NUCLEUS

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)= V_N/V_C-V_N(V_N are V_C 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.