Eukarya

Eukarya, or eukaryotes, constitutes one of the three domains of life. Organisms in this domain are defined by cells that contain a true nucleus enclosed by a nuclear membrane and by membrane-bound organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria, and, in many lineages, chloroplasts. Eukaryotic cells typically have linear chromosomes organized with histones and a complex cytoskeleton that supports cell shape, movement, and intracellular transport.

Key cellular features include the presence of an internal membrane system, a nuclear envelope surrounding genetic material, and a generally larger and more complex genome compared with prokaryotes. Most eukaryotes reproduce sexually through meiosis and fertilization, though many also undergo asexual reproduction via mitosis. Mitochondria are a central energy-producing organelle in nearly all eukaryotes, and plastids such as chloroplasts are present in photosynthetic lineages, having originated through endosymbiosis with cyanobacteria.

Evolutionarily, eukaryotes are believed to have arisen when an archaeal host cell formed a symbiotic relationship with bacteria, giving rise to the nucleus and other organelles. Mitochondria and chloroplasts trace their origins to endosymbiotic events. This lineage gave rise to a vast diversity, including the kingdoms Animalia, Plantae, Fungi, and numerous protist groups.

Taxonomically, Eukarya is the domain that encompasses a wide range of life forms, from single-celled protists to complex multicellular organisms. Classifications vary, with traditional kingdoms such as Protista, Fungi, Plantae, and Animalia, while modern phylogenies emphasize major clades and their relationships.