First described and christened by Robert brown 1831, a nucleus is the largest and most important organelle of eukaryotic cells. It is membrane bound and generally located near the centre of the cells. It contains the basic genetic material DNA which guides and controls the entire structural pattern and working of the cell in accordance with the coded information or genetic blueprint contained in it different compartments of this controlling material and the material in which the controlled processes occur has enabled eukaryotic cells to achieve high degree of structural complexity and functional efficiency as integrated units of life .Study of nuclei is now a separate branch of biology, named karyology.
Typically, the eukaryotic cells are mononucleate, i.e. Each cells has a single nucleus but binucleate and multinucleate cells are also found. Some animals possess multinucleate protoplasmic masses called syncytia. A syncytium may be formed by merger of uninucleate cells as in skeletal muscles. Or rarely by repeated nuclear division without corresponding cytoplasmic divisions coenocytic mass or plasmodium .
Shape, size and structure. In some cells the nucleus is indented and irregular in shape, or its shape corresponds to that of the cells itself , but mostly it is spheroidal or ellipsoidal .Its size depends upon its protein and DNA contents which are in turn , related to the structural complexity and functional activities of the cell. However, since all cytoplasmic activities are controlled by nuclear activities cytoplasmic and nuclear masses remain in a state of optimum equilibrium called nucleoplasmic index NP or KP , This index is numerically expressed as
NP(=KP)= VN/VC-VN(VN are VC are the volumes respectively of the nucleus and the cell)
When, due to growth the cytoplasmic mass increases, the karyoplasmic index gets disturbed. Then the cells has to divide to restore equilibrium.
Nuclear structure can be described under four subheads viz nuclear envelope , nucleoplasm, Chromatin filaments and nucleolus.