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Nucleus is the most conspicuous organelle located somewhat in the center of cytoplasm controlling all the vital activities of the cell. It was discovered in the year 1831 and named by Robert Brown. It has a delicate nuclear membrane that encloses a relatively dense nucleoplasm. In the nucleoplasm, there are thin thread-like structures referred to as chromatin fibres or chromosomes. 

Structure of the nucleus
It is composed of the following structures :

(i) Nuclear envelope: The nuclear envelope separates the nucleoplasm from the cytoplasm. It is made of two unit membranes: an outer membrane and an inner membrane. The space between the two membranes is called perinuclear space.

(ii) Nuclear sap or nucleoplasm:  The space between the nuclear envelope and the nucleolus is filled with a transparent semisolid, granular matrix known as nucleoplasm. It is composed of mainly nucleoprotein and also inorganic and organic substance.

(iii) Nuclear matrix: There is a network of fibrils which joins to the nuclear envelope by their ends and is called as nuclear matrix.

(iv) Chromatin: The thread like structures found in the chromosome are called chromatin fibers.

(v) Nucleolus: It was discovered by Fontana in 178. The number of nucleoli in the nucleus depends on the species and number of chromosomes.

Function of the nucleus

The nucleus is not only an integral part of the eukaryotic cell but also its controlling centre. The genes carried on the chromosomes are responsible for the determination of characters. Cellular metabolic activities are controlled by various enzymes. The synthesis and regulation of the activity of these enzymes are controlled by the nucleus. The mammalian red blood cells which are devoid of nuclei have only a limited lifespan and cannot undergo division or any repairs. If an amoeba is nucleated (removal of nucleus) it can survive for a few more days, but it cannot reproduce and will eventually fail to survive.

Nuclear envelope

In a living cell there is a steady flow of materials like RNA and ribosomal materials to the cytoplasm through the nuclear membrane. It is known that signals from the cytoplasm are passed back to the nucleus. The nucleocytoplasmic interaction is essential for the proper regulation of the metabolic activities of the cell.


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