The structure and Characteristics of Nucleus - Cytology

THE NUCLEUS 
Every normal cell has one nucleus, and one only. Exceptions to this rule are rare, except in Fungi and in some reproductive structures. The Bacteria and the Cyanophyceae have no nucleus as it is understood in higher plants, though their cells may contain nuclear substances. In some Thallo­phyta large cells occur containing many small nuclei, and these structures are called coenocytes on the supposition that they may be really compound cells.
The fully developed nucleus, like the chloroplast, is an organ of remark­able uniformity, not only over great groups of the plant kingdom but throughout most of the animal 'world as well. This means that the structure of the nucleus is fundamental to the organization of the living cdl, and that conclusions drawn from its study must have a close bearing on the general problems of vitality. The nucleus is in many ways the centre of the life of the cell. If a cell is deprived of its nucleus it may continue to respire for some time, but it does not grow or reproduce. Whether the nucleus or the cytoplasm is the more ancient, that is to say which came first in evolution, cannot be definitely answered, but as some lowly organisms have no nuclei it seems possible that the nucleus was secondary, at least in its fully organized form.
Nucleus and cytoplasm are separated by the nuclear membrane, which seems to be a cytoplasmic structure like the tonoplast, but more delicate and less permanent. The nucleus it surrounds is usually more or less spherical. Inside the membrane is the karyolymph or nuclear sap, resembling the cell sap, except that it is very viscous and sometimes almost solid. In some respects therefore the nucleus may be looked upon as a special kind of vacuole. Where it differs from other vacuoles is in containing the substance chromatin, which plays an entirely special role in the cell. In it are located those factors which control the development of the cell and hence the development of the whole organism, and serve, moreover, to transmit the same characteristics to the follpowing generation. In short, the chromatin is the seat of the mechanism of heredity which maintains the continuity of the species.
The chromatin is visible as a spherical network of fine threads inside the nuclear membrane and in contact with it. This is called the reticulum of the nucleus, and it consists of simple proteins of the protamine and histone types. These are basic substances and they therefore stain easily with acid dyes. The nuclear surface is probably the seat of protein synthesis in the cell so that these proteins are formed in situ. At many points of the reticulum are seen lumps or knots of material which stain differently, chiefly with basic dyes. This is due to the fact that they contain thymo-nucleic acid, and they are spoken of as heterochromatin, while the rest of the reticulum is called euchromatin.
Thymo-nucleic acid differs from the ribo-nucleic acid that we spoke of connection with mitochondria. The chief difference lies in the sugar which forms part of the molecule. In ribo-nucleic acid there is, as the e implies, ribose, a stable sugar which can be isolated as a pure substance.
In thymo-nucleic acid, on the other hand, ribose is replaced by a molecular group, desoxy-ribose, which only exists in combination with other molecules, which has a strong tendency towards the linking of its molecules to one oilier in long chains to produce complexes or "polymers" of as many molecules, arranged in a similar manner to the molecules in the uunine proteins. This substance can be identified microscopically by a staining reaction known as the Feulgen Test, and its presence is taken as indictive of the existence of chromatin in a cell.
In addition to the reticulum each nucleus contains one or two rounded bodies nucleoli, which are attached to part of the reticulum. The nucleolus is not formed of chromatin. It consists of histone protein, with phospho­lip ins and ribo-nucleic acid, that is to say, it has chemical affinities with the mitochondria. It is ahl'ays attached to one part of the chromatin reticulum, and there seems to be an active exchange of material between the two.
The nucleus in this condition is properly called the metabolic nucleus, for its chief activity is in connection with cell metabolism. To distinguish it from the nucleus in active division it is sometimes also described as the· resting nucleus or the interphase nucleus, though the latter term only applies properly to tissues in which nuclear division is still active.

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