The
nucleus is a highly organised globular, ellipsoidal, spherical or disc-shaped protoplasmic body enclosed within the cytoplasm. The density of
nucleus is greater than that of cytoplasm.
The
nucleus generally lies in the central position of the cell and occupies a considerable proportion of the volume of the cell, sometimes as much as 2/3rd or 3/4th-but this proportion becomes less during differentiation. Due to the development of a big vacuole in the Centre of the cell the nucleus moves to the periphery. Nucleus is always formed from the pre existing one.
According to the number of nucleus present in a cell, a cell may be uninucleate binucleate or multinucleate. (a) Uninucleate-One nucleus is present in a cell, it is very common. (b) Binucleate-Two nuclei are present in a cell e.g. some tapetum cells of anther. (c) Multinucleate-Many nuclei are present in a cell, e.g. some cells of fungi.
A.
Structure of a Nucleus-
(a) The nucleus is surrounded by a thin fine protoplasmic membrane called nuclear membrane or nuclear envelope Which separates the nucleus from the cytoplasm. Under electron microscope, the nuclear membrane appears as a double membrane structure having pores at regular intervals. The nuclear membrane is connected with the endoplasmic reticulum.The contents of the nucleus merge with the surrounding cytoplasm through the pores of the nuclear membrane.
(b) The cavity of the nucleus is filled up with a colourless, jelly-like and non-staining or slightly chromophilic fluid called nuclear gel, nuclear sap, karyolymph or nucleoplasm. Nucleoplasm resembles the cytoplasm but is often of a different density. Nucleoplasm consists of a pattern of irregularly shaped particles or granules and is essentially protein in character.
(c) Within the nucleoplasm, a network or reticulate structure known as nuclear reticulum, karyotin network or chromonemata is present in dispersed state. The delicate threads of the nuclear reticulum i.e. chromonemata are the consituents of the chromosomes This thread-like reticulum of the chromosomes at the interphase stage (i.e.at the resting stage) of the nucleus is also referred to as chromatin which means an intensely staining substance, because of the affinity of the complex of DNA and protein (present in the chromosomes) to the basxc dyes. At this stage condensed and deeply stained chromatin masses are often seen in the nucleoplasm-this chromatin is called heterochromatin, but the chromatin which stains less deeply than heterochromatin is called euchromatin. Chromosomes consist of nucleoproteins in which the nucleic acid component is mainly DNA (deoxyribonucleic acid)-the carrier of genetic information in the form of segments called genes. In addition to DNA, chromosomes also contain proteins
including histones, as well as lipid substances.
(d) One or more spherical, thick, prominent and highly refractory bodies are also found in dispersed conditions within the nucleoplasm of each nucleus, those bodies are called nucleoli (singular : nucleolus). Nucleoli are formed during the telophase stage of nuclear division in association with specific regions of specific chromosomes of the complement. Nucleoli are not bounded by membranes. Nucleolus generally stains differently from the chromatin and is composed of a mixture of ribonucleic acid (RNA), protein and lipid. The function of nucleolus is not yet definitely known.
B.
Function of the nucleus-
The nucleus controls the development of the cell by means of its DNA which can regulate the synthesis of a type of RNA known as messenger RNA. Therefore nucleus controls all the vital functions of the cell. Experiments involving nuclear transplants have shown that the nucleus is not autonomous, but has an intimate relationship with the cytoplasm. Through the nucleus parent characters are transferred to the offspring.
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