Nucleus is specialized protoplasmic body much denser than the cytoplasm and is commonly spherical or oval in shape. It always lies embedded in the cytoplasm. Nucleus is universally present in all living cells. In the higher plants there is single nucleus in each cell, while in many algae and fungi numerous nuclei may be present. In lower organisms like bacteria and blue green algae true nuclei are absent but there is a corresponding nuclear material. Nuclei may vary widely in sizes from 1 to 500 microns. A nucleus can never be newly formed but it multiplies in number by division of the pre existing one.
STRUCTURE
Each nucleus is surrounded by a thin transparent membrane known as the nuclear membrane which separates the nucleus from the surrounding cytoplasm within the membrane, completely filling up the space. There is a dense but clear mass of protoplasm known as the nuclear sap or nucleoplasm.
Suspended in the nucleoplasm there are numerous fine crooked threads loosely connected here and there forming a sort of network called the nuclear reticulum or chromatin network. The threads are made of a substance known as chromatin or nuclein which is strongly stainable.
The chromatin or nuclein is a nucleoprotein. One or more highly refractive relatively large and usually spherical bodies may be seen in the nucleoplasm. These are known as the nuclei.
CHEMICAL POSITION
Chemical position of nucleus is more or less similar to that of the cytoplasm. The nucleus however is predominantly composed of nucleoprotein which is made of phosphorus containing nucleic acids and certain specific types of proteins. All plant proteins contain C, H, O, N and S and sometimes P. Two important nucleic acids are DNA (Deoxyribo nucleic acid) and RNA (ribo nucleic acid). DAN occurs in the nuclear reticulum and RNA in the nucleolus, chromosome and cytoplasm. Nucleus also contains some amount of lipids, particularly phospholipids. Inorganic salts such as those of Ca, Mg, Fe and Zn are also present in the nucleus.
Nucleic acids are universally present in the nucleus and in the cytoplasm of all living cells and form the chemical basis of life. They are very complex organic compounds made of phosphate, 5 carbon (pentose) sugar (ribose in RNA) or (Deoxyribose in DNA) and nitrogen bases.
DNA AND RNA
DNA and RNA particularly the former are now known to be most important constituents of living cells. RNA occurs in the nucleoli chromosomes are cytoplasm (about 90% of the cells RNA occur in the cytoplasm) while DNA occurs in the chromosomes (nuclear reticulum). They are related chemically, consisting of 5 carbon ribose sugar and phosphate but DNA has deoxyribose with one less oxygen atom in its molecule. Both are large molecules or macromolecules but DNA is a double stranded molecule while RNA is singled stranded one. All secrets of life are embodied in DNA or in other words it is the chemical basis of life. DNA is the controlling centre of all the vital activities of the cell. DNA is sole genetic (hereditary) material (migrating intact from generation to generation through the reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls the biosynthetic processes of the cell including protein synthesis. RNA is a chemical messenger and plays a key role in the process of protein synthesis.
FUNCTIONS
Nucleus is controlling centre of vital activities of the cell in many ways.
(1) Nucleus takes direct part in reproduction.
(2) Nucleus takes initiative in cell division.
(3) Nucleus is the bearer of hereditary characters.
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