NUCLEUS

NUCLEUS

1. The nucleus is a small, round and membrane-bound structure found in cells.
2. And the fluid inside the nucleus surrounded by the nuclear membrane is called nucleoplasm.
3. It controls cell's growth and reproduction as it contains cell's hereditary information.
4. It's covered by dual layer called a nuclear membrane.
5. It's the most important part of the cell which controls and regulate all the vital activities of the cell .

Robert Brown (1881) discovered the nucleus.

J. Hammerling (1953) proved the hereditary role of the nucleus on a green alga Acetabularia

• Distribution : Present in all eukaryotic cells. In certain cells, they get disintegrated when cells attain maturity e.g., RBC in mammals, no true nucleus is present in prokaryotes.

• Number : Generally a cell contains a single nucleus i.e., mononucleated condition, sometimes certain cells show binucleated or multinucleated conditions

• Shape : The shape of the nucleus is variable and the shape depends on the functional state of cell. The shape may be discoid in case of flattened squamous epithelium cells, spherical, cuboidal or polyhedral cells, ovoid in columnar cells, bilobed or multilobed in leucocytes.

Position of nucleus

Structure of a nucleus

Nucleus has the following important parts:

• Nuclear membrane
• Nucleoplasm

Flow chart of Nucleus showing its constituents

Nuclear Membrane

• The nucleus is bounded by a double membrane structure called nuclear envelope or karyotheca.

• Nuclear membrane is semi-permeable and helps in the exchange of dissolved matter between protoplasm and nucleus while nuclear pores are the pathway for the exchange of micromolecules.

• It is a thin double membrane surrounding the nucleus.Each membrane is 90 Å thick and both are separated by a 120-300 Å, this space is known as perinuclear space. It contain many small nuclear pores of 400 Å diameter. They are surrounded by a thin membrane and protoplasmic Annulus.

• Inside the nucleus, a clear or slightly acidophilic mass is present, which is called karyoplasm or nuclear sap.

Nucleoplasm

• It is transparent, homogeneous and granular dense fluid. It possesses high quantities of phosphorous and proteins.

• Many thread like structures are irregularly present in nucleoplasm. Their shape changes during cell division.

It has following structures in it :

• Nuclear chromatin
• Nucleolus

Nuclear chromatin or chromatin material

• The nucleoplasm contains many thread like, coiled and much elongated structures known as Nuclear reticulum and is made up of chromatin, it is also known as chromatin net.

• These thread like structures contain small granular bodies known as chromomeres. At the time of cell division chromatin net thickens and distinguishes into rod like structures known as chromosomes.

• Each chromosome remains in the form of chromatin network but actually consists of two parts known as Hetero-chromatin and Euchromatin.

• It is made up of two chromonemata and each chromonemata thread contain numerous small button shaped particles known as genes.

• They are made up of DNA and help in the transfer of heredity characters from one generation to another.

Nucleolus

• The nucleus contains a large, spherical, dense, granular, and stable structure known as nucleolus.

• These are one or more round structures that are not bound by a membrane.

• They are rich in RNA and proteins.

• It is known as factory of ribosomes.

Functions of Nucleus

• Control centre : It controls all the metabolic activities of the cell. If it is removed the protoplasm dries up and dies.
• Genetic information : It is responsible for the transmission of hereditary traits.
• It regulates cell cycle.
• Ribosomes are formed by nucleolar part of nucleus.
• Nucleus plays a central role in cellular reproduction (process by which single cell divides to form two new cells).