Nucleus

It is the most important part of the cell which directs and controls all the cellular activities. It also contains genetic information of the cell and also helps in transmission of characters to the future generation. A true nucleus is absent in prokaryotic cell and present in all eukaryotic cell except RBC, sieve tube cell (phloem). The number of a nucleus is variable in different types of cell. Most of the cells are uninucleate and some are binucleate . Eg: Paramecium and some are multinucleated like coenocytic hypha of mucor.

It is the largest cell organelles and its size is 5-25\(\mu\)m. It occurs in different shapes like rounded, oval, elliptical or lobed. In position, it occurs centrally in animal cell nd peripherally in the plant cell.

Structure of Nucleus

Structurally it consists of four major important parts which are as follows:

Nuclear Envelope
The nucleus is surrounded by a double membrane covering called nuclear envelope. It consists of two membranes outer and inner. The outer membrane is continuous with the endoplasmic reticulum and contains many ribosomes on its outer surface. The nuclear envelope has many small gaps called nuclear pores. It is lipo- proteinous in chemical composition.

Functions:
It gives shape and protection to the nucleus.
It regulates the flow of materials in and out of the nucleus.

Nucleoplasm
It is clear, transparent, semifluid ground substance which contains water, minerals, sugar, protein, nucleotides, RNA (mRNA, tRNA, rRNA ) and enzymes.

Functions:
It is the seat of synthesis of RNA and DNA.
It holds nucleolus and chromatin reticulum.
It acts as the nuclear skeleton.
It helps in the formation of spindle proteins for the cell division.

Nucleolus
It is dense dark- stained naked, rounded structure present inside the nucleus. It is chemically composed of RNA and protein. Its number may vary from 1 to 4 in a nucleus.

Functions:
It synthesises and stores RNA.
It forms sub- units of ribosomes.
It forms spindle during cell division.

Chromatin reticulum
It is the network of thin threadlike chromatin fibres which condensed into chromosomes during cell division. At interphase chromosomes remain in the form of chromatin fibres. The chromatin fibres are differentiated into two distinct regions called heterochromatin and euchromation. The differences between heterochromatin and euchromatin are as follows:

Chromosomes

The term 'chromosome' was given by Waldaye( 1888 A.D.).A chromosome is a long, stringy aggregate of genes that carries heredity information (DNA) and is formed from condensed chromatin. The electron microscopic structure of each chromosome includes following parts:

Chromonema: The metaphaisc chromosome is made up of two subunits called chromatids consists of two sub chromatids known as the chromonemata.

Centromere; They are constricted regions in chromosomes whose positions varies in different chromosomes. Depending upon their positions, they are categorised as metacentric, submetacentric, acrocentric and telocentric chromosomes.Nuclear organiser (secondary constriction I): The constriction near one end of the chromosome is called nuclear organiser which are necessary for the formation of the nucleolus.Secondary constriction; In addition to the centromere, the arms of the chromosomes may show one or more secondary constriction. It represents the site of breakage and subsequent fusion.

Nuclear organiser (secondary constriction I): The constriction near one end of the chromosome is called nuclear organiser which are necessary for the formation of the nucleolus.

Secondary constriction; In addition to the centromere, the arms of the chromosomes may show one or more secondary constriction. It represents the site of breakage and subsequent fusion.

Satellite: This is the part of chromosome beyond the nucleolar organiser which is very short like a sphere.

Telomeres: telomeres are the tip of chromosomes and prevent the ends of the chromosome from sticking together.

Function of Nucleus

• It contains genetic or hereditary materials or genes.
• It carries all the genetic information for growth, development, metabolism, differentiates, reproduction and behaviour.
• It controls all the cellular activities.
• It regulates protein synthesis.
• It forms ribosomes.
• It induces genetic changes and brings variation which is essential for organic evolution.