A eukaryote is an organism whose cells contain complex structures enclosed within membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried.
Theory of eukaryotes
Def. any "of the single-celled or multicellular organisms, of the taxonomic domain Eukaryota, whose cells contain at least one distinct nucleus" is called a eukaryote.
A nuclear membrane (also known as the nuclear envelope, nucleolemma or karyotheca) is a double lipid bilayer that encloses the genetic material in eukaryotic cells. The nuclear membrane also serves as the physical barrier, separating the contents of the nucleus (DNA in particular) from the cytoplasm. Many nuclear pores are inserted in the nuclear envelope, which facilitate and regulate the exchange of materials (proteins such as transcription factors, and RNA) between the nucleus and the cytoplasm.
The perinuclear space (PNS) is the space between the inner and outer nuclear membranes (lipid bilayers). The PNS is joined with the lumen of the rough endoplasmic reticulun (RER). The width of the PNS is about 20 - 40 nm. The perinulcear space is also called the perinuclear cisterna or nuclear envelope lumen.
The diagram at right indicates the various molecular structures of or associated with the nuclear envelope.
Nuclear pore complexes
There are about an average of 2000 nuclear pore complexes (NPCs), in the nuclear envelope of a vertebrate cell, but it varies depending on cell type and the stage in the life cycle. The proteins that make up the nuclear pore complex are known as nucleoporins. About half of the nucleoporins typically contain either an alpha solenoid or a beta-propeller fold, or in some cases both as separate structural domains. Each NPC contains at least 456 individual protein molecules and is composed of 30 distinct proteins (nucleoporins).
- There are organisms between prokaryotes and eukaryotes that have nuclei somewhat separated from cellular cytoplasm.
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