Genomes

Abstract

A deep split within prokaryotes, separating Archaea and Bacteria, is well documented. Eukaryotes likely arose by symbiogenic origin due to the fusion of an archaean with a bacterium, suggesting that a ring of life best describes the major relationships of living organisms. Eukaryotes are characterized by the presence of a nucleus and mitochondria, with the latter evolved via endosymbiosis. Mitochondria are likely a prerequisite of organismal complexity, which is obviously more pronounced in eukaryotes. Fittingly, eukaryotes bear substantially larger genomes than archaeans and bacteria, but within eukaryotes there is no correlation between complexity and genome size. Eukaryote genomes are characterized by the presence of introns within coding genes and usually by a large amount of transposable elements. Two classes of transposable elements are distinguished based on their mode of transposition. Whereas DNA transposons are found in eukaryotes and prokaryotes, retrotransposons propagate via RNA intermediates and are mainly restricted to eukaryotes.