CELL THE UNIT OF LIFE Nucleus NEET NCERT
Channel: ISHTA’s BIO ACADEMY
Date: 2021-09-01

Nucleus as a cell organelle was first described by Robert Brown as early
as 1831. Later the material of the nucleus stained by the basic dyes was
given the name chromatin by Flemming.
The interphase nucleus (nucleus of a
cell when it is not dividing) has highly
extended and elaborate nucleoprotein
fibres called chromatin, nuclear matrix and
one or more spherical bodies called
nucleoli (sing.: nucleolus) (Figure 8.10).
Electron microscopy has revealed that the
nuclear envelope, which consists of two
parallel membranes with a space between
(10 to 50 nm) called the perinuclear space,
forms a barrier between the materials
present inside the nucleus and that of the
cytoplasm. The outer membrane usually
remains continuous with the endoplasmic
reticulum and also bears ribosomes on it.
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 movement of RNA and protein
molecules takes place in both directions between the nucleus and the
cytoplasm. Normally, there is only one nucleus per cell, variations in the
number of nuclei are also frequently observed. Can you recollect names
of organisms that have more than one nucleus per cell? Some mature
cells even lack nucleus, e.g., erythrocytes of many mammals and sieve
tube cells of vascular plants. Would you consider these cells as ‘living’?
The nuclear matrix or the nucleoplasm contains nucleolus and
chromatin. The nucleoli are spherical structures present in the
nucleoplasm. The content of nucleolus is continuous with the rest of the
nucleoplasm as it is not a membrane bound structure. It is a site for
active ribosomal RNA synthesis. Larger and more numerous nucleoli are
present in cells actively carrying out protein synthesis