants,s are extremely rare. They are usually much smaller than eukaryotic cells.
Prokaryotes have a single chromosome, contained within a nucleoid region rather than a membrane-bound nucleus, but may also have various small circular pieces of DNA called plasmidPlasmids are (typically) circular double stranded DNA molecules that are separate from the chromosomal DNA (Fig. They usually occur in bacteria, sometimes in eukaryotic organisms (e. the 2-micrometre-ring in Saccharomyces cerevisiae . Their size varies frs spread throughout the cell. Reproduction is exclusively asexual, through binary fissionIn biology, binary fission is the asexual reproductive process used by prokaryotes and results in the reproduction of a living cell by division into two equal, or near equal, parts. It begins when the DNA of the cell is duplicated. Duplication of DNA insi, where the chromosome is duplicated and attaches to the cell membrane, and then the cell divides in two. However, they show a variety of parasexual processes where DNA is transferred between cells, such as transformationTransformation is the genetic alteration of a cell resulting from the introduction, uptake and expression of foreign DNA. This is a common technique in molecular biology. The effect was first demonstrated in 1944 by Oswald Avery, Collin Macleod, and Macly and transduction.
It is generally accepted that the first living cells were some form of prokaryote, and they are known as fossils from over 3.5 billion years ago. Some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes, but this is extremely doubtful.
