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Cell nucleus

The meaning of «cell nucleus»

In cell biology, the nucleus (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm; and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton supports the cell as a whole.

The cell nucleus contains all of the cell's genome, except for the small amount of mitochondrial DNA and, in plant cells, plastid DNA. Nuclear DNA is organized as multiple long linear molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell.

Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes.

Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, and a number of nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the nucleolus, which is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate messenger RNA.

The nucleus contains nearly all of the cell's DNA, surrounded by a network of fibrous intermediate filaments and enveloped in a double membrane called the "nuclear envelope". The nuclear envelope separates the fluid inside the nucleus, called the nucleoplasm, from the rest of the cell. The size of the nucleus depends on the size of the cell it is contained in, with a nucleus typically occupying about 8% of the total cell volume.[1] The nucleus is the largest organelle in animal cells.[2]:12 In mammalian cells, the average diameter of the nucleus is approximately 6 micrometres (µm).[3]

The nuclear envelope consists of two membranes, an inner and an outer nuclear membrane.[4]:649 Together, these membranes serve to separate the cell's genetic material from the rest of the cell contents, and allow the nucleus to maintain an environment distinct from the rest of the cell. Despite their close apposition around much of the nucleus, the two membranes differ substantially in shape and contents. The inner membrane surrounds the nuclear content, providing its defining edge.[2]:14 Embedded within the inner membrane, various proteins bind the intermediate filaments that give the nucleus its structure.[4]:649 The outer membrane encloses the inner membrane, and is continuous with the adjacent endoplasmic reticulum membrane.[4]:649 As part of the endoplasmic reticulum membrane, the outer nuclear membrane is studded with ribosomes that are actively translating proteins across membrane.[4]:649 The space between the two membranes, called the "perinuclear space", is continuous with the endoplasmic reticulum lumen.[4]:649

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