Characteristics of Eukarya
Eukaryotes (members of of the domain Eukarya – stemming from the Greek eu, “good” and karyon, “seed”) are one of the three domains of life. Members of Eukarya all have a membrane-bound nucleus and membrane bound organelles. While eukaryotes include multicellular species (i.e. plants, animals, and fungi), many unicellular eukaryotes exists, known as protists.
The first eukaryotes are hypothesized to have emerged sometime between 1.6 and 2.1 billion years ago. Eukarya is widely considered (but not uncontested) to be a monophyletic group, stemming from a common ancestor. Phylogenetic analyses have supported this. It is also supported with the internal anatomy of eukaryotes.
All eukaryotes have a nucleus with linear DNA; prokaryotes (in the domains Bacteria and Archaea) do not have a nucleus and their DNA is circular. Eukaryotes also have a wide variety of membrane-bound organelles not found in prokaryotes: endoplasmic reticula, Golgi apparatus, mitochondria, lysosomes, and vacuoles.
While nearly all eukaryotes have all these organelles, some species have evolutionarily lost these organelles (likely due to a mistake in cytokinesis). For example, the fungi (Saccharomyces cerevisiae), better known as Brewer’s yeast, has lost its mitochondria. Subsequently yeast is not capable of undergoing the citric acid cycle or the electron transport chain of cellular respiration.