Describe the nucleus in detail.

(N/A) $\Rightarrow$ The nucleus as a cell organelle was first described by Robert Brown in $1831$. Later,the material of the nucleus stained by basic dyes was given the name chromatin by Flemming.
$\Rightarrow$ The interphase nucleus has highly extended and elaborate nucleoprotein fibres called chromatin,nuclear matrix,and one or more spherical bodies called nucleoli.
$\Rightarrow$ The nuclear envelope consists of two parallel membranes with a space between them ($10$ to $50 \ nm$) called the perinuclear space. This forms a barrier between the materials present inside the nucleus and those of the cytoplasm.
$\Rightarrow$ The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
$\Rightarrow$ At a number of places,the nuclear envelope is interrupted by minute pores,which are formed by the fusion of its two membranes. These nuclear pores are the passages through which the movement of $RNA$ and protein molecules takes place in both directions between the nucleus and the cytoplasm.
$\Rightarrow$ Nucleoli: These are spherical structures present in the nucleoplasm. The content of the nucleolus is continuous with the rest of the nucleoplasm. It is a site for active ribosomal $RNA$ synthesis.
$\Rightarrow$ Large and more numerous nucleoli are present in cells actively carrying out protein synthesis.