Nucleus structure class 11 best note:

Introduction to nucleus:

The control center of the cell , nucleus,is the most prominent and largest cellular organelle. In all the eukaryotic cell this cellular organelle is present.

The diameter of the nucleus is 10-22 μ and occupies about 10℅of total volume of the cell.

Except the red blood cell ,nucleus is present in all the cell in the body . The cells with nucleus are called eukaryotes and the cell without nucleus are called prokaryotes.

In the process of cell division, nucleus has the most important role , because of this reason presence of nucleus in the cell is necessary.

The cells ,in which only one nucleus is present is known as uni-nucleated cells. And the cells in which multiple number of nucleus is present is known as multinucleated cells such as skeletal muscle cells.

In general this cellular organelle is present in the center of the cell. It is mostly spherical in nature.

However the shape and situation of the nucleus vary in some cells.

Diagram human cell nucleus, Image by-LadyofHats

Structure of nucleus:

The nucleus consists of nuclear membrane, nucleoplasm and nucleoli.

Nuclear membrane:

The nuclear membrane also known as nuclear envelope is a double layerd porous structure having a 40-70nm wide space known as perinuclear space or perinuclear cistern.

The special feature of this perinuclear cistern is ,it is continuous with the lumen of the endoplasmic reticulum.

The outer layer of the nuclear membrane is continuous with the endoplasmic reticulum of the cell cytoplasm.

Pores present in the nuclear membrane of the nucleus are gurded by protein molecules. Diameter of these pores is about 80-100nm .

However,it is decreased to about 7-9nm , because the protein molecules are attached with the periphery of the pores.

The exchange of materials between nucleoplasm and cytoplasm occurs through these pores.

Nucleoplasm:

The nucleoplasm is also known as nuclear matrix,is highly viscous in nature,and which helps in the formation of ground substance of the nucleus. The nucleoplasm contains large quantity of of genetic material in the form of DNA .

During the absence of a cell division,the nucleoplasm appears as dark staining thread like material called nuclear chromatin.

Large number of DNA molecules are involved in the formation of chromatin. Chromatin is referred as DNA-histone complex.

It forms the major bulk of the DNA material.

When the cell divides,the nuclear chromatin is converted into the rod-shaped structures,the chromosomes. Chromosome carries a complete blueprint of all the hereditary characteristics of that species.

Normally the chromosomes are not visible in the nucleus under microscope. But during cell division,the chromosomes are visible under microscope.

There is present 46chromosomes(23pairs) in all the dividing cells of the body except the reproductive cells ,which contains only 23 chromosomes (haploid number).

Structure of nucleus

Nucleolus:

Small , round granular structure of the nucleus is known as nucleolus. There is present one or more nucleoli in a nucleus . The nucleolus,unlike most other organelles does not have a limiting membrane .

The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes. With the help of five different pairs of chromosomes,the RNA is synthesized and which later stored in the nucleus.

Which later condensed to form the subunits of ribosomes. In the growing cell or in the cell which actively synthesize proteins ,the nucleoli are common.