All eukaryotic cells contain a large, pale, spherical or oval body in their surrounding cytoplasm, the nucleus.
It is most prominent structure within the cell as it contains the genetical material and also functions in the control of cellular activities such as protein synthesis, cellular growth, and cellular reproduction.
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The nucleus is usually found located in the centre of the cell but variations have been observed in the location or position of the nucleus depending upon the metabolic state of the cell.
The nucleus is bounded by a delicate but clearly defined membrane, the nuclear membrane. Recent electron microscopic studies show that the nuclear membrane is composed of two unit membranes, viz., an outer unit membrane and an inner unit membrane.
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Each unit membrane is about 75 to 90A in thickness and lipoproteinous in nature (Picken, 1960; Kashing and Kasper. 1969; and Smith et. al., 1969). The outer and inner unit membranes remain separated by a space of 100 to 150A (Robertis et al., 1970) 100 to 300 A (Cohn 1970) or 400 to 700A'(Burke, 1970).
This inter membranous space is known as perinuclear space or cisterna. Penetrating the surface of the nuclear membrane are a large number of nuclear pores of 1000 A diameter.
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Because of the presence of these openings, which may be closed with some type of diaphragm, the nuclear membrane is not considered as a complete barrier between the cytoplasm and nucleoplasm.
These pores apparently permit only specific large molecules to go through, thus keeping the chemical composition of the nuclear material different from that of the cytoplasm.
It is believed that the nuclear membrane is composed of terminal parts of the endoplasmic reticulum or cisternae surrounding the nucleus (Watson, 1959, 1965; Grell, 1964) and that the pores which provide direct continuity between the nucleus and the cytoplasmic sap represent gaps between the “end-feet” of the endoplasmic reticulum.
The nuclear membrane is of profound importance because during interphase the nucleus can pass information to the cytoplasm only through the nuclear membrane in controlling or in directing the synthetic activities occurring in the cytoplasm of the cell (Zbarskii, 1969).
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Within the nuclear membrane there is a semifluid medium, the nuclear sap or nucleoplasm or karyolymph.
It is composed of mainly the nucleoproteins but it also contains other inorganic and organic substances including nucleic acids, proteins, enzymes, and minerals.
The nucleoplasm contains a tangle of fine coiled and elongated threads called the chromatin.
Such chromatin fibres are observed when the cell is not dividing, i.e., at interphase. During the cell division (mitosis or meiosis) chromatin fibres become thick ribbon-like structures which are known as chromosomes. Chemically the chromosomes are made up of DNA and histone proteins.
The most conspicuous body within the nucleus is the nucleolus. It is a large, spherical and acidophilic granule reported first by Fontona in 1781.
The nucleolus, which is composed of DNA and protein, like the chromosomes, is formed in association with certain portions (called the nucleolar organizers) of certain chromosomes.
It also contains long fibres, presumably composed of protein and perhaps serving as structural elements. It is the site at which a particular kind of RNA-the ribosomal RNA-is formed; hence, the nucleolus is rich in RNA. The nucleolus is most prominent when the cell is synthesizing protein.
The nucleus in general performs two crucial functions for the cell. First, it carries the hereditary information for the cell; second, it regulates the ongoing activities of the cell, ensuring that the complex molecules the cell requires are synthesized in the number and of the kind needed.
These molecules are involved in carrying out the various activities of the cell and also in forming organelles and other structures.