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Containing the code for all of a cell’s enzymes and other proteins, the nucleus is the command center of the cell. The nucleus also holds the molecular machinery to replicate the DNA and to synthesize and process all types of RNA. Between cell divisions pore complexes in the membrane enclosing the nucleus regulate macromolecular transfer between the nuclear and cytoplasmic compartments. Mature RNA molecules pass into the cytoplasm for their roles in protein synthesis, while proteins needed for nuclear activities are imported there from the cytoplasm. Restricting protein synthesis to the cytoplasm helps ensure that newly made RNA molecules do not become involved in translation before processing is completed.
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COMPONENTS OF THE NUCLEUS
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The nucleus usually appears as a large rounded or oval structure, often near the cell’s center (Figure 3–1). Typically, the largest structure within a cell, it consists of a nuclear envelope enclosing the chromatin, the mass of DNA, and its associated proteins, with one or more specialized chromatin regions called nucleoli. In specific tissues, the size and shape of cell nuclei normally tend to be uniform.
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The nuclear envelope forms a selectively permeable barrier between the nuclear and cytoplasmic compartments. Electron microscopy reveals that the envelope has two concentric membranes separated by a narrow (30–50 nm) perinuclear space (Figures 3–2 and 3–3). This space and the outer nuclear membrane are continuous with the extensive cytoplasmic network of the rough endoplasmic reticulum (RER). Closely associated with the inner nuclear membrane is a highly organized meshwork of proteins called the nuclear lamina (Figure 3–4), which stabilizes the nuclear envelope. Major components of this layer comprise the class of intermediate filament proteins called lamins that bind to membrane proteins and associate with chromatin in nondividing cells.
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