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 contains the molecular machinery to replicate the DNA and to synthesize and process all types of RNA. During interphase, 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 complete.
COMPONENTS OF THE NUCLEUS
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 containing chromatin, the mass of DNA and its associated proteins, with one or more specialized regions of chromatin called nucleoli. In specific tissues the size and shape of nuclei normally tend to be uniform.
Nuclei of large, active cells.
Liver cells have large, central nuclei. One or more highly basophilic nucleoli are visible within each nucleus, indicating intense protein synthesis by these cells. Most of the chromatin is light staining or euchromatic, with small areas of more darkly stained heterochromatin scattered throughout the nucleus and just inside the nuclear envelope. This superficial heterochromatin allows the boundary of the organelle to be seen more easily by light microscopy. One cell here has two nuclei, which is fairly common in the liver. (X500; Pararosaniline–toluidine blue)
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 are the class of intermediate filament proteins called lamins that bind to membrane proteins and associate with chromatin in nondividing cells.
Relationship of nuclear envelope to the rough ER (RER).
Three-dimensional representation of a cell nucleus shows a single large nucleolus and the distribution of the nuclear pores in the nuclear envelope. The outer membrane of the nuclear envelope is continuous with the RER. (TEM X20,000)
Ultrastructure of a nucleus.