Eukaryota

Eukaryota, also called Eukarya in some classifications, is one of the three domains of life, comprising organisms with complex cellular organization. A defining feature is the presence of a membrane-bound nucleus and other membrane-bound organelles, organized cytoskeleton, and more complex regulation of the genome than in prokaryotes. The name derives from Greek eu meaning true and karyon meaning nucleus.

Most eukaryotes are composed of eukaryotic cells that contain a nucleus enclosed by a nuclear envelope, an internal membrane system (endoplasmic reticulum and Golgi apparatus), mitochondria, and a cytoskeleton. They typically have linear chromosomes and histone-associated DNA, and their cytoplasmic ribosomes are predominantly 80S, with organellar ribosomes often 70S. Many lineages also possess plastids derived from endosymbiotic events, enabling photosynthesis in plants and algae.

Eukaryotes reproduce in a variety of ways, most commonly sexually through meiosis and fertilization, though asexual reproduction occurs in many groups. They are believed to have arisen from an archaeal ancestor, acquiring mitochondria through endosymbiosis with an alpha-proteobacterium about 1.6–2.1 billion years ago; in plants and some algae, plastids originated from cyanobacterial symbionts.

Taxonomically, Eukaryota includes animals, plants, fungi, and numerous protists. Classification within Eukaryota is dynamic and often organized into supergroups or clades rather than rigid kingdoms. The domain is distinct from Bacteria and Archaea, the other two primary domains of life.