eukaryooteissa

Eukaryooteissa refers to eukaryotes, organisms whose cells contain a true nucleus and other membrane-bound organelles. They form the domain Eukaryota (also called Eukarya) and include animals, plants, fungi, and a large and diverse set of protists, ranging from single-celled organisms to multicellular forms.

A defining feature is cellular compartmentalization. The genetic material is housed within a membrane-bound nucleus. The cell contains membrane-bound organelles such as mitochondria, and in photosynthetic lineages chloroplasts. An elaborate endomembrane system, a cytoskeleton, and complex mechanisms for transcription, RNA processing, and protein sorting distinguish eukaryotic cells from prokaryotic ones. Ribosomes in the cytosol are typically 80S, while mitochondria and plastids retain their own 70S ribosomes.

Most eukaryotes reproduce sexually, via meiosis and fertilization, though many also reproduce asexually. Their genomes are organized into linear chromosomes associated with histone proteins, enabling extensive regulation of gene expression and development.

Origin and evolution: eukaryotes are believed to have arisen through endosymbiotic events in which ancestral cells acquired mitochondria, and, in photosynthetic lineages, chloroplasts. Mitochondria are derived from alpha-proteobacteria and chloroplasts from cyanobacteria. The earliest confidently dated eukaryotes appear around 1.6–2.1 billion years ago.

Taxonomy has shifted from traditional kingdoms toward clade-based classifications, but the term eukaryote remains the standard way to denote all organisms with true nuclei and organelles. Diversity includes unicellular organisms such as ciliates and diatoms as well as multicellular groups like animals, plants, and fungi.