Eukaryotes belong to the taxon Eukarya or Eukaryota. The defining feature that sets eukaryotic cells apart from prokaryotic cells (Bacteria and Archaea) is that they have membrane-bound organelles, especially the nucleus, which contains the genetic material, and is enclosed by the nuclear envelope. The presence of a nucleus gives eukaryotes their name, which comes from the Greekεὖ (eu, “well”) and κάρυον (karyon, “nut” or “kernel”). Eukaryotic cells also contain other membrane-bound organelles such asmitochondria and the Golgi apparatus. In addition, plants and algae contain chloroplasts. Eukaryotic organisms may be unicellular, ormulticellular. Only eukaryotes have many kinds of tissue made up of different cell types.
Eukaryotes can reproduce both by asexual reproduction through mitosis and sexual reproduction through meiosis. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells each with half the number of chromosomes as the original parent cell (haploid cells). These act as sex cells (gametes – each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes) resulting from genetic recombination during meiosis.
The domain Eukaryota appears to be monophyletic, and so makes up one of the three domains of life. The two other domains,Bacteria and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things;even the cells in a human’s body are outnumbered ten to one by bacteria in the gut. However, due to their much larger size, eukaryotes’ collective worldwide biomass is estimated at about equal to that of prokaryotes. Eukaryotes first developed approximately 1.6–2.1 billion years ago.
A prokaryote is a single-celled organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle. The word prokaryote comes from the Greek πρό (pro) “before” and καρυόν (karyon) “nut orkernel“. Prokaryotes can be divided into two domains, Archaea and Bacteria. Species with nuclei and organelles are placed in the domain Eukaryota.
In the prokaryotes all the intracellular water-soluble components (proteins, DNA and metabolites) are located together in thecytoplasm enclosed by the cell membrane, rather than in separate cellular compartments. Bacteria, however, do possess protein-based bacterial microcompartments, which are thought to act as primitive organelles enclosed in protein shells.Some prokaryotes, such as cyanobacteria may form large colonies. Others, such as myxobacteria, have multicellular stages in their life cycles.
Molecular studies have provided insight into the evolution and interrelationships of the three domains of biological species.Eukaryotes are organisms, including humans, whose cells have a well defined membrane-bound nucleus (containing chromosomal DNA) and organelles. The division between prokaryotes and eukaryotes reflects the existence of two very different levels of cellular organization. Distinctive types of prokaryotes include extremophiles and methanogens; these are common in some extreme environments.