Table of Contents
Introduction Of Nucleus
Nucleus is defined as the DNA containing region of the eukaryotic cells. Nucleus control the chemical activities of the cell and carries Herediatory information.
The nucleus or the karyon was first observed by Robert Brown in 1831.
All eukaryotic cells except mature red blood cells contain nucleus.
1. Position, Shape And Size
The nucleus is a comparatively large structure mostly found near the centre of the cell.
It shape is variable, like spherical ovoid, and sometimes irregular (as in neutrophil leucocytes).
Nucleus occupies about 1/10 of the volume of the cell. The size of the nucleus is correlated with the amount of DNA or number of Chromosomes present in it.
The haploid cells contain small sized nuclei in comparison to the nuclei of diploid, triploid or tetraploid cells.
Structure Of Nucleus
The interphase nucleus of the eukaryotic cells contains following structures:
- The nuclear envelope
- The nuclear sap or the nucleoplasm
- The chromatin and chromocenter
- The Nucleolus
1. The Nuclear Envelope
The nuclear contents remain Separated from the cell cytoplasm by the nuclear enveloped.
The nuclear envelope contains two Phospholipid membranes. Each membrane is about 10 nanometer thick.
The two membranes remain separated by a fluid filled space. This space is called the perinuclear space.
2. The Nuclear or The Nucleoplasm
The fluid substance present in the nucleus is called Nucleoplasm Or karyolymph Or nuclear matrix. It is composed of a net work of thin protein containing fibrils.
3. The Chromatin Net and Chromocenters
During interphase the chromosomes decondense and form a network called the charomatin net. On staining the nucleus with chromosomal stains (basic dyes) some areas of the nucleus stain darkly. These are called Chromocenters Chromocenters are the parts of chromosomes that remain condensed even in the interphase.
Types of Charomatin
Depending on the staining properties, two different types of chromatin can be recognised in the interphase nucleus.
(a). Euchromatin
Lightly stained parts of chromatin net are called euchromatin. These represent less condensed portions of chromosomes. Euchromatin is also called as dispersed chromatin.
(b). Heterochromatin
Darkly stained parts of the chromatin net are called heterochromatin. These represent highly condensed part of chromosomes. Chromosomes in condensed condition remain inactive that is they are not transcribed.
Usually there is some heterochromatin around the nucleus and some inside the nucleus. Both are chromatins appear to the connected together they are called as nucleolar chromatin.
4.The Nucleolus
Nucleolus was the first structure to be observed in the cell by light microscopy by fontona in 1774.
It is spherical in shape. It number per cell nucleus varies. A cell may contain one or more nucleoli in its nucleus. Its size varies according to the synthetic activity of the cell.
Its size is very small in cells not synthesizing proteins (sperm cells, blastomeres, muscle cells, dormant plant cells etc) it is large and may occupy 25%of the total nuclear volume in cells making large amount of protein (oocytes, neurons, and secretory cells)
Structure
Electron microscopic studies show that the nucleolus has no surrounding membrane. It contains the following structure
1. Matrix Or Pars Amorpha
The matrix or pars amorpha of the nucleolus is amorphous and is composed of proteins. All other nucleolar structure lie in the matrix.
2. Chromatin
The nucleolus contains large loops of chromosomes whose ribosomal RNA (rRNA) genes are transcribed at a rapid rate by RNA polymerase enzyme found in the nucleolus.
Such a loop of DNA is called as a nucleolar organizer region. According to the position the chromatin is named as perinucleolar chromatin (along the edges of nucleolus ), and intranucleolar chromatin (inside the nucleolus).