Eukaryotes

'Eukaryote' comes from the Greek Eu, meaning true, and karyon, nucleus - and a eukaryotic cell is defined precisely by having a true nucleus, with its DNA properly enclosed inside a nuclear membrane. Within that nucleus, the DNA is not loose the way it is in a prokaryote; it is wound around histone proteins, and that DNA-plus-histone complex is what forms the structures we call chromosomes. Eukaryotic cells are also, in general, far more internally complex than prokaryotic ones, because they contain numerous distinct membrane-bound organelles - mitochondria, endoplasmic reticulum, Golgi apparatus, and, in plants and algae, chloroplasts - none of which a prokaryotic cell has. Interestingly, some of these very organelles are themselves believed to have started out as separate, free-living prokaryotic cells: the endosymbiosis theory proposes that mitochondria and chloroplasts descend from once-independent bacteria that were eng …