A eukaryote is an organism belonging to the evolutionary lineage characterized by cells with a membrane-bound nucleus and an elaborate system of internal compartments. Eukaryotes include animals, plants, fungi, and the diverse organisms informally called protists. They range from single-celled yeasts and amoebas to large multicellular organisms. Their cellular organization distinguishes them from prokaryotes, which comprise bacteria and archaea and lack a membrane-bound nucleus. (openstax.org)
Cellular organization
The nucleus encloses most of a eukaryotic cell’s DNA within a double membrane called the nuclear envelope. Nuclear pores regulate exchanges between the nucleus and cytoplasm. Nuclear DNA is usually arranged in multiple linear chromosomes, associated with proteins that organize it into chromatin. A region within the nucleus, the nucleolus, participates in assembling ribosomal subunits. Eukaryotic cells are generally larger than bacterial and archaeal cells, although their size ranges overlap, so size alone does not reliably identify them. (openstax.org)
Like all cells, eukaryotic cells have a cell membrane, cytoplasm, and ribosomes. Their internal membranes divide cellular activities among specialized organelles. The endoplasmic reticulum participates in protein and lipid production, while the Golgi apparatus modifies and sorts materials for transport. Lysosomes and related compartments break down macromolecules; peroxisomes perform particular oxidative reactions. Compartmentalization allows different chemical environments and processes to coexist within one cell. (openstax.org)
The cytoskeleton provides structural organization and supports intracellular transport, movement, and chromosome segregation. Plant cells typically possess a cellulose-rich wall and a large central vacuole, whereas animal cells lack a cell wall. These differences illustrate that eukaryotic organization is a shared framework rather than a single uniform cellular design. (openstax.org)
Energy-producing organelles
Most eukaryotes contain mitochondria, which carry out important stages of aerobic cellular respiration and also perform other metabolic functions. Mitochondria have their own genomes, although many proteins needed for their operation are encoded by nuclear genes and imported from the cytoplasm. Anaerobic eukaryotes may possess highly modified mitochondrial derivatives rather than conventional mitochondria. A few lineages, including Monocercomonoides, have secondarily lost the organelle altogether; this does not imply that their ancestors lacked it. (openstax.org)
Plants and many algae also contain chloroplasts, members of the broader plastid family, which perform photosynthesis. Chloroplasts retain their own DNA and depend heavily on nuclear-encoded proteins. Neither chloroplasts nor photosynthetic activity define eukaryotes: animals, fungi, and many protists obtain organic nutrients without photosynthesis, and photosynthetic bacteria lack chloroplasts. (ncbi.nlm.nih.gov)
Genetic information and reproduction
The nuclear envelope separates transcription, the production of RNA from DNA templates, from most translation, the synthesis of proteins by ribosomes. RNA molecules can therefore undergo processing before entering the cytoplasm. This spatial separation is an important feature of eukaryotic gene expression, although mitochondria and plastids possess their own genetic-expression machinery. (ncbi.nlm.nih.gov)
During ordinary cell division, DNA replication precedes mitosis, in which duplicated chromosomes are distributed to daughter nuclei. Division of the cytoplasm usually follows, but nuclear division can occur without it, producing multinucleate cells. In unicellular organisms, division can constitute asexual reproduction; in multicellular organisms, it also supports growth and tissue maintenance. (openstax.org)
Sexual life cycles involve meiosis and fertilization. Meiosis reduces the number of chromosome sets and introduces genetic variation through chromosome assortment and crossing over. Fertilization restores the combined chromosome complement. The timing of these events varies: animals generally produce gametes through meiosis, whereas plants produce meiotic spores that develop into gamete-producing stages. Many eukaryotes alternate sexual and asexual reproduction. (openstax.org)
Evolutionary origin
Eukaryotic evolution involved both the development of complex cellular organization and the integration of formerly separate organisms. According to the endosymbiotic theory, mitochondria descend from an alphaproteobacterial partner that became established inside an ancestral host. Their bacterial affinities, residual genomes, and distinctive cellular machinery support this origin. The last common ancestor of living eukaryotes is inferred to have possessed mitochondria, even though some descendants subsequently reduced or lost them. (nature.com)
Genomic analyses identify Asgard archaea as the closest known archaeal relatives of eukaryotes. These archaea encode homologues of several proteins involved in eukaryotic cellular organization. Many evolutionary reconstructions place the host lineage within Archaea, rather than treating eukaryotes as an entirely separate primary branch. The precise relationship within Asgard and the sequence of steps leading to the nucleus, internal membranes, and mitochondrial integration remain subjects of research. (nature.com)
Plastids have a separate endosymbiotic history involving cyanobacteria. Photosynthetic eukaryotes subsequently became partners in further endosymbioses, spreading plastids into additional lineages. Thus, the histories of eukaryotic hosts and their organelles do not always follow the same branching pattern. (openstax.org)
Diversity and classification
Eukaryotes are commonly designated Eukarya or Eukaryota. Animals, fungi, and plants represent only part of their diversity. “Protist” is a useful informal designation, not a single natural evolutionary group: some protists are more closely related to animals, fungi, or plants than to other protists. Classification therefore increasingly uses molecular evidence to identify shared ancestry rather than grouping organisms solely by appearance or nutrition. (openstax.org)
Eukaryotic lifestyles include photosynthesis, ingestion of prey, absorption of dissolved nutrients, and combinations of these strategies. Their bodies may be unicellular, colonial, or multicellular. Multicellularity arose in several lineages rather than being a defining property of the entire group, and microscopic eukaryotes display substantial structural and metabolic diversity. (openstax.org)