eukaryote (yōkârˈē-ōtˌ) [key], a cell or organism composed of cells that have a membrane-bound nucleus and organelles (mitochondria, chloroplasts; see cell, in biology) and genetic material organized in chromosomes in which the DNA is combined with histone proteins. Eukaryotes are contrasted with the prokaryotes (see Monera). Eukaryotes formed through the merger of prokaryotes, which predate them in the fossil record by some 2 billion years. In the five-kingdom system of classification, the eukaryotes have comprised the taxonomic kingdoms Protista, Fungi, Plantae (see plant), and Animalia (see animal). In a recently proposed system they are called the eukarya and classified as an overarching group (domain) above the kingdom level.