Cell nucleus is spherical or semi spherical body present usually in the center of eukaryotic cells. In prokaryotic cell, nuclear material is dispersed throughout the cytoplasm without a definite membrane. Nucleus is often called 'Brain Of The Cell' as it contains cell's hereditary material DNA and coordinates its all metabolic activities including growth, metabolism, protein synthesis and reproduction (cell division) etc. As far as its size is concerned it occupies about 10 percent of total eukaryotic cell's volume.
Nuclear membrane is nuclear envelope that separates the nuclear material from the cellular cytoplasm. It is double layered membrane that encloses the nuclear material during most of the cell's life cycle. These two unit membranes run parallel to each other with a space between them called perinuclear space which is an electron transport area. At some places outer membrane gives out tubular structures called endoplasmic reticulum. Annuli are openings in nuclear envelope through which nucleoplasm communicates with the cytoplasm. These nuclear pores regulate the passage of molecules between the nucleus and cytoplasm allowing them to pass through the nuclear membrane. It is believed that these pores can open and close but its definite mechanism is not clear. During cell division (mitosis), the nuclear membrane disintegrates into vesicles which disappear in the cytoplasm. But soon at the reformation stage when two cells complete their formation and the chromatin begins to appear these vesicles again combine to reform nuclear membrane.
The semi-fluid matrix bounded by nuclear membrane is called nucleoplasm. Within nucleoplasm, a mass of thin fibrils called chromatin is present. This chromatin network is the actual site of genetic activities as it contains DNA and become highly coiled at the time of cell division to form chromosomes. Chromatin is so organized that it gives reticulate appearance. About 6 feet of DNA is present in every human cell, divided into 46 individual molecules (chromosomes). Chromosomes are chemically composed of DNA, RNA, Histone (basic proteins) and Hertones (non histone proteins). Calcium is also present in it. Genes, the fundamental units of heredity, are present on chromosomes on specific position called locus.
The nucleus also contains one or more dark, spherical, colloidal bodies called nucleoli (singular; nucleolus). Nucleolus is associated with a special region called 'Nucleolar Organizing Region' present in the chromosome. It is rich in RNA which passes to cytoplasm to take part in protein synthesis. Nucleoli are involved in manufacturing ribosomes. A nucleolus consists of RNA, acidic and basic dyes, phospholipids and alkaline phosphates. Under microscope nucleolus looks like a large dark spot within the nucleus. A nucleus may contain several nucleoli which sometimes merge to form a single and large nucleolus.