Nucleus- Definition, Structure, Composition, Functions, Worksheet

What is Nucleus?

Nucleus Properties

  • The Nucleus is an important cell organelle that is present in the eukaryotic cell.
  • It is derived from the Latin word, ‘nucleus’ or ‘ nuculeus’. It means kernel or seed.
  • It is first described by Robert Brown as the cell organelle.
  • It was named chromatin by the Flemming.
  • It is double-membraned.
  • It consists of genetic material. 
  • The presence of a nucleus is one of the distinguishing features of the eukaryotic cell. 
  • All the cellular activities are directed and coordinated by the nucleus.
  • In the prokaryotic cell, the true nucleus is absent.
  • In the case of a eukaryotic cell, the nucleus is present in all cells except the Red Blood Cell (RBC) and sieve tube cell (phloem). 
  • In the different types of cells, there is a different number of the nucleus. 
  • Most of the cells consist of only one type of nucleus i.e uninucleated while some of them consist of two nuclei i.e binucleate. 
  • The multinucleated cell consists of two or more nuclei. Example: slime mold.
  • The size of the nucleus is 5-25 µm. So, it is considered to be the largest organelles. 
  • It occupies about 8% of the total cell volume.
  • There is a variation in the shape as well as the position too. 
  • The nucleus is of different shapes like round, oval, elliptical, or lobed.
  • The Nucleus is present in a different position.
  • In the animal cell, it is present in the center.
  • In the plant cell, it is present in the periphery. It is because in the center there is the presence of a large water-filled vacuole.


Structure of Nucleus

The structure of the nucleus consists of the following parts. They are:  

Nuclear envelope

  • It is of the double- membrane and surrounds the nucleus.
  • Outer and inner membrane are present in it. The outer membrane of the nucleus is continuous with the ER ( Endoplasmic reticulum ). On its outer surface, there is the presence of many ribosomes. 
  • Perinuclear space is present between the membranes.
  • It is impermeable to large molecules like proteins and RNA.
  • Small molecules and ions can move freely.
  • Nuclear pores are present in the nuclear envelope. These pores are the small gaps present in the envelope. 
  • Chemical composition: lipo-proteinous

The function of the nuclear envelope

  • It gives the shape to the nucleus.
  • It protects the internal constituents of the nucleus.
  • It controls and regulates the movement of the substances which enter and exits the nucleus.


  • During the cell division, it disappears in the late prophase. 
  • In the Telophase stage, it reappears. 
  • The structure of the nucleolus consists of three main regions. They are :
  • Fibrillar centers: In the form of partly condensed chromatin, ribosomal RiboNucleic Acids (rRNA) genes are present.
  • Fibrillar component:  It surrounds the fibrillar centers where RNA molecules are present.
  • Granular regions: It consists of the mature ribosomal precursor particles. These are the outermost regions.
  • In the nucleus, the nucleolus may be present 1 to 4 in number.
  • It is rounded in structure and naked.
  • It is dense and is stained dark in color.
  • Chemical composition: RNA and protein

Functions of the Nucleolus

  • RNA is synthesized and stored in it.
  • Sub-units of ribosomes are formed.
  • During the cell division, it forms the spindle.

Nucleus Structure

Other nuclear bodies

Different types of nuclear bodies are present in the nucleus. They are:

  • Cajal bodies
  • Gemini of Cajal bodies
  • PIKA ( Polymorphic interphase karyosomal association)
  • PML ( Promyelocytic leukemia) bodies
  • Paraspeckles
  • Splicing speckles

Chromatin reticulum

  • Inside the nucleus, there is the presence of chromatins.
  • During the cell division,  it is condensed in the chromosome. 
  • It is dense in structure and is thread-like.
  • It consists of proteins and DNA.
  • Chromosome remains in the form of chromatin fibers at the interphase stage of cell division. 
  • They are differentiated into two distinct regions as heterochromatin and euchromatin.

Comparison between heterochromatin and euchromatin can be done based on the following properties:

  • Stain: Heterochromatin is dark stained and euchromatin is a lightly stained region of chromatin reticulum.
  • Condensation: Heterochromatin is highly condensed and euchromatin is a less condensed region.
  • Proportion: Heterochromatin forms the small part whereas euchromatin forms the major part of the chromatin reticulum.
  • Activeness: Heterochromatin is genetically inactive and euchromatin is genetically active.


  • In 1888 A.D Waldaye gave the term chromosome. 
  • Chromosome consists of the gene and genes consist of the DNA.
  • All the genetic information, heredity characters are present in it. 
  • A Chromosome consists of the following parts which are revealed by the electron microscope. They are:
  • Chromonemata: It is known as the subchromatid. There are two subunits. Chromatids are the two subunits of the metaphasic chromosome.
  • Centromere: In the different chromosomes, there is the presence of the constricted regions in different places. Based on the positions they are categorized as metacentric, submetacentric, acrocentric, telocentric chromosomes. 

Metacentric chromosome: Centromere is present in the middle and it forms two equal arms of a chromosome.

Sub-metacentric chromosome: Centromere is present nearer to one end of the chromosome due to which resulting arms are unequal. One is a long arm while another is a shorter arm.

Acrocentric chromosome: Centromere is present close to the end. It also results in unequal arms of a chromosome. One will be very short whereas one will be a very long arm.

Telocentric chromosome: It has got a terminal centromere.

Nuclear organizer (secondary constriction I)

  • A Nuclear organizer is a constriction that is present near one end of the chromosome. It is necessary for the formation of the nucleolus.
  • One or more secondary constrictions may be present.

Satellite: It is very short like the sphere. It is present beyond the nucleolar organizer. It is the non-staining secondary constriction and is the small fragment. It is present only in a few chromosomes.

Telomeres: It is the tip of the chromosome. It prevents the ends of the chromosome from sticking together.

Nuclear matrix

  • It is also called a nuclear scaffold.
  • With the use of non-ionic detergents, nucleases, and high salt buffers, extraction of the nuclear matrix from the cell’s nucleus is possible.
  • It consists of the nuclear lamina which is a network of intermediate filaments.
  • It consists of lamin proteins.
  • It consists of the nuclear matrix which provides t mechanical support to the nucleus. It acts like the cytoskeleton. 
  • A Nuclear matrix is a network of fibers and filaments.


  • Nucleoplasm is a clear and transparent, gelatinous substance.
  • It is also known as the karyoplasm.
  • It surrounds the nucleolus and chromosomes.
  • It consists of water, minerals, sugar, protein, nucleotides, enzymes, and RNA.
  • Nucleoplasm is a semifluid substance.

Function of nucleoplasm

  • It forms the spindle proteins which aids in cell division.
  • It protects the contents of the nucleus.
  • It provides the medium by which the enzymes and nucleotides get transported throughout the nucleus.
  • RNA and DNA are synthesized in it.
  • Nucleolus and chromatin reticulum are held by nucleoplasm.
  • It provides support by acting as the nuclear skeleton.

Nucleus Structure Free Worksheet

Nucleus Structure Worksheet


Functions of the Nucleus

Storing genetic material

  • Genetic material like DNA is stored in the nucleus. 
  • It is the cellular hereditary material. 
  • The information which is encoded in the DNA is passed to the offsprings from the parents.

DNA replication

  • Replication is the process of copying the parental DNA.
  • It occurs in the cell nucleus. 
  • It takes place in the S phase of the interphase of the cell cycle.


  • It provides the site for genetic transcription. 
  • It allows the level of gene regulation which are not available to prokaryotes. It consists of a variety of proteins. 
  • It either mediates the transcription process directly or is they may be involved in regulating the process. 
  • Different proteins involved in it are helicases, RNA polymerases, topoisomerases, etc.

Other functions

  • The nucleus controls the gene expression and helps in the replication of DNA during the cell cycle.
  • Coordinates and regulates cellular activities like cell division, protein synthesis, and growth.
  • The formation of ribosomes occurs in the nucleolus.
  • Through the nuclear pores only selective transportation is allowed. 
  • Organic evolution: It involves variation and can induce genetic change.
  • Nucleolus stores the proteins and RNA.
  • In the nucleus, during the cell division, chromatins are arranged into chromosomes.

References and Sources

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