Prokaryote

Prokaryotes are single-celled organisms such as bacteria that have no distinct nucleus. In addition to the lack of a nucleus, prokaryotes lack many of the other small organelles found in the larger eukaryotic cells.

A typical prokaryote is bound by a plasma membrane and a cell wall. Within this double boundary, the fluid material inside the cell (the cytoplasm) is studded with small, rounded bodies called ribosomes. The ribosomes are composed of nucleic acids and proteins, and function in protein synthesis. The chromosomes containing the hereditary material of prokaryotes are concentrated within a region called the nucleoid. Because the nucleoid is not separated from the rest of the cytoplasm by a membrane, it is not considered a true nucleus. Dissolved in the cytoplasm of prokaryotes are the various chemicals needed by the cell to function.

Prokaryotes were the first organisms to evolve on Earth, predating eukaryotes in the fossil record by about one billion years. Appearing on Earth 3.5 billion years ago, the first prokaryotes were probably bacteria that performed photosynthesis (cyanobacteria), which is a process that produces carbohydrates from sunlight, water, and carbon dioxide.

Eukaryotes are thought to have evolved when cells engulfed prokaryotic cells, and incorporated them into their cytoplasm. Some of the eukaryotic organelles, particularly mitochondria (the organelle that contains energy-producing enzymes) and chloroplasts (the organelle that contains photosynthetic enzymes in photosynthetic cells) resemble individual free-living prokaryotic cells. Supporting this theory (called the endosymbiotic theory) is the fact that mitochondria and chloroplasts have their own DNA sequences, as if they were once separate organisms in their own right.

Prokaryotes are divided taxonomically into two large groups: the archaebacteria and the eubacteria. Archaebacteria are probably little changed from the organisms that first evolved billions of years ago. They are capable of living in extremely harsh environments,

such as salt marshes, hot springs, or even beneath the ice. Eubacteria evolved later. Some are photosynthetic bacteria; some are chemosynthetic bacteria, making carbohydrates from other chemicals besides carbon dioxide; and some are heterotrophic bacteria, deriving nutrients from the environment. Heterotrophic prokaryotes include some pathogens, bacteria that cause diseases, such as pneumonia, food poisoning, and tuberculosis.

The relative simplicity of prokaryotes, as compared to eukaryotes, extends to the genetic level. The prototypical bacterial species Escherichia coli contains approximately 5,000 genes. On average, about one in every 200 bacteria is likely to have a mutation in at least one of the genes. In a 100 milliliter culture containing one million bacteria per milliliter, this translates to 500,000 mutant bacteria. This ability of prokaryotes to mutant and so quickly adapt to a changing environment is the principle reason for their success through time.

The ecological distribution of the Prokaryotae is vast. Bacteria have adapted to live almost everywhere, in environments as diverse as the thermal deep-sea vents to the boiling hot springs of Yellowstone National Park, from the soil to the intestinal tract of man and animals. The diversity of bacteria led to the design of a classification system just for them. David Hendricks Bergey spearheaded this classification scheme in the first half of the twentieth century. His efforts culminated in the publication (and ongoing revisions) of the Bergey’s Manual of Systematic Bacteriology.

Prokaryote

Prokaryotes are single-celled organisms such as bacteria that have no distinct nucleus. In addition to the lack of a nucleus, prokaryotes lack many of the other small organelles found in the larger eukaryotic cells.

A typical prokaryote is bound by a plasma membrane and a cell wall. Within this double boundary, the fluid material inside the cell (the cytoplasm) is studded with small, rounded bodies called ribosomes . The ribosomes are composed of nucleic acids and proteins , and function in protein synthesis. The chromosomes containing the hereditary material of prokaryotes are concentrated within a region called the nucleoid. Because the nucleoid is not separated from the rest of the cytoplasm by a membrane, it is not considered a true nucleus. Dissolved in the cytoplasm of prokaryotes are the various chemicals needed by the cell to function.

Prokaryotes were the first organisms to evolve on Earth , predating eukaryotes in the fossil record by about one billion years. Appearing on Earth 3.5 billion years ago, the first prokaryotes were probably bacteria that performed photosynthesis (cyanobacteria), which is a process that produces carbohydrates from sunlight, water , and carbon dioxide .

Eukaryotes are thought to have evolved when cells engulfed prokaryotic cells, and incorporated them into their cytoplasm. Some of the eukaryotic organelles, particularly mitochondria (the organelle that contains energy-producing enzymes) and chloroplasts (the organelle that contains photosynthetic enzymes in photosynthetic cells) resemble individual free-living prokaryotic cells. Supporting this theory (called the endosymbiotic theory) is the fact that mitochondria and chloroplasts have their own DNA sequences, as if they were once separate organisms in their own right.

Prokaryotes are divided taxonomically into two large goups: the archaebacteria and the eubacteria . Archaebacteria are probably little changed from the organisms that first evolved billions of years ago. They are capable of living in extremely harsh environments, such as salt marshes, hot springs, or even beneath the ice . Eubacteria evolved later. Some are photosynthetic bacteria; some are chemosynthetic bacteria, making carbohydrates from other chemicals besides carbon dioxide; and some are heterotrophic bacteria, deriving nutrients from the environment. Heterotrophic prokaryotes include some pathogens , bacteria that cause diseases, such as pneumonia , food poisoning , and tuberculosis .

See also Eukaryotae.

prokaryote (formerly Monera) Any member of the biological kingdom (Prokaryotae); it includes bacteria and cyanobacteria (formerly blue-green algae). They have more simple cells than other organisms. DNA is not contained in chromosomes in the nucleus, but lies in a distinct part of the cytoplasm, called the nucleoid. They have no distinct membrane-surrounded structures (organelles). Cell division is simple and, in the rare cases where sexual reproduction occurs, genetic material is simply transferred from one partner to another; there are no separate sex cells. In photosynthetic prokaryotes, photosynthesis takes place on the cell membrane. At present, two subkingdoms are recognized: Archaebacteria and Eubacteria. See also asexual reproduction; eukaryote; symbiosis

prokaryote A type of organism that is mainly unicellular and in which the cells lack a true nucleus; in all known prokaryotes the DNA is present as a loop in the cytoplasm. Other prokaryotic features include the lack of chloroplasts and mitochondria and the possession of small ribosomes. The prokaryotes comprise two groups (domains): Eubacteria (the bacteria, including the cyanobacteria, formerly known as ‘blue-green algae’) and Archaea (see EXTREMOPHILES).

pro·kar·y·ote / prōˈkarēˌōt/ (also pro·car·y·ote) • n. Biol. a microscopic single-celled organism, including the bacteria and cyanobacteria, that has neither a distinct nucleus with a membrane nor other specialized organelles. Compare with eukaryote.DERIVATIVES: pro·kar·y·ot·ic / prōˌkarēˈätik/ adj.