Eukaryotes

Eukaryotes are organisms whose cells contain a nucleus enclosed by a membrane and a variety of membrane-bound organelles. They include animals, plants, fungi, and many protists, spanning unicellular and multicellular forms. Eukaryotes differ from prokaryotes in having a true nucleus, a complex cytoskeleton, and extensive internal compartmentalization that allows specialized biochemistry.

Cell organization centers on the nucleus, which houses most of the genome and is bordered by a nuclear envelope with pores. The cytoplasm contains organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria, lysosomes, and, in plants and algae, chloroplasts. The cell membrane encloses the cell, while the cytoskeleton provides structure, intracellular transport, and mechanisms of cell division.

Genomes are usually organized into multiple linear chromosomes within the nucleus, and mitochondria and chloroplasts carry their own DNA. Eukaryotic cells divide by mitosis for growth and maintenance and by meiosis for sexual reproduction. Life cycles vary; most eukaryotes can reproduce both sexually and asexually.

Evolutionary origin: eukaryotes are believed to have arisen through endosymbiosis, with mitochondria derived from an ancestral aerobic bacterium and, in photosynthetic lineages, chloroplasts from cyanobacteria. Evidence includes double membranes, organellar ribosomes similar to bacteria, and circular DNA.

Classification and diversity: Eukaryotes constitute the domain Eukarya. Traditional schemes group animals, plants, and fungi, along with numerous protists; modern taxonomy emphasizes clades and phylogenetic relationships that cut across traditional ranks.